PSYCH NOTES 3.1-3.9 PDF: Developmental Psychology

Module 3.1 Themes and Methods in Developmental Psychology Learning Targets 3.1-1 Explain three themes that have engaged developmental psychologists and two methods they typically use to study human development over time. My [DM’s] story, and yours, began when a man and a woman together contributed...

Module 3.1 Themes and Methods in Developmental Psychology Learning Targets 3.1-1 Explain three themes that have engaged developmental psychologists and two methods they typically use to study human development over time. My [DM’s] story, and yours, began when a man and a woman together contributed 20,000+ genes to an egg that became a unique person. Those genes coded the protein building blocks that, with astonishing precision, form our body and predispose our traits. My grandmother handed down to my mother a rare hearing-loss pattern, which she, in turn, gave to me (the least of her gifts). My father was an amiable extravert, and sometimes I forget to stop talking (although as a child, my talking was impeded by embarrassing stuttering, for which Seattle Public Schools provided speech therapy). Along with my parents’ nature, I also received their nurture. Like you, I was born into a particular family and culture, with its own way of viewing the world. My values have been shaped by a family culture filled with talking and laughter, by a religious culture that speaks of love and justice, and by an academic culture that encourages critical thinking (asking, What do you mean? How do you know?). 1051 We are formed by our genes and by our contexts, so our stories all differ. But in many ways we are each like nearly everyone else on Earth. Being human, you and I have a need to belong. My mental video library, which began after age 4, is filled with scenes of social attachment. Over time, my attachments to parents loosened as peer friendships grew. After lacking confidence to date in high school, I fell in love with a college classmate and married at age 20. Natural selection predisposes us to survive and perpetuate our genes. Sure enough, two years later a child entered our lives, and I experienced a new form of love that surprised me with its intensity. But life is marked by change. That child and his brother now live 2000 miles away, and their sister has found her calling in South Africa. The tight rubber bands linking parent and child have loosened, as yours likely have as well. Change also marks most vocational lives, which for me transitioned from a teen working in the family insurance agency, to a premed chemistry major and hospital aide, to (after discarding my half- completed medical school applications) a psychology professor and author. I predict that in 10 years you, too, will be doing things you do not currently anticipate. Stability also marks our development: Our life situations change, but we experience a continuous self. When I look in the mirror, I do not see the person I once was, but I feel like the person I have always been. I am the same person who, as a late teen, played basketball and discovered love. Sixty years later, I still enjoy basketball and still 1052 love (with less passion but more security) the life partner with whom I have shared life’s griefs and joys. We experience a continuous self, but that self morphs through stages — for me, growing up, raising children, enjoying a career, and, eventually, life’s final stage, which will demand my presence. As I wend my way through this cycle of life and death, I am mindful that life’s journey is a continuing process of development, seeded by nature and shaped by nurture, animated by love and focused by work, begun with wide-eyed curiosity and completed, for those blessed to live to a good old age, with peace and never-ending hope. Across the lifespan, we grow from newborn to toddler, from toddler to teenager, and from teenager to mature adult. At each stage of life there are physical, cognitive, and social-emotional milestones. Developmental Psychology’s Major Themes 3.1-1 What three themes have engaged developmental psychologists, and what are two methods they typically use to study human development over time? Researchers find human development interesting for the same reasons most of us do — they want to understand more about how we’ve become our current selves, and how we may change in the years ahead. Developmental psychology examines our physical, cognitive, and social-emotional development across the lifespan. 1053 Developmental psychologists study both the chronological order of this development and its key themes. They often do cross-sectional studies (comparing people of different ages) and longitudinal studies (following people across time) to explore three major themes: 1. Nature and nurture: How does our genetic inheritance (our nature) interact with our experiences (our nurture) to influence our development? How have your nature and your nurture influenced your life story? 2. Continuity and stages: Which parts of development are gradual and continuous, like riding an escalator? Which parts change abruptly in separate stages, like climbing rungs on a ladder? 3. Stability and change: Which of our traits persist through life? How do we change as we age? developmental psychology a branch of psychology that studies physical, cognitive, and social-emotional development throughout the lifespan. cross-sectional study research that compares people of different ages at the same point in time. longitudinal study research that follows and retests the same people over time. AP® Science Practice Research Developmental psychology often features longitudinal and cross-sectional studies — because these research design methods help us learn about development across the lifespan (changes over time). 1054  AP® Exam Tip All three of these themes are important for development. Nature and nurture, of course, weave their way through almost every module. Look for the nature/nurture idea throughout this text (and on the AP® exam). Nature and Nurture The unique gene combination created when our mother’s egg engulfed our father’s sperm helped form us as individuals. Genes predispose both our shared humanity and our individual differences. But our experiences also shape us, in the womb and in the world. Our families and peer relationships teach us how to think and act. Even differences initiated by our nature may be amplified by our nurture. We are not formed by either nature or nurture, but by the interaction between them. Biological, psychological, and social- cultural forces interact. Mindful of how others differ from us, however, we often fail to notice the similarities stemming from our shared biology. Regardless of our culture, we humans share the same life cycle. We speak to our infants in similar ways and respond similarly to their coos and cries (Bornstein et al., 1992a,b). Although ethnic groups have differed in some ways, including average school achievement, these differences are “no more than skin deep.” To the extent that family structure, peer influences, and parental education predict behavior in one of 1055 these ethnic groups, they do so for the others. Compared with the person-to-person differences within groups, between-group differences are small. Continuity and Stages Do adults differ from infants as a giant redwood differs from its seedling — a difference created by gradual, cumulative growth? Or do they differ as a butterfly differs from a caterpillar — a difference of distinct stages? Stages of the life cycle Researchers who emphasize experience and learning typically see development as a slow, continuous shaping process. Those who emphasize biological maturation tend to see development as a 1056 sequence of genetