Chapter 4 Physical Development in Infancy PDF

for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 121 Chapter Overview Growth and Stability Physical Growth: The Rapid Advances of Infancy The Nervous System and Brain: The Foundations of Development Nutrition in Infancy: Fueling Mo...

for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 121 Chapter Overview Growth and Stability Physical Growth: The Rapid Advances of Infancy The Nervous System and Brain: The Foundations of Development Nutrition in Infancy: Fueling Motor Development Breast or Bottle? The Development of the Senses Visual Perception: Seeing the World Integrating the Bodily Systems: The Life Cycles of Infancy Auditory Perception: The World of Sound SUID and SIDS: Unanticipated Killers Smell and Taste Motor Development Reflexes: Our Inborn Physical Skills Motor Development in Infancy: Landmarks of Physical Achievement Sensitivity to Pain and Touch Multimodal Perception: Combining Individual Sensory Inputs Liz and Seth Kaufman are so exhausted they have a hard time staying awake through dinner. The problem? Their 3-month-old son, Evan, showed no signs of adopting normal patterns of eating and sleeping any time soon. “I thought babies were these big sleep fanatics, but Evan takes little cat naps of an hour throughout the night, and then stays awake all day,” Liz says. “I’m running out of ways to entertain him because all I want to do is sleep.” Evan’s feeding schedule was hard on Liz, too. “He wants to nurse every hour for 5 hours in a row, which makes it hard to keep up my milk supply. Then he goes another 5 hours not wanting to nurse, and I’m positively, painfully engorged.” Seth tries to help out, walking at night with Evan when he won’t sleep, offering him a bottle of Liz’s expressed milk at 3 a.m. “But sometimes he just refuses the bottle,” Seth says. “Only mommy will do.” The pediatrician has assured the Kaufmans that their son is healthy and blossoming. “We’re pretty sure Evan will come out of this just fine,” Liz says. “It’s us we’re wondering about.” Looking Ahead Evan’s parents can relax. Their son will settle down. Sleeping through the night is just one of the succession of milestones that characterize the dramatic physical attainments of infancy. In this chapter we consider physical development during infancy, a period that starts at birth and continues until the second birthday. We begin by discussing the pace of growth, noting obvious changes in height and weight as well as less apparent changes in the nervous system. We also consider how infants quickly develop increasingly stable patterns in such basic activities as sleeping, eating, and attending to the world. Our discussion then turns to infants’ thrilling gains in motor development as skills emerge that eventually allow them to roll over, take the first step, and pick up a cookie crumb from the floor—skills that ultimately form the basis of even more complex behaviors. We start with basic, genetically determined reflexes and consider how even these may be modified through experience. We also discuss the nature and timing of the development of particular physical skills, look at whether their emergence can be speeded up, and consider the importance of early nutrition to their development. Finally, we explore how infants’ senses develop. We investigate how sensory systems such as hearing and vision operate, and how infants sort through the raw data from their sense organs and transform it into meaningful information. Antonio Guillem/Shutterstock Prologue: Dreaming of Sleep 122 PART 2 Infancy: Forming the Foundations of Life Growth and Stability Average newborns weigh slightly more than 7 pounds, which is less than the weight of the average Thanksgiving turkey. They measure about 20 inches in length, shorter than a loaf of French bread. They are helpless; if left to fend for themselves, they could not survive. Yet after just a few years, the story is quite different. Babies grow much larger, they are mobile, and they become increasingly independent. How does this growth happen? We can answer this question first by describing the changes in weight and height that occur over the first 2 years of life and then by examining some of the principles that underlie and direct that growth. Physical Growth: The Rapid Advances of Infancy LO 4.1 Describe how the human body develops in the first 2 years of life, including the four principles that govern its growth. Infants grow at a rapid pace over the first 2 years of their lives (see Figure 4-1). By the age of 5 months, the average infant’s birthweight has doubled, and they weigh around 15 pounds. By the first birthday, the baby’s weight has tripled to about 22 pounds. Although the pace of weight gain slows during the second year, by the end of their second year, the average child weighs around 4 times as much as they did at birth. Of course, there is a good deal of variation among infants. Height and weight measurements, which are taken regularly at physician’s visits during a baby’s first year, provide a way to spot problems in development. Figure 4-1 Height and Weight Growth Although the greatest increase in height and weight occurs during the first year of life, children continue to grow throughout infancy and toddlerhood. Weight Height cm 130 In 125 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 In Birth 120 115 110 105 100 95 90 85 80 75 70 65 60 55 50 45 cm kg 14 Boys 13 Girls 24 10 22 9 20 8 18 4 3 2 kg 3 Boys 28 26 5 2 30 11 6 Age (years) 32 12 7 1 lb Girls 16 14 12 10 8 6 4 lb Birth 1 2 Age (years) 3 for more ebook/ testbank/ solution manuals requests: The weight gains of infancy are matched by increased length. By the end of the first year, the typical baby grows almost a foot and is about 30 inches tall. By their second birthdays, children average a height of 3 feet. Not all parts of an infant’s body grow at the same rate. For instance, as we saw first in Chapter 2, at birth the head accounts for one-quarter of the newborn’s entire body size. During the first 2 years of life, the rest of the body begins to catch up. By age 2, the baby’s head is only one-fifth of body length, and by adulthood it is only one-eighth (see Figure 4-2). There are also sex and racial differences in weight and length. Females generally are slightly shorter and weigh slightly less than males—differences remain throughout childhood—and, as we will see later in the book, the disparities become considerably greater during adolescence. Furthermore, Asian infants tend to be slightly smaller than North American white infants, and Black American infants tend to be slightly bigger than North American white infants. The disproportionately large size of infants’ heads at birth is an example of one of four major principles (summarized in Table 4-1) that govern growth: FOUR PRINCIPLES OF GROWTH. email [email protected] Chapter 4 Physical Development in Infancy 123 Figure 4-2 Decreasing Proportions At birth, the head represents one-quarter of the neonate’s body. By adulthood, the head is only one-eighth the size of the body. Why is the neonate’s head proportionally so much larger? 1/4 1/5 1/6 1/7 1/8 Newborn 2 6 Age (years) 12 25 The cephalocaudal principle states that growth follows a direction and pattern that begins with the head and upper body parts and then proceeds to the rest of the body. The cephalocaudal growth principle means that we develop visual abilities (located in the head) well before we master the ability to walk (closer to the end of the body). The proximodistal principle states that development proceeds from the center of the body outward. The proximodistal principle means that the trunk of the body grows before the extremities of the arms and legs. Furthermore, development of the ability to use various parts of the body also follows the proximodistal principle. For instance, effective use of the arms precedes the ability to use the hands. The principle of hierarchical integration states that simple skills typically develop separately and independently but that these simple skills are integrated into more complex ones. Thus, the relatively complex skill of grasping something in the hand cannot be mastered until the developing infant learns how to control—and integrate—the movements of the individual fingers. Finally, the principle of the independence of systems suggests that different body systems grow at different rates. For instance, the patterns of growth for body size, the nervous system, and sexual maturation are quite different. cephalocaudal principle the principle that growth follows a pattern that begins with the head and upper body parts and then proceeds down to the rest of the body proximodistal principle the principle that development proceeds from the center of the body outward principle of hierarchical integration the principle that simple skills typically develop separately and independently but are later integrated into more complex skills principle of the independence of systems the principle that different body systems grow at different rates Table 4-1 The Major Principles Governing Growth Cephalocaudal Principle Proximodistal Principle Principle of Hierarchical Integration Principle of the Independence of Systems Growth follows a pattern that begins with the head and upper body parts and then proceeds to the rest of the body. Based on Greek and Latin roots meaning “head to tail.” Development proceeds from the center of the body outward. Based on the Latin words for “near” and “far.” Simple skills typically develop separately and independently. Later they are integrated into more complex skills. Different body systems grow at different rates. 124 PART 2 Infancy: Forming the Foundations of Life The Nervous System and Brain: The Foundations of Development LO 4.2 neuron the basic nerve cell of the nervous system synapse the gap at the connection between neurons, through which neurons chemically communicate with one another Describe how the nervous system and brain develop in the first 2 years of life and how the environment affects such development. When Rina was born, she was the first baby among her parents’ circle of friends. These young adults marveled at the infant, “oohing” and “aahing” at every sneeze and smile and whimper, trying to guess at their meaning. Whatever feelings, movements, and thoughts Rina was experiencing, they were all brought about by the same complex network: the infant’s nervous system. The nervous system is composed of the brain and the nerves that extend throughout the body. Neurons are the basic cells of the nervous system. Figure 4-3 shows the structure of an adult neuron. Like all cells in the body, neurons have a cell body containing a nucleus. But unlike other cells, neurons have a distinctive ability: They can communicate with other cells, using a cluster of fibers called dendrites at one end. Dendrites receive messages from other cells. At their opposite end, neurons have a long extension called an axon, the part of the neuron that carries messages destined for other neurons. Neurons do not actually touch one another. Rather, they communicate with other neurons by means of chemical messengers, neurotransmitters that travel across the small gaps, known as synapses, between neurons. Figure 4-3 The Neuron The basic element of the nervous system, the neuron, has a number of components. (Source: Van de Graaff, 2000.) Dendrites Cell body Axon Myelin sheath Movement of electrical impulse Terminal buttons for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 125 Although estimates vary, infants are born with between 100 billion and 200 billion neurons. To reach this number, neurons multiply at an amazing rate prior to birth. At some points in prenatal development, cell division creates some 250,000 additional neurons every minute. At birth, most neurons in an infant’s brain have relatively few connections to other neurons. During the first 2 years of life, however, a baby’s brain will establish billions of new connections between neurons. Furthermore, the network of neurons becomes increasingly complex, as illustrated in Figure 4-4. The intricacy of neural connections continues to increase throughout life. In adulthood, a single neuron is likely to have a minimum of 5,000 connections to other neurons or other body parts. Babies are actually born with many more neurons than they need. In addition, although synapses are formed throughout life based on our changing experiences, the billions of new synapses infants form during the first 2 years are more numerous than necessary. What happens to the extra neurons and synaptic connections? Like a farmer who, to strengthen the vitality of a fruit tree, prunes away unnecessary branches, brain development enhances certain capabilities in part by a “pruning down” of unnecessary neurons. Neurons that do not become interconnected with other neurons as the infant’s experience of the world increases become unnecessary. They eventually die out, increasing the efficiency of the nervous system. As unnecessary neurons are being reduced, connections between remaining neurons are expanded or eliminated as a result of their use or disuse during the baby’s experiences. If a baby’s experiences do not stimulate certain nerve connections, these, like unused neurons, are eliminated—a process called synaptic pruning. The result of synaptic pruning is to allow established neurons to build more elaborate communication networks with other neurons. Unlike most other aspects of growth, then, the development of the nervous system proceeds most effectively through the loss of cells (Zong et al., 2015; Athanasiu et al., 2017; Wen et al., 2020). After birth, neurons continue to increase in size. In addition to growth in dendrites, the axons of neurons become coated with myelin, a fatty substance that, like the insulation on an electric wire, provides protection and speeds the transmission of nerve impulses. So, even though many neurons are lost, the increasing size and complexity of the remaining ones contribute to impressive brain growth. A baby’s brain triples its weight during the first 2 years of life, and it reaches more than three-quarters of its adult weight and size by the age of 2. SYNAPTIC PRUNING. Figure 4-4 Neuron Networks Over the first 2 years of life, networks of neurons become increasingly complex and interconnected. Why are these connections important? (Source: Conel, 1930/1963.) At birth 1 month 3 months 15 months 24 months synaptic pruning the elimination of neurons as the result of nonuse or lack of stimulation myelin a fatty substance that helps insulate neurons and speeds the transmission of nerve impulses 126 PART 2 Infancy: Forming the Foundations of Life cerebral cortex the upper layer of the brain Figure 4-5 Shaken Baby Scott Camazine/Alamy Stock Photo This CT scan shows areas of the brain indicating (in red) severe brain injury in an infant suspected of being abused by a caretaker. plasticity the degree to which a developing structure or behavior is modifiable due to experience As they grow, the neurons also reposition themselves, becoming arranged by function. Some move into the cerebral cortex, the upper layer of the brain, whereas others move to subcortical levels, which are below the cerebral cortex. The subcortical levels, which regulate such fundamental activities as breathing and heart rate, are the most fully developed at birth. As time passes, however, the cells in the cerebral cortex, which are responsible for higher-order processes such as thinking and reasoning, become more developed and interconnected. For example, synapses and myelinization experience a growth spurt at around 3 to 4 months in the area of the cortex involving auditory and visual skills (areas called the auditory cortex and the visual cortex). This growth corresponds to the rapid increase in auditory and visual skills. Similarly, areas of the cortex related to body movement grow rapidly, allowing for improvement in motor skills. Although the brain is protected by the bones of the skull, it is highly sensitive to some forms of injury. One particularly devastating injury comes from a form of child abuse called shaken baby syndrome in which an infant is shaken by a caretaker or parent, usually out of frustration or anger due to a baby’s crying. Shaking can lead the brain to rotate within the skull, causing blood vessels to tear and destroying the intricate connections between neurons (Figure 4-5). The results of shaken baby syndrome can be devastating, leading to severe medical problems, including long-term physical disabilities such as blindness, hearing impairment, and speech disabilities. Some children experience learning disabilities and behavior disorders. In the most severe cases, the shaking leads to death. Approximately 20 out of 100,000 babies sustain injury due to shaken baby syndrome each year in the United States. One-quarter of babies who are shaken ultimately die, and 80 percent of survivors have permanent brain damage (Narang & Clarke, 2014; Grinkevičiūtė et al., 2016; Centers for Disease Control and Prevention [CDC], 2017a; Richey & Rilling, 2020). ENVIRONMENTAL INFLUENCES ON BRAIN DEVELOPMENT. Brain development, much of which unfolds automatically because of genetically predetermined patterns, is also strongly susceptible to environmental influences. In fact, the brain’s plasticity, the degree to which a developing structure or behavior is modifiable due to experience, is a significant attribute of it. The brain’s plasticity is greatest during the first several years of life. Because many areas of the brain are not yet devoted to specific tasks, if one area is injured, other areas can take over for the injured area. For example, preterm infants who suffer damage due to bleeding in the brain can recover almost entirely by age 2. In addition, even when particular parts of infants’ brains are injured due to accidents, other parts of the brain can compensate, aiding in recovery (Guzzetta et al., 2013; Rocha-Ferreira & Hristova, 2016). Similarly, because of the high degree of plasticity in the brain, infants who suffer brain injuries typically are less affected and recover more fully than adults who have experienced similar types of brain injuries, showing the infants’ high degree of plasticity. Of course, not even the brain’s inherent plasticity can fully protect against severe injuries, such as those resulting from the violent shaking typical of shaken baby syndrome (Stiles, 2012; Inguaggiato et al., 2017). Infants’ sensory experiences affect both the size of individual neurons and the structure of their interconnections. Consequently, compared with those brought up in more enriched environments, infants raised in severely restricted settings are likely to show differences in brain structure and weight (Glaser, 2012; Musacchia et al., 2017; de Almeida et al., 2020). Work with nonhumans has helped reveal the nature of the brain’s plasticity. Studies have compared rats raised in an unusually visually stimulating environment to those raised in more typical, and less interesting, cages. Results of such research show that areas of the brain associated with vision are both thicker and heavier for the rats reared in enriched settings (Degroot, Wolff, & Nomikos, 2005; Axelson et al., 2013; Stephany et al., 2016). In contrast, environments that are unusually barren or in some way restricted may impede the brain’s development. Again, work with nonhumans provides some for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy intriguing data. In one classic study, young kittens were fitted with goggles that restricted their vision so that they could view only vertical lines (Hirsch & Spinelli, 1970). When the cats grew up and had their goggles removed, they were unable to see horizontal lines, although they saw vertical lines perfectly well. Analogously, kittens whose goggles restricted their vision of vertical lines early in life were effectively blind to vertical lines during their adulthood—although their vision of horizontal lines was accurate. In contrast, when goggles are placed on older cats that have lived relatively normal lives as kittens, such results are not seen after the goggles are removed. The conclusion is that there is a sensitive period for the development of vision. As we noted in Chapter 1, a sensitive period is a specific but limited time, usually early in an organism’s life, during which the organism is particularly susceptible to environmental influences relating to some particular facet of development. A sensitive period may be associated with a behavior—such as the development of full vision—or with the development of a structure of the body, such as the configuration of the brain (Hartley & Lee, 2015; Opendak et al., 2017; Frankenhuis & Walasek, 2020). From a Social Worker’s Perspective What are some cultural or subcultural influences that might affect parents’ child-rearing practices? The existence of sensitive periods raises several important issues. For one thing, it suggests that unless infants receive a certain level of early environmental stimulation during a sensitive period, they may suffer damage or fail to develop capabilities, an effect that may never be fully remedied. If this is true, providing successful later intervention for such children may prove to be particularly challenging (Gottlieb & Blair, 2004; Zeanah, 2009). Support for this view comes from studies of the brains of children raised in poverty. For example, research has shown that children raised under impoverished circumstances have subtle differences in the structure of their brains. The surface area of the outer layer of cells in the brain is slightly smaller, particularly in areas of the brain associated with language and impulse control. Furthermore, some studies have found that the hippocampus, which is associated with memory and learning, has lower volume in children raised in poverty (Hair et al., 2015; Dufford et al., 2019; Katsnelson, 2021). Of course, the differences in the brains of children raised in impoverished circumstances do not mean that they are inevitably damaged. For example, moving children out of poverty can improve their outcomes. Ongoing studies are examining whether reducing poverty can lead to positive changes in brain development and social and emotional development (Baby’s First Years, 2021). Ultimately, determining how unusually impoverished or enriched environments affect later development is one of the major questions addressed by developmental researchers as they try to find ways to maximize opportunities for developing children. In the meantime, many developmentalists suggest that there are many ways parents and caregivers can provide a stimulating environment that will encourage healthy brain growth. Cuddling, talking and singing to, and playing with babies all help enrich their environment. In addition, holding children and reading to them is important because it simultaneously engages multiple senses, including vision, hearing, and touch (Shoemark, 2014; Dreyer, 2020). Integrating the Bodily Systems: The Life Cycles of Infancy LO 4.3 Explain the body rhythms and states that govern an infant’s behavior in the first 2 years of life. If you happen to overhear new parents discuss their newborns, chances are one or several bodily functions will be the subject. In the first days of life, infants’ body rhythms—waking, eating, sleeping, and eliminating—govern the infant’s behavior, often at seemingly random times. 127 sensitive period a specific but limited time, usually early in an organism’s life, during which the organism is particularly susceptible to environmental influences relating to some particular facet of development qingqing/Shutterstock Intellistudies/Shutterstock 128 PART 2 Infancy: Forming the Foundations of Life Infants cycle through various states, including crying and alertness. These states are integrated through bodily rhythms. These most basic activities are controlled by a variety of bodily systems. Although each of these individual behavioral patterns probably is functioning quite effectively, it takes some time and effort for infants to integrate the separate behaviors. One of the neonate’s major missions is to make its individual behaviors work in harmony, helping the neonate, for example, to sleep through the night (Waterhouse & DeCoursey, 2004). rhythms repetitive, cyclical patterns of behavior state the degree of awareness an infant displays to both internal and external stimulation RHYTHMS AND STATES. One of the most important ways that behavior becomes integrated is through the development of various rhythms, which are repetitive, cyclical patterns of behavior. Some rhythms are immediately obvious, such as the change from wakefulness to sleep. Others are more subtle but still easily noticeable, such as breathing and sucking patterns. Still other rhythms may require careful observation to be noticed. For instance, newborns may go through periods in which they jerk their legs in a regular pattern every minute or so. Although some of these rhythms are apparent just after birth, others emerge slowly over the first year as the neurons of the nervous system become increasingly integrated (Blumberg et al., 2014; Xiao et al., 2017). One of the major body rhythms is that of an infant’s state, the degree of awareness it displays to both internal and external stimulation. As can be seen in Table 4-2, such states include various levels of wakeful behaviors, such as alertness, fussing, and crying, and different levels of sleep. Each change in state brings about an alteration in the amount of stimulation required to get the infant’s attention (Anzman-Frasca et al., 2013; Miller et al., 2019). Some of the different states that infants experience produce changes in electrical activity in the brain. These changes are reflected in different patterns of electrical brain waves, which can be measured by a device called an electroencephalogram, or EEG. Starting at 3 months before birth, these brain wave patterns are relatively irregular. However, by the time an infant reaches the age of 3 months, a more mature pattern emerges and the brain waves become more regular (Cuevas et al., 2015; Pavlidis et al., 2020). SLEEP: PERCHANCE TO DREAM? At the beginning of infancy, the major state that occupies a baby’s time is sleep—much to the relief of exhausted parents, who often regard sleep as a welcome respite from caregiving responsibilities. On average, newborn infants sleep some 16 to 17 hours a day. However, there are wide variations. Some sleep more than 20 hours, whereas others sleep as little as 10 hours a day (de Graag et al., 2012; Korotchikova et al., 2016). Infants sleep a lot, but you probably shouldn’t ever wish to “sleep like a baby.” The sleep of infants comes in fits and starts. Rather than covering one long stretch, sleep for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 129 Table 4-2 Primary Behavioral States States Characteristics Time When in State (%) Awake States Alert Attentive or scanning, the infant’s eyes are open, bright, and shining. 6.7 Nonalert waking Eyes are usually open but dull and unfocused. Varied, but typically high motor activity. 2.8 Fuss Fussing is continuous or intermittent, at low levels. 1.8 Cry Intense vocalizations occurring singly or in succession. 1.7 Transition States Between Sleep and Waking Drowse Infant’s eyes are heavy-lidded but opening and closing slowly. Low level of motor activity. 4.4 Daze Open but glassy and immobile eyes. State occurs between episodes of alert and drowse. Low level of activity. 1.0 Sleep–wake transition Behaviors of both wakefulness and sleep are evident. Generalized motor activity; eyes may be closed or they open and close rapidly. State occurs when baby is awakening. 1.3 Active sleep Eyes closed; uneven respiration; intermittent rapid eye movements. Other behaviors: smiles, frowns, grimaces, mouthing, sucking, sighs, and sigh-sobs. 50.3 Quiet sleep Eyes are closed, and respiration is slow and regular. Motor activity is limited to occasional startles, sigh-sobs, or rhythmic mouthing. 28.1 Sleep States Transitional Sleep States Active–quiet transition sleep During this state, which occurs between periods of active sleep and quiet sleep, the eyes are closed and there is little motor activity. Infant shows mixed behavioral signs of active sleep and quiet sleep. 1.9 (Source: Adapted from Thoman, E. B., & Whitney, M. P. (1990) Behavioral states in infants: Individual differences and individual analyses, In J. Colombo & J. Fagen (eds.) Individual Differences in Infancy: Reliability, Stability, Prediction. Hillsdale, N.J.: Lawrence Erlbaum Associates.) initially comes in spurts of around 2 hours, followed by periods of wakefulness. Because of this, infants—and their sleep-deprived parents—are “out of sync” with the rest of the world, for whom sleep comes at night and wakefulness during the day (Burnham et al., 2002; Blomqvist et al., 2017; Barry, 2021). In addition, most babies do not sleep through the night for several months after birth. Parents’ sleep is interrupted, sometimes several times a night, by the infant’s cries for food and physical contact. Luckily for their parents, infants gradually settle into a more adult-like pattern. After a week, babies sleep a bit more at night and are awake for slightly longer periods during the day. Typically, by the age of 16 weeks, infants begin to sleep as much as 6 continuous hours at night, and daytime sleep falls into regular napping patterns. Most infants sleep through the night by the end of the first year, and the total amount of sleep they need each day is down to about 15 hours (Magee et al., 2014; De Beritto, 2020). Hidden beneath the supposedly tranquil sleep of infants is another cyclic pattern. During periods of sleep, infants’ heart rates increase and become irregular, their blood pressure rises, and they begin to breathe more rapidly. Sometimes, though not always, their closed eyes begin to move in a back-and-forth pattern, as if they were viewing an action-packed scene. This period of active sleep is similar, though not identical, to the rapid eye movement (REM) sleep that is found in older children and adults and is associated with dreaming (Blumberg et al., 2013). At first, this active, REM-like sleep takes up around one-half of an infant’s sleep, compared with just 20 percent of an adult’s sleep (see Figure 4-6). However, the quantity of active sleep quickly declines, and by the age of 6 months, it amounts to just one-third of total sleep time (Burnham et al., 2002; Staunton, 2005; Ferri et al., 2017). The appearance of active sleep periods that are similar to REM sleep in adults raises the intriguing question of whether infants dream during those periods. No one knows the rapid eye movement (REM) sleep the period of sleep that is found in older children and adults that is associated with dreaming 130 PART 2 Infancy: Forming the Foundations of Life answer, although it seems unlikely. First of all, young infants do not have much to dream about, given their relatively limited experiAs we age, the proportion of REM sleep decreases. In ences. Furthermore, the brain waves of sleeping infants appear to addition, the total amount of sleep falls as we get older. be qualitatively different from those of adults who are dreaming. It (Source: Based on Roffwarg et al., 1966.) is not until the baby reaches 3 or 4 months of age that the wave pat24 terns become similar to those of dreaming adults, suggesting that REM sleep young infants are not dreaming during active sleep—or at least are 20 Non-REM sleep not doing so in the same way as adults (Zampi et al., 2002). 16 What, then, is the function of REM sleep in infants? Although we don’t know for certain, some researchers think it provides a 12 means for the brain to stimulate itself—a process called autostimulation (Roffwarg et al., 1966). Stimulation of the nervous system 8 would be particularly important in infants, who spend so much 4 time sleeping and relatively little in alert states. Infants’ sleep cycles seem largely preprogrammed by genetic 0 50 80 1–15 3–5 6–23 2–3 3–5 5–9 10 –13 14–18 19–30 33–35 factors, but environmental influences also play a part. For Age instance, both long- and short-term stressors in infants’ environDays Months Years ments (such as a heat wave) can affect their sleep patterns. When environmental circumstances keep babies awake, sleep, when at last it comes, is apt to be less active (and quieter) than usual (Goodlin-Jones et al., 2000; Galland et al., 2012). Cultural practices also affect infants’ sleep patterns. For example, among the Kipsigis of Africa, infants sleep with their mothers at night and are allowed to nurse whenever they wake. In the daytime, they accompany their mothers during daily chores, often napping while strapped to their mothers’ backs. Because they are often out and on the go, Kipsigis infants do not sleep through the night until much later than babies in Western societies, and for the first 8 months of life, they seldom sleep longer than 3 hours at a stretch. In comparison, 8-month-old infants in the United States may sleep as long as 8 hours at a time. One reason for these cultural differences may relate to the use of artificial light and shades used to manage natural light that varies across cultures (Lightfoot et al., 2018; Ball et al., 2019). Hours Figure 4-6 REM Sleep Through the Life Span SUID and SIDS: Unanticipated Killers LO 4.4 Discuss SUID and SIDS and how they can be prevented. Like many parents of young infants, Samuel Hanke was exhausted. After positioning his 3-week-old son Charlie on his chest, facing belly down, Samuel fell asleep while watching television. When he woke up a few minutes later, he found to his horror that his son was dead (Carroll, 2018). sudden unexpected infant death (SUID) the abrupt death of an infant less than 1 year old that is unanticipated and unpredicted sudden infant death syndrome (SIDS) the unexplained death of a seemingly healthy baby The cause of Charlie’s death could never be ascertained, and his death was determined to be an example of sudden unexpected infant death, or SUID. Sudden unexpected infant death (SUID) is defined as death of an infant younger than 1 year old and that has no immediately obvious cause. About a third of SUID can be attributed to accidental suffocation and strangulation, and the most common SUID is sudden infant death syndrome, or SIDS. Sudden infant death syndrome (SIDS) is a disorder in which seemingly healthy infants die in their sleep. Put to bed for a nap or for the night, an infant simply never wakes up (see Figure 4-7). SIDS strikes about 1,250 infants in the United States each year. Although it seems to occur when the normal patterns of breathing during sleep are interrupted, scientists have been unable to discover why that might happen. It is clear that infants don’t smother or choke; they die a peaceful death, simply ceasing to breathe. Although no totally reliable means for preventing the syndrome has been found, the good news is that there is increasing research showing how SIDS can be avoided. The American Academy of Pediatrics (AAP) suggests that babies sleep on their backs for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 131 Figure 4-7 Trends in Sudden Unexpected Infant Death This graph shows the trends in Sudden Unexpected Infant Death (SUID) rates in the United States from 1990 through 2019. (Source: CDC/NCHS, National Vital Statistics System, Compressed Mortality File 2021.) 180 Deaths per 100,000 Live Births 160 Combined SUID rate 140 120 100 80 60 Sudden infant death syndrome Unknown cause 40 20 0 1990 Accidental suffocation and strangulation in bed 1994 1998 2002 2006 2010 2014 2018 rather than on their sides or stomachs—called the back-to-sleep guideline. The AAP also suggests that infants share a bedroom with their parents (but not the same sleeping surface) until the baby turns 1 year of age and, if not that, at least for the first 6 months. Sharing a room decreases the risk of SIDS by as much as 50 percent (Moon, 2016). The latest AAP guidelines also suggest avoiding the use of soft bedding, including no crib bumpers, blankets, pillows, or soft toys. In other words, the crib should be bare except for a tight-fitting sheet. In fact, a simple cardboard baby box may be preferable to a crib for young infants. Such boxes have been used in Finland for decades, and that country has one of the lowest infant mortality rates in the world (and half that of the United States; Ball & Volpe, 2013; Catalini, 2017; Fodaro, 2017). Although the AAP guidelines have not completely eradicated SIDS, they have been responsible for a significant reduction in the number of deaths due to the condition. Specifically, SIDS rates fell from 130 deaths per 100,000 live births in 1990 to 33 deaths per 100,000 live births in 2019. Still, SIDS remains the leading cause of death in children younger than the age of 1 year (Middlemiss et al., 2019; Centers for Disease Control and Prevention [CDC], 2021). Some infants are more at risk for SIDS than are others. For instance, males and Black Americans are at greater risk. In addition, low birth weight and low Apgar scores found at birth are associated with SIDS, as is having a mother who smokes during pregnancy. Some evidence also suggests that a brain abnormality in the hippocampus, the area of the brain that affects breathing, may produce SIDS. In a small number of cases, child abuse may be the actual cause. Other hypotheses have suggested infants who die from SIDS may have had undiagnosed sleep disorders, nutritional deficiencies, problems with reflexes, or undiagnosed illnesses. Still, there is no clear-cut factor that explains why some infants Cardboard baby boxes may be preferable to cribs for young die from the syndrome. SIDS is found in children of every race infants. Milla Kontkanen/Alamy Stock Photo Year 132 PART 2 Infancy: Forming the Foundations of Life and socioeconomic group and in children who have had no apparent health problems (Gollenberg & Fendley, 2018; Keywan et al., 2021). Because parents are unprepared for the death of an infant in a case of SUID, the event is particularly devastating. Parents often feel guilt, fearing that they were neglectful or somehow contributed to their child’s death (Behm et al., 2012; Horne, 2017; Pabayo et al., 2019). Module 4.1 Review LO 4.1 Describe how the human body develops in the first 2 years of life, including the four principles that govern its growth. LO 4.3 Explain the body rhythms and states that govern an infant’s behavior in the first 2 years of life. Infants grow at a rapid pace over the first 2 years of life. The major principles of growth are the cephalocaudal principle, the proximodistal principle, the principle of hierarchical integration, and the principle of the independence of systems. Babies integrate their individual behaviors by developing rhythms—repetitive, cyclical patterns of behavior. A major rhythm relates to the infant’s state—the awareness it displays to internal and external stimulation. LO 4.2 LO 4.4 Describe how the nervous system and brain develop in the first 2 years of life and how the environment affects such development. The development of the nervous system first entails the development of billions of neurons and interconnections among them. Later, the numbers of both neurons and connections decrease as a result of the infant’s experiences. Brain plasticity, the susceptibility of a developing organism to environmental influences, is relatively high. Researchers have identified sensitive periods during the development of body systems and behaviors—limited periods when the organism is particularly susceptible to environmental influences. Discuss SUID and SIDS and how they can be prevented. SUID and SIDS are disorders in which seemingly healthy infants die in their sleep. Experts now urge that infants sleep on their backs to avoid SUID and SIDS. Journal Prompt Applying Lifespan Development: What evolutionary advantage could there be for infants to be born with more nerve cells than they actually need or use? Motor Development Suppose a genetic engineering firm hired you to redesign newborns and charged you with replacing the current version with a new, more mobile one. The first change you’d probably consider in carrying out this (luckily fictitious) job would be in the conformation and composition of the baby’s body. The shape and proportions of newborn babies are simply not conducive to easy mobility. Their heads are so large and heavy that young infants lack the strength to raise them. Because their limbs are short in relation to the rest of the body, their movements are further impeded. Furthermore, their bodies are mainly fat, with a limited amount of muscle; the result is that they lack strength. Fortunately, it doesn’t take too long before infants begin to develop a remarkable amount of mobility. Actually, even at birth they have an extensive repertoire of behavioral possibilities brought about by innate reflexes, and their range of motor skills grows rapidly during the first 2 years of life. Reflexes: Our Inborn Physical Skills LO 4.5 Explain how the reflexes that infants are born with help them adapt to their surroundings and protect themselves. When her father pressed 3-day-old Christina’s palm with his finger, she responded by tightly winding her small fist around his finger and grasping it. When he moved his finger upward, she held on so tightly that it seemed he might be able to lift her completely off her crib floor. for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 133 Table 4-3 Some Basic Reflexes in Infants Reflex Approximate Age of Disappearance Rooting reflex Description Possible Function 3 weeks Neonate’s tendency to turn its head toward things that touch its cheek. Food intake Stepping reflex 2 months Movement of legs when held upright with feet touching the floor. Prepares infants for independent locomotion Swimming reflex 4–6 months Infant’s tendency to paddle and kick in a sort of swimming motion when lying facedown in a body of water. Avoidance of danger Grasping reflex 5–6 months Infant’s fingers close around an object placed in its hands. Provides support Moro reflex 6 months Activated when support for the neck and head is suddenly removed. The arms of the infant are thrust outward and then appear to grasp onto something. Similar to primates’ protection from falling Babinski reflex 8–12 months An infant fans out its toes in response to a stroke on the outside of its foot. Unknown Startle reflex Remains in different form An infant, in response to a sudden noise, flings out its arms, arches its back, and spreads its fingers. Protection Eye-blink reflex Remains Rapid shutting and opening of eye on exposure to direct light. Protection of eye from direct light Sucking reflex Remains Infant’s tendency to suck at things that touch its lips. Food intake Gag reflex Remains An infant’s reflex to clear its throat. Prevents choking Her father was right: Christina probably could have been lifted in this way. The reason for her resolute grip was activation of one of the dozens of reflexes with which infants are born. Reflexes are unlearned, organized, involuntary responses that occur automatically in the presence of certain stimuli. Newborns enter the world with a repertoire of reflexive behavioral patterns that help them adapt to their new surroundings and serve to protect them. As we can see from the list of reflexes in Table 4-3, many reflexes clearly represent behavior that has survival value, helping to ensure the well-being of the infant. For instance, the swimming reflex makes a baby who is lying facedown in a body of water paddle and kick in a sort of swimming motion. The obvious consequence of such behavior is to help the baby move from danger and survive until a caregiver can come to its rescue. Similarly, the eye-blink reflex seems designed to protect the eye from too much direct light, which might damage the retina. Given the protective value of many reflexes, it might seem beneficial for them to remain with us for our entire lives. In fact, some do: The eye-blink reflex remains functional throughout the full life span. But quite a few reflexes, such as the swimming reflex, disappear after a few months. Why should this be the case? Researchers who focus on evolutionary explanations of development attribute the gradual disappearance of reflexes to the increase in voluntary control over behavior that occurs as infants become more able to control their muscles. In addition, it may be that reflexes form the foundation for future, more complex behaviors. As these more intricate behaviors become well learned, they encompass the earlier reflexes (Lipsitt, 2003; Reid et al., 2019). THE BASIC REFLEXES. RACIAL AND CULTURAL DIFFERENCES AND SIMILARITIES IN REFLEXES. Although reflexes are, by definition, genetically determined and universal throughout all infants, there are actually some cultural variations in the ways they are displayed. For instance, consider the Moro reflex, which is activated when support for the neck and head is reflexes unlearned, organized, involuntary responses that occur automatically in the presence of certain stimuli LECA/BSIP SA/Alamy Stock Photo 134 PART 2 Infancy: Forming the Foundations of Life Blend Images - Mike Kemp/ Getty Images (a) Jules Selmes/Pearson Education Ltd (b) (c) Infants showing (a) the grasping reflex, (b) the startle reflex, and (c) the Moro reflex. suddenly removed. The Moro reflex consists of the infant’s arms thrusting outward and then appearing to seek to grasp onto something. Most scientists feel that the Moro reflex represents a leftover response that we humans have inherited from our nonhuman ancestors. The Moro reflex is an extremely useful behavior for monkey babies, who travel about by clinging to their mothers’ backs. If they lose their grip, they fall down unless they are able to grasp quickly onto their mother’s fur—using a Moro-like reflex (Zafeiriou, 2004; Rousseau et al., 2017). The Moro reflex is found in all humans, but it appears with significantly different vigor in different children. Some differences reflect cultural and racial variations (Freedman, 1979). For instance, white infants show a pronounced response to situations that produce the Moro reflex. Not only do they fling out their arms, but they also cry and respond in a generally agitated manner. In contrast, Navajo Indian babies react to the same situation much more calmly. Their arms do not flail out as much, and they cry only rarely. In some cases, reflexes can serve as helpful diagnostic tools for pediatricians. Because reflexes emerge and disappear on a regular timetable, their absence—or presence—at a given point of infancy can provide a clue that something may be amiss in an infant’s development. (Even for adults, physicians include reflexes in their diagnostic bags of tricks, as anyone knows who has had their knee tapped with a rubber mallet to see if the lower leg jerks forward.) Reflexes evolved because, at one point in humankind’s history, they had survival value. For example, the sucking reflex automatically helps infants obtain nourishment, and the rooting reflex helps them search for the presence of a nipple. In addition, some reflexes also serve a social function, promoting caregiving and nurturance. For instance, Christina’s father, who found his daughter gripping his finger tightly when he pressed her palm, probably cares little that she is simply responding with an innate reflex. Instead, he will more likely view his daughter’s action as responsiveness to him, a signal perhaps of increasing interest and affection on her part. As we will see in Chapter 6, when we discuss the social and personality development of infants, such apparent responsiveness can help cement the growing social relationship between an infant and its caregivers. Motor Development in Infancy: Landmarks of Physical Achievement LO 4.6 Summarize the landmarks of motor skill development in infancy. Probably no physical changes are more obvious—and more eagerly anticipated—than the increasing array of motor skills that babies acquire during infancy. Most parents can remember their child’s first steps with a sense of pride and awe at how quickly their child changed from a helpless infant, unable even to roll over, into a person who could navigate quite effectively in the world. Even though the motor skills of newborn infants are not terribly sophisticated, at least compared with attainments that will soon appear, young infants still are able to accomplish some kinds of movement. For instance, when placed on their stomachs, they wiggle their arms and legs and may try to lift their heavy heads. As their strength increases, they are able to push hard enough against the surface on which they are resting to propel their bodies in different directions. They often end up moving backward rather than forward, but by the age of 6 months they become rather accomplished at moving themselves in particular directions. These initial efforts are the forerunners of crawling, in which babies coordinate the motions of their arms and legs and propel themselves forward. Crawling develops typically between 8 and 10 months. Figure 4-8 provides a summary of some of the milestones of normal motor development. Walking comes later. At around the age of 9 months, most infants are able to walk by supporting themselves on furniture, and half of all infants can walk well by the end of their first year of life. GROSS MOTOR SKILLS. for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 135 Figure 4-8 Milestones of Motor Development Fifty percent of children are able to perform each skill at the month indicated in the figure. However, the specific timing at which each skill develops varies widely. For example, one-quarter of children are able to walk well at 11.1 months; by 14.9 months, 90 percent of children are walking well. Is knowledge of such average benchmarks helpful or harmful to parents? (Source: Adapted from Frankenburg et al., 1992.) 3.2 months: rolling over 3.3 months: grasping rattle 5.9 months: sitting without support 11.5 months: standing alone well 12.3 months: walking well 14.8 months: building tower of two cubes 7.2 months: standing while holding on 16.6 months: walking up steps 8.2 months: grasping with thumb and finger 23.8 months: jumping in place At the same time infants are learning to move around, they are perfecting the ability to remain in a stationary sitting position. At first, babies cannot remain seated upright without support. But they quickly master this ability, and most are able to sit without support by the age of 6 months. It is also the case that child rearing practices related to motor development vary considerably, both within and across different cultures. We discuss one difference in From Research to Practice. As infants are perfecting their gross motor skills, such as sitting upright and walking, they are also making advances in their fine motor skills. For instance, by the age of 3 months, infants show some ability to coordinate the movements of their limbs. Furthermore, although infants are born with a rudimentary ability to reach toward an object, this ability is neither very sophisticated nor very accurate, and it disappears around the age of 4 weeks. A different, more precise form of reaching reappears at 4 months. It takes some time for infants to coordinate successful grasping after they reach out, but in fairly short order they are able to reach out and hold onto an object of interest (Daum et al., 2011; Foroud, & Whishaw, 2012; Libertus et al., 2016). The sophistication of fine motor skills continues to grow. By the age of 11 months, infants are able to pick up off the ground objects as small as marbles—something caregivers need to be concerned about because the next place such objects often go is the mouth. By the time they are 2 years old, children can carefully hold a cup, bring it to their lips, and This infant demonstrates his fine motor skills. take a drink without spilling a drop. Jules Selmes/Pearson Education Ltd FINE MOTOR SKILLS. 136 PART 2 Infancy: Forming the Foundations of Life From Research to Practice Differences in Infant Mobility: Does Practice Really Make Perfect? You have probably heard that strength training increases your athletic performance. But did you know that this applies to babies as well? As babies roll, attempt to sit, or practice crawling, they are building the necessary muscular strength to control their body further, enhancing the development of motor skills. Gaining more control of your voluntary movements and displacement, in turn, facilitates the development of various cognitive, perceptual, and social skills. For instance, being able to sit and control hand movements encourages infants’ physical exploration of the immediate environment, which supports conceptual development (Adolph & Hoch, 2019). Is the importance of independent movement the same across diverse cultural communities, given differences in baby handling? Caregivers across cultures vary in the ways they carry and handle babies, which lead to differences in how much movement is allowed early in development. Some cultures, such as various indigenous communities in the South American Andes, restrict movement in the first few months after birth by swaddling the infant tightly in a waltha. Other cultural groups such as some African and Caribbean communities promote movement and early motor development through rough handling and exercising of babies. Still, the early differences observed in motor development tend to disappear by the time children reach their second birthday. A recent study documenting a different baby-handling practice challenges our ideas about this universal caregiving characteristic and raises questions about our current understanding of motor development. Psychologist Lana Karasik and her colleagues traveled to the Central Asian country of Tajikistan to describe the traditional custom of cradling through a gahvora, a wooden transportable rocking cradle, that severely restricts babies’ movement throughout infancy (Karasik et al., 2018). Their results showed widespread adoption of the practice (i.e., 98% of the families) with the overwhelming majority of families starting cradling right after birth. The researchers were surprised by the consistency across families in how infants were cradled. Babies were swaddled, blanketed, and were prepared in such a way that their waste was collected outside of the gahvora without removing the baby from the cradle. The bounding of babies restricted all movement except for slight sideway head movements. Also, the infants were fed while in the gahvora. Despite these consistencies, the researchers found differences in the length of time and time of the day infants were kept in the gahvora. About half of the babies across all ages, were cradled at night for sleeping, for an average of 14 hours per day. About a third across various age groups were cradled only during the day for short periods of time averaging about 6 hours in a day. Finally, a third group, composed of mostly younger infants, were cradled for longer periods of time both during the day and night for an average of 19 hours. This study documents a child-rearing practice that differs considerably from the practices adopted in many other cultures. Yet children still learn to function affectively, ultimately learning to walk, talk, and play. Future research needs to identify whether differences in gahvora use has long-lasting effects on children’s physical as well as psychological development. Shared Writing Prompt What effects might cradling have on children’s long-term motor development? How about on other areas of development? Grasping, like other motor advances, follows a sequential developmental pattern in which simple skills are combined into more sophisticated ones. For example, infants first begin picking things up with their whole hand. As they get older, they use a pincer grasp, where thumb and index finger meet to form a circle. The pincer grasp allows for considerably more precise motor control (Thoermer et al., 2013; Senna et al., 2017; Karl et al., 2019). DYNAMIC SYSTEMS THEORY: HOW MOTOR DEVELOPMENT IS COORDINATED. Although it is easy to think about motor development in terms of a dynamic systems theory a theory of how motor skills develop and are coordinated series of individual achievements, the reality is that each of these skills does not develop in a vacuum. Each skill (such as a baby’s ability to pick up a spoon and guide it to their lips) advances in the context of other motor abilities (such as the ability to reach out and lift the spoon in the first place). Furthermore, as motor skills are developing, so also are nonmotor skills such as visual capabilities. Developmentalist Esther Thelen has created an innovative theory to explain how motor skills develop and are coordinated. Dynamic systems theory describes how motor behaviors are assembled. By “assembled,” Thelen means the coordination of a variety of skills that develop in a child, ranging from the development of an infant’s muscles to its perceptual abilities and nervous system, as well as its motivation to carry out particular motor activities, and support from the environment (Thelen & Smith, 2006; Perone & Simmering, 2017). for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 137 According to dynamic systems theory, motor development in a particular sphere, such as beginning to crawl, is not just dependent on the brain initiating a “crawling program” that permits the muscles to propel the baby forward. Instead, crawling requires the coordination of muscles, perception, cognition, and motivation. The theory emphasizes how children’s exploratory activities, which produce new challenges as they interact with their environment, lead them to advancements in motor skills (Corbetta & Snapp-Childs, 2009; Adolph & Hoch, 2019). Dynamic systems theory is noteworthy for its emphasis on a child’s own motivation (a cognitive state) in advancing important aspects of motor development. For example, infants need to be motivated to touch something out of their reach to develop the skills they need to crawl to it. Furthermore, recent research builds on dynamic systems theory by showing how the acquisition of new motor skills helps infants to better access their surroundings, which in turn leads to new opportunities for learning. And this new learning paves the way for the development of critical cognitive skills such as language acquisition (which we’ll discuss in the next chapter). In short, there is a developmental cascade during the first year, leading to a snowballing surge in motor and language skills (Iverson, 2021). Keep in mind that the timing of the milestones in motor development that we have been discussing is based on norms. Norms represent the average performance of a large sample of children of a given age. They permit comparisons between a particular child’s performance on a particular behavior and the average performance of the children in the norm sample. For instance, one of the most widely used techniques to determine infants’ normative standing is the Brazelton Neonatal Behavioral Assessment Scale (NBAS), a measure designed to determine infants’ neurological and behavioral responses to their environment. The NBAS provides a supplement to the traditional Apgar test that is given immediately following birth. Taking about 30 minutes to administer, the NBAS includes 27 separate categories of responses that constitute four general aspects of infants’ behavior: interactions with others (such as alertness and cuddliness), motor behavior, physiological control (such as the ability to be soothed after being upset), and responses to stress (Canals, et al., 2003; Ohta & Ohgi, 2013; Barlow et al., 2018). Although the norms provided by scales such as the NBAS are useful in making broad generalizations about the timing of various behaviors and skills, they must be interpreted with caution. Because norms are averages, they mask substantial individual differences in the timing of attaining various achievements. For example, some children may be ahead of the norm. Other perfectly normal children may be a bit behind. Norms also may hide the fact that the sequence in which various behaviors are achieved may differ somewhat from one child to another. Finally, scores on measures such as the NBAS have not always been shown to be closely associated with future outcomes (Boatella-Costa et al., 2007; Noble & Boyd, 2012; Barlow et al., 2018). Furthermore, norms are useful only to the extent that they are based on data from a large, heterogeneous, culturally diverse sample of children. Unfortunately, many of the norms on which developmental researchers have traditionally relied have been based on groups of infants who are predominantly white and from the middle and upper socioeconomic strata. The reason: Much of the research was conducted on college campuses, using the children of graduate students and faculty. This limitation would not be critical if no differences existed in the timing of development in children from different cultural, racial, and social groups. But they do. For example, as a group, Black babies show more rapid motor development than white babies throughout infancy. Moreover, there are significant variations related to cultural factors, as we discuss in Developmental Diversity and Your Life (Wu et al., 2008; Mendonça et al., 2016; Halpin et al., 2019). DEVELOPMENTAL NORMS: COMPARING ONE INFANT TO ANOTHER. norm the average performance of a large sample of children of a given age Brazelton Neonatal Behavioral Assessment Scale (NBAS) a measure designed to determine infants’ neurological and behavioral responses to their environment 138 PART 2 Infancy: Forming the Foundations of Life Developmental Diversity and Your Life The Cultural Dimensions of Motor Development Among the Ache people, who live in the rain forest of South America, infants face an early life of physical restriction. Because the Ache live in a series of tiny camps in the rain forest, open space is at a premium. Consequently, for the first few years of life, infants spend nearly all their time in direct physical contact with their mothers. Lucian Coman/Shutterstock *** Infants among the Kipsigis people, who live in a more open environment in rural Kenya, Africa, lead quite a different existence. Their lives are filled with activity and exercise. Parents seek to teach their children to sit up, stand, and walk from the earliest days of infancy. Parents begin to teach their children to walk starting at the 8th week of life. The infants are held with their feet touching the ground, and they are pushed forward. Clearly, the infants in these two societies lead very different lives (Super, 1976; Kaplan & Dove, 1987). But do the relative lack of early motor stimulation for Ache infants and the efforts of the Kipsigis to encourage motor development really make a difference? In fact, Ache infants tend to show delayed motor development, relative both to Kipsigis infants and to children raised in Western societies. They tend to begin walking at around 23 months, about a year later than the typical child in the United States. In contrast, Kipsigis children, who are encouraged in their motor development, learn to sit up and walk several weeks earlier, on average, than American children. As we see with the Ache and Kipsigis babies, variations in the timing of motor skills seem to depend in part on parental expectations of what is the “appropriate” schedule for the emergence of specific skills. Thus, cultural factors help Cultural influences affect the rate of the development of motor skills. determine the time at which specific motor skills appear. Activities that are an intrinsic part of a culture are more apt to be purposely taught to infants in that culture, leading to the potential of their earlier emergence (Suir et al, 2019). Still, there are certain limitations on how early a skill can emerge. It is physically impossible for 1-month-old infants to stand and walk, regardless of the encouragement and practice they may get within their culture. Parents who are eager to accelerate their infants’ motor development, then, should be cautioned not to hold overly ambitious goals. Although some parents may take pride in a child who walks earlier than other babies (just as some parents may be concerned over a delay of a few weeks), in the long run, the timing of this activity will probably make no difference. Nutrition in Infancy: Fueling Motor Development LO 4.7 Summarize the role of nutrition in the physical development of infants. Rosa sighed as she sat down to nurse the baby—again. She had fed 4-week-old Juan about every hour today, and he still seemed hungry. Some days, it seemed as if all she did was breastfeed her baby. “Well, he must be going through a growth spurt,” she decided, as she settled into her favorite rocking chair and put the baby to her nipple. The rapid physical growth that occurs during infancy is fueled by the nutrients that infants receive. Without proper nutrition, infants cannot reach their physical potential, and they may suffer cognitive and social consequences as well (Costello et al., 2003; Gregory, 2005; Poinsot et al., 2020). Although there are vast individual differences in what constitutes appropriate nutrition—infants differ in terms of growth rates, body composition, metabolism, and activity levels—some broad guidelines do hold. In general, infants should consume about 50 calories per day for each pound they weigh—an allotment that is twice the suggested caloric intake for adults (Skinner et al., 2004). Typically, though, it’s not necessary to count calories for infants. Most infants regulate their caloric intake quite effectively on their own. If they are allowed to consume as much as they seem to want, and not pressured to eat more, they will do fine. for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy Malnutrition, the condition of having an improper amount and balance of nutrients, produces several results—none of them good. For instance, malnutrition is more common among children living in many developing countries than among children who live in more industrialized, affluent countries. Malnourished children in these countries begin to show a slower growth rate by the age of 6 months. By the time they reach the age of 2 years, their height and weight are only 95 percent of the height and weight of children in more industrialized countries. In addition, children who have been chronically malnourished during infancy later score lower on IQ tests and tend to do less well in school. These effects may linger even after the children’s diet has improved substantially (Ratanachu-Ek, 2003; Waber et al., 2014; Peter et al., 2016). The problem of malnutrition due to food insecurity is greatest in underdeveloped countries, where overall 10 percent of infants are severely malnourished (see Figure 4-9). In some countries, the problem is especially severe. Undernutrition is a problem across the globe and is particularly widespread in Asia and Africa (UNICEF, 2016). For example, 1 in 10 North Korean children younger than 5 years old is underweight and 1 in 5 are stunted from chronic malnutrition (Chaudhary & Sharma, 2012; United Nations World Food Programme, 2013; UNICEF, 2019). Problems of malnourishment are not restricted to developing countries, however. In the United States, around 20 percent of children live in poverty, which puts them at risk for malnutrition. More than 1 in 7 children live in households that are food insecure, which means that they lack reliable access to sufficient quantities of nutritious food. Moreover, Black and Hispanic children are twice as likely to live in food-insecure households as white children (Children’s Defense Fund, 2021). Overall, some 26 percent of families who have children 3 years old and younger live in poverty, and 6 percent of Americans live in extreme poverty, meaning their income is $10,000 a year or less (see Figure 4-10; U.S. Department of Agriculture, 2020). MALNUTRITION. Figure 4-9 Undernutrition by Country Undernutrition is widespread across the globe. What countries in Africa have higher and lower rates of underweight children? What about Asia? (Source: United Nations Children’s Fund (UNICEF), World Health Organization, International Bank for Reconstruction and Development/The World Bank. Levels and trends in child malnutrition: Key Findings of the 2020 Edition of the Joint Child Malnutrition Estimates. Geneva: World Health Organization, 2020.). 2.6 North America 17.6 12.6 8.1 Caribbean 7.3 South America * 34.5 31.5 Middle Africa 29.0 Southern Africa 4.5 12.7 Western Asia 27.7 Western Africa * Central Asia * Northern Africa Central America 9.9 * * * Eastern Africa 31.7 Southern Asia Eastern Asia 24.7 Southeastern Asia 38.4 Oceania 139 140 PART 2 Infancy: Forming the Foundations of Life Figure 4-10 Children Living in Poverty Members of Black, American Indian, and Hispanic households are more likely to live in poverty than members of white and Asian families. (Source: Koball & Jiang, 2018.) 70 65 64 61 60 Percent (%) 50 41 40 30 31 28 19 20 19 13 10 0 5 Asian Black Hispanic %Low Income nonorganic failure to thrive Chris Hondros/Getty Images a disorder in which infants stop growing due to a lack of stimulation and attention as the result of inadequate parenting Malnourishment at an early age can lower IQ scores, even if diet improves later. How might this deficit be overcome? 10 6 Native American White Other %Deep Poverty Furthermore, the COVID-19 pandemic of 2020 pushed additional families into poverty. Six months into the pandemic, 1 in 10 families in the United States reported not having sufficient food. Black and Hispanic families were particularly affected (Picchi, 2020). From an Educator’s Perspective Think of reasons why malnourishment, which slows physical growth, also harms IQ scores and school performance. How might malnourishment affect education in developing countries? A variety of social service programs, such as the federal Supplemental Nutrition Assistance Program (SNAP), have been created to combat this issue. These programs mean that children rarely become severely malnourished, but such children remain susceptible to undernutrition, in which there is some deficiency in diet. Some surveys find that as many as a quarter of 1- to 5-year-old children in the United States have diets that fall below the minimum caloric intake recommended by nutritional experts. Although the consequences are not as severe as those of malnutrition, undernutrition also has long-term costs. For instance, cognitive development later in childhood is affected by even mild to moderate undernutrition (Lian et al., 2012; Turesky et al., 2019). Severe malnutrition during infancy may lead to several disorders. Malnutrition during the first year can produce marasmus, a disease in which infants stop growing. Marasmus, attributable to a severe deficiency in proteins and calories, causes the body to waste away; it ultimately results in death. Older children are susceptible to kwashiorkor, a disease in which a child’s stomach, limbs, and face swell with water. To a casual observer, it appears that a child with kwashiorkor is actually chubby. However, this is an illusion: The child’s body is in fact struggling to make use of the few nutrients that are available (Douglass & McGadney-Douglass, 2008; Alou et al., 2021). In some cases, infants who receive sufficient nutrition act as though they have been deprived of food. Looking as though they suffer from marasmus, they are underdeveloped, listless, and apathetic. The real cause, however, is emotional: They lack sufficient love and emotional support. In such cases, known as nonorganic failure to thrive, for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 141 children stop growing not for biological reasons but due to a lack of stimulation and attention from their parents. Usually occurring by the age of 18 months, nonorganic failure to thrive can be reversed through intensive parent training or by placing children in a foster home where they can receive emotional support. It is clear that malnourishment has potentially disastrous consequences for an infant. Less clear, however, are the effects of obesity, defined as a body mass index (BMI) at or above the 95th percentile for children of the same age and sex. (BMI is calculated by dividing a child’s weight in kilograms by the square of their height in meters). Although there is no clear association between obesity during infancy and obesity during adolescence, some research suggests that overfeeding during infancy may lead to the creation of an excess of fat cells, which remain in the body throughout life and may predispose a person to be overweight. Weight during infancy is associated with weight at age 6 and adult obesity, suggesting that obesity in babies ultimately may be found to be associated with adult weight problems. A clear link between overweight babies and overweight adults, however, has not yet been found (Carnell et al., 2013; Murasko, 2015; Mallan et al., 2016). Although the evidence linking infant obesity to adult obesity is inconclusive, it’s plain that the societal view that “a fat baby is a healthy baby” is not necessarily correct. Indeed, cultural myths about food clearly lead to overfeeding. But other factors are related to obesity in infants. For example, infants delivered via cesarean section are twice as likely to become obese as infants born vaginally (Huh et al., 2011; Zhang et al., 2021). Given the lack of clarity regarding infant obesity, parents should concentrate less on their baby’s weight and more on providing appropriate nutrition. But just what constitutes proper nutrition? Probably the biggest question revolves around whether infants should be breastfed or given a formula of commercially processed cow’s milk with vitamin additives, as we consider next. OBESITY. Summarize the benefits of breastfeeding in infancy. Fifty years ago, if a mother asked her pediatrician whether breastfeeding or bottle-feeding was better, she would have received a simple and clear-cut answer: Bottle-feeding was the preferred method. Starting around the 1940s, the general belief among childcare experts was that breastfeeding was an obsolete method that put children unnecessarily at risk. With bottle-feeding, the argument went, parents could keep track of the amount of milk their baby was receiving and could thereby ensure that the child was taking in sufficient nutrients. In contrast, mothers who breastfed their babies could never be certain just how much milk their infants were getting. Use of the bottle was also supposed to help mothers keep their feedings to a rigid schedule of one bottle every 4 hours, the recommended procedure at that time. Today, however, a mother would get a different answer to the same question. Childcare authorities agree: For the first 12 months of life, there is no better food for an infant than breast milk. Breast milk not only contains all the nutrients necessary for growth, but it also seems to offer some immunity to a variety of childhood diseases, such as respiratory illnesses, ear infections, diarrhea, and allergies. Breastfeeding for as little as 4 months reduces infections by an average of 45 percent, and the reduction in infection is 65 percent lower for 6 months of breastfeeding compared to formula-fed babies. Breast milk is more easily digested than cow’s milk or formula, and it improves infants’ digestion. Moreover, breast milk is convenient for the mother to dispense. There is even some evidence that breast milk may enhance cognitive growth, leading to high adult intelligence, although the evidence is far from certain (Rogers & Blissett, 2017; Oster, 2019; Ottolini et al., 2020). Photodisc/Getty Images LO 4.8 Oksana Kuzmina/Shutterstock Breast or Bottle? Breast or bottle? Although infants receive adequate nourishment from breast- or bottle-feeding, most authorities agree that “breast is best.” 142 PART 2 Infancy: Forming the Foundations of Life Breast milk also contains complex carbohydrates called oligosaccharides (which are also found in such foods as asparagus, onions, wheat, and barley). Humans can’t digest oligosaccharides, but bacteria can, pointing to the role of breast milk in nurturing the bacteria that normally thrive in the human gut and provide important protective functions. It turns out that these oligosaccharides are very specific: Only one species of bacterium, called Bifidobacterium longum bv. infantis, has all the enzymes necessary to digest them. This enables that species of bacteria to dominate any others inhabiting the infant gut and crowding out other, potentially harmful, species of bacteria. In short, the oligosaccharides found in breast milk may provide long-term health benefits and aid in the avoidance of future problems such as diabetes and immune system difficulties (Vinukonda et al., 2016; Qiu et al., 2016; Moubareck, 2021). Breastfeeding also offers significant emotional advantages for both mother and child. Most mothers report that the experience of breastfeeding brings about feelings of well-being and intimacy with their infants, perhaps because of the production of endorphins in mothers’ brains. Breastfed infants are also more responsive to their mothers’ touch and their mothers’ gaze during feeding, and they are calmed and soothed by the experience. As we’ll see in Chapter 7, this mutual responsiveness may lead to healthy social development (Gerrish & Mennella, 2000; Zanardo et al., 2001; Jones et al., 2020). Breastfeeding may even be advantageous to mothers’ health. For instance, research suggests that women who breastfeed may have lower rates of ovarian cancer and breast cancer prior to menopause. Furthermore, the hormones produced during breastfeeding help shrink the uteruses of women following birth, enabling their bodies to return more quickly to a prepregnancy state. These hormones also may inhibit ovulation, reducing (but not eliminating!) the chance of becoming pregnant, and thereby helping to space the birth of additional children (Kim et al., 2007; Pearson et al., 2011; Kornides & Kitsantas, 2013). Still, breastfeeding is not a cure-all for infant nutrition and health, and the millions of individuals who have been raised on formula should not be concerned that they have suffered irreparable harm—or any harm at all. But it does seem that the popular slogan used by groups advocating the use of breastfeeding is on target: “Breast Is Best” (Ludlow et al., 2012; Oster, 2019; Feldman-Winter et al., 2020). Although pediatricians agree that breast milk is the ideal initial food, at some point infants require more nutrients than breast milk alone can provide. Although the American Academy of Pediatrics and the American Academy of Family Physicians recommends exclusive breastfeeding for about 6 months, followed by continued breastfeeding for 1 year or longer, solid foods can begin to be introduced after 6 months (American Academy of Pediatrics, 2013); Centers for Disease Control and Prevention [CDC], 2017). Solid foods are introduced into an infant’s diet gradually, one at a time, to identify preferences and allergies. Most often cereal comes first, followed by strained fruits. Vegetables and other foods typically are introduced next, although the order varies significantly from one infant to another. The timing of weaning, the gradual cessation of breast- or bottlefeeding, varies greatly. In developed countries such as the United States, weaning frequently occurs as early as 3 or 4 months. However, Infants generally start solid foods at around 4 to 6 some mothers continue breastfeeding for 2 or 3 years (Lee, 2017; months, gradually working their way up to a variety of different foods. Bovbjerg et al., 2021). Copyright Crezalyn Nerona Uratsuji/ Moment/Getty Images INTRODUCING SOLID FOODS: WHEN AND WHAT? for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 143 Module 4.2 Review LO 4.5 Explain how the reflexes that infants are born with help them adapt to their surroundings and protect themselves. Reflexes are unlearned, automatic responses to stimuli that help newborns survive and protect themselves. LO 4.6 Summarize the landmarks of motor skill development in infancy. During infancy, children reach a series of milestones in their physical development on a fairly consistent schedule, with some individual and cultural variations. Training and cultural expectations affect the timing of the development of motor skills. LO 4.7 Summarize the role of nutrition in the physical development of infants. Nutrition strongly affects physical development. Malnutrition can slow growth, affect intellectual performance, and cause diseases such as marasmus and kwashiorkor. The victims of undernutrition also suffer negative effects. LO 4.8 Summarize the benefits of breastfeeding in infancy. The advantages of breastfeeding are numerous, including nutritional, immunological, emotional, and physical benefits for the infant, and physical and emotional benefits for the mother. Journal Prompt Applying Lifespan Development: What advice might you give a friend who is concerned that their infant is still not walking at 14 months, when every other baby they know started walking by the first birthday? The Development of the Senses William James (1842–1910), one of the founding fathers of psychology, believed that the world of the infant is a “blooming, buzzing confusion” (James, 1890/1950). Was he right? In this case, James’s wisdom failed him. The newborn’s sensory world does lack the clarity and stability that we can distinguish as adults, but day by day, the world grows increasingly comprehensible as the infant’s ability to sense and perceive the environment develops. In fact, as we’ll see in this section, babies appear to thrive in an environment enriched by pleasing sensations. Visual Perception: Seeing the World LO 4.9 Describe the capabilities of infants in the realm of visual perception. The processes that underlie infants’ understanding of the world around them are sensation and perception. Sensation is the physical stimulation of the sense organs, and perception is the mental process of sorting out, interpreting, analyzing, and integrating stimuli from the sense organs and the brain. The study of infants’ capabilities in the realm of sensation and perception challenges the ingenuity of investigators. And researchers have developed a number of procedures for understanding sensation and perception in different realms. Take, for instance Lee Eng, a typical infant. From the time of Lee Eng’s birth, everyone who met him felt that he gazed at them intently. His eyes seemed to meet those of visitors. They seemed to bore deeply and knowingly into the faces of people who looked at him. How good, in fact, was Lee’s vision, and what, precisely, could he make out of his environment? Quite a bit, at least up close. According to some estimates, a newborn’s distance vision ranges from 20/200 to 20/600, which means that an infant can see with accuracy visual material up to 20 feet that an adult with normal vision is able to see with similar accuracy from a distance of between 200 and 600 feet (Haith, 1991; Jones et al., 2015). These figures indicate that infants’ distance vision is one-tenth to one-third that of the average adult’s. This isn’t so bad, actually: The vision of newborns provides the same degree of distance acuity as the uncorrected vision of many adults who wear eyeglasses or contact lenses. (If you wear glasses or contact lenses, remove them to get a sense of sensation the physical stimulation of the sense organs perception the sorting out, interpretation, analysis, and integration of stimuli involving the sense organs and the brain 144 PART 2 Infancy: Forming the Foundations of Life Sporrer/Rupp/Image Source/Getty Images what an infant can see of the world.) Furthermore, infants’ distance vision grows increasingly acute. By 6 months of age, the average infant’s vision is already 20/20—in other words, identical to that of adults (Cavallini et al., 2002; Corrow et al., 2012). Other visual abilities grow rapidly. For instance, binocular vision, the ability to combine the images coming to each eye to see depth and motion, is achieved at around 14 weeks. Before then, infants do not integrate the information from each eye. Depth perception is a particularly useful ability, helping babies acknowledge heights and avoid falls. In a classic study by developmental psychologists Eleanor Gibson and Richard Walk (Gibson & Walk, 1960; Rodkey, 2015), infants were placed on a sheet of heavy glass. A checkered pattern appeared under one-half of the glass sheet, making it seem that the infant was on a stable floor. However, in the middle of the glass sheet, the pattern dropped down several feet, forming an apparent “visual cliff.” Gibson and Walk asked this question: Would infants willingly crawl across the cliff when called by their mothers (see Figure 4-11)? The results were unambiguous. Most of the infants in the study, who ranged in age from 6 to 14 months, could not be coaxed over the apparent cliff. Clearly, the ability to perceive depth had already developed in most of them by that age. However, the experiment did not pinpoint when depth perception emerged because only infants who had already learned to crawl could be tested. But other experiments, in which infants of 2 and 3 months were placed on their stomachs above the apparent floor and above the visual cliff, revealed differences in heart rate between the two positions (Kretch & Adolph, 2013; Adolph et al., 2014; LoBue & Adolph, 2019). Still, it is important to keep in mind that such findings do not permit us to know whether infants are responding to depth itself or merely to the change in visual stimuli that occurs when they are moved from a lack of depth to depth. Although an infant’s vision is poorer than the average adult’s, the vision of newborns provides the same degree of distance acuity as the uncorrected vision of many adults who wear eyeglasses or contact lenses. Figure 4-11 Visual Cliff Photo Researchers/Science History Images/Alamy Stock Photo The “visual cliff” experiment examines the depth perception of infants. Most infants in the age range of 6 to 14 months cannot be coaxed to cross the cliff, apparently responding to the fact that the patterned area drops several feet. for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy Figure 4-12 Preferring Complexity In a classic experiment, researcher Robert Fantz found that 2- and 3-month-old infants preferred to look at more complex stimuli than simple ones. (Source: Adapted from Fantz, 1961.) abc def Percent of Total Fixation Time Infants also show clear visual preferences, preferences that are present from birth. Given a choice, infants reliably prefer to look at stimuli that include patterns than to look at simpler stimuli (see Figure 4-12). How do we know? Developmental psychologist Robert Fantz (1963) created a classic test. He built a chamber in which babies could lie on their backs and see pairs of visual stimuli above them. Fantz could determine which of the stimuli the infants were looking at by observing the reflections of the stimuli in their eyes. Fantz’s work was the impetus for a great deal of research on the preferences of infants, most of which points to a critical conclusion: Infants are genetically preprogrammed to prefer particular kinds of stimuli. For instance, just minutes after birth, they show preferences for certain colors, shapes, and configurations of various stimuli. They prefer curved over straight lines, three-dimensional figures to two-dimensional ones, and human faces to nonfaces. Such capabilities may reflect the existence of highly specialized cells in the brain that react to stimuli of a particular pattern, orientation, shape, and direction of movement (Hubel & Wiesel, 2004; Kellman & Arterberry, 2006; Gliga et al., 2009). Genetics is not the sole determinant of infant visual preferences. Just a few hours after birth, infants have already learned to prefer their own mother’s face to other faces. Similarly, between the ages of 6 and 9 months, infants become more adept at distinguishing between the faces of humans, but they become less able to distinguish faces of members of other species (see Figure 4-13). They also distinguish between male and female faces. Such findings provide another clear piece of evidence of how heredity and environmental experiences are woven together to determine an infant’s capabilities (Quinn, Uttley, Lee, et al., 2008; Otsuka et al., 2012; Bahrick et al., 2016). 145 Auditory Perception: The World of Sound LO 4.10 Describe the capabilities of infants in the realm of auditory sensation and perception. What is it about a mother’s lullaby that helps soothe a crying, fussy baby? Some clues emerge when we look at the capabilities of infants in the realm of auditory sensation and perception. Infants hear from the time of birth—and even before. As noted in Chapter 2, the ability to hear begins prenatally. Even in the womb, the fetus responds to sounds outside of its mother. Furthermore, infants are born with preferences for particular sound combinations (Trehub, 2003; Pundir et al., 2012; Missana et al., 2017). Because they have had some practice in hearing before birth, it is not surprising that infants have reasonably good auditory perception after they are born. Infants actually are more sensitive to certain very high and very low frequencies than adults—a sensitivity that seems to increase during the first 2 years of life. However, infants are initially less sensitive than adults to middle-range frequencies. Eventually, however, their capabilities within the middle range improve (Fernald, 2001; Homae et al., 2012; Lee & Kisilevsky, 2014). It is not fully clear what, during infancy, leads to the improvement in sensitivity to middle-frequency sounds, although it may be related to the maturation of the nervous system. More puzzling is why, after infancy, children’s ability to hear very high and low frequencies gradually declines. One explanation may be that exposure to high levels of noise may diminish capacities at the extreme ranges (Stewart et al., 2003; Mishra et al., 2021). Figure 4-13 Distinguishing Faces Examples of faces used in a study found that 6-month-old infants distinguished human or monkey faces equally well, whereas 9-month-olds were less adept at distinguishing monkey faces as compared to human faces. (Source: Pascalis, de Haan, & Nelson, 2002, p. 1322.) Marcos Mesa Sam Wordley/Shutterstock 146 PART 2 Infancy: Forming the Foundations of Life In addition to the ability to detect sound, infants need several other abilities to hear effectively. For instance, sound localization permits us to pinpoint the direction from which a sound is emanating. Compared to adults, infants have a slight handicap in this task because effective sound localization requires use of the slight difference in the times at which a sound reaches our two ears. Sound that we hear first in the right ear tells us that the source of the sound is to our right. Because infants’ heads are smaller than those of adults, the difference in timing of the arrival of sound at the two ears is less than it is in adults, so they have difficulty determining from which direction sound is coming (Winkler et al., 2016). Despite the potential limitation caused by their smaller heads, infants’ sound localization abilities are fairly good even at birth, and they reach adult levels of success by the age of 1 year. Interestingly, their improvement is not steady: Although we don’t know why, studies show that the accuracy of sound localization declines between birth and 2 months of age, but then begins to increase (Slugocki & Trainor, 2014). Infants can discriminate groups of different sounds, in terms of their patterns and other acoustical characteristics, quite well. For instance, infants as young as 6 months old can detect the change of a single note in a six-tone melody. They also react to changes in musical key and rhythm. In sum, they listen with a keen ear to the melodies of lullabies their mothers and fathers sing to them (Masataka, 2006; Trehub & Hannon, 2009; Suppanen et al., 2019). Even more important to their ultimate success in the world, young infants are capable of making the fine discriminations that their future understanding of language will require (Bijeljac-Babic et al., 1993; Gervain et al., 2008). For instance, in one classic study, a group of 1- to 4-month-old infants sucked on nipples that activated a recording of a person saying “ba” every time they sucked. At first, their interest in the sound made them suck vigorously. Soon, though, they became acclimated to the sound (through a process called habituation, discussed in Chapter 3) and sucked with less energy. However, when the experimenters changed the sound to “pa,” the infants immediately showed new interest and sucked with greater vigor once again. The clear conclusion: Infants as young as 1 month old could make the distinction between the two similar sounds (Miller & Eimas, 1995; Wang & Feigenson, 2021). Even more intriguing, young infants are able to discriminate one language from another. By the age of 4½ months, infants are able to discriminate their own names from other, similar-sounding words. By the age of 5 months, they can distinguish the difference between English and Spanish passages, even when the two are similar in meter, number of syllables, and speed of recitation. Some evidence suggests that even 2-day-olds show preferences for the language spoken by those around them over other languages (Chonchaiya, Tardif, Mai, et al., 2013; Pejovic & Molnar, 2017). In fact, such preferences may start in the womb: Research shows that fetuses are sensitive to the rhythms of different languages and are able to discriminate between them, as evidenced by changes in heart rate depending on the kind of language they hear (Minai et al., 2017). Given their ability to discriminate a difference in speech as slight as the difference between two consonants, it is not surprising that infants can distinguish different people on the basis of voice. From an early age they show clear preferences for some voices over others. For instance, in one experiment newborns were allowed to suck a nipple that turned on a recording of a human voice reading a story. The infants sucked significantly longer when the voice was that of their mother than when the voice was that of a stranger (DeCasper & Fifer, 1980; Fifer, 1987). By the age of 4 months, infants are able to discriminate their How do such preferences arise? One hypothesis is that prenatal own names from other, similar sounding, words. What are exposure to the mother’s voice is the key. As support for this consome ways infants are able to discriminate their name from other words? jecture, researchers point to the fact that newborns do not show a for more ebook/ testbank/ solution manuals requests: email [email protected] Chapter 4 Physical Development in Infancy 147 preference for their fathers’ voices over other male voices. Furthermore, newborns prefer listening to melodies sung by their mothers before they were born to melodies that were not sung before birth. It seems, then, that the prenatal exposure to their mothers’ voices— although muffled by the liquid environment of the womb—helps shape infants’ listening preferences (Jardri et al., 2012; Swingley & Humphrey, 2017; Carvalho et al., 2019). Smell and Taste What do infants do when they smell a rotten egg? Pretty much what adults do—crinkle their noses and generally look unhappy. By contrast, the scents of bananas and butter both produce a pleasant reaction on the part of infants (Steiner, 1979; Pomares et al., 2002). The sense of smell is so well developed, even among very young infants, that at least some 12- to 18-day-old babies can distinguish their mothers on the basis of smell alone. For instance, in one experiment, infants were exposed to the smell of gauze pads worn under the arms of adults the previous evening. Infants who were being breastfed were able to distinguish their mothers’ scent from those of other adults. However, not all infants could do this: Those who were being bottle-fed were unable to make the distinction. Moreover, Infants’ sense of smell is so well develboth breastfed and bottle-fed infants were unable to distinguish their fathers on the basis of oped they can distinguish their mothodor (Allam et al., 2006; Lipsitt & Rovee-Collier, 2012; Leleu et al., 2020; Rekow et al., 2021). ers on the basis of smell alone. Infants seem to have an innate sweet tooth (even before they have teeth!), and they show facial expressions of disgust when they taste something bitter. Very young infants smile when a sweet-tasting liquid is placed on their tongues. They also suck harder at a bottle if it is sweetened. Because breast milk has a sweet taste, it is possible that this preference may be part of our evolutionary heritage, retained because it offered a survival advantage. Infants who preferred sweet tastes may have been more likely to ingest sufficient nutrients and to survive than those who did not (Blass & Camp, 2015; Borowitz, 2021). Infants also develop taste preferences based on what their mothers drank while they were in the womb. For instance, one study found that women who drank carrot juice while pregnant had children who had a preference for the taste of carrots during infancy (Gerrish & Mennella, 2000). Sensitivity to Pain and Touch LO 4.12 Describe the nature of pain and touch in infants. When Eli Rosenblatt was 8 days old, he participated in the ancient Jewish ritual of circumcision. As he lay nestled in his father’s arms, the foreskin of his penis was removed. Although Eli shrieked in what seemed to his anxious parents as pain, he soon settled down and went back to sleep. Others who had watched the ceremony assured his parents that at Eli’s age, babies don’t really experience pain, at least not in the same way that adults do. Were Eli’s relatives accurate in saying that young infants don’t experience pain? In the past, many medical practitioners would have agreed. Because they assumed th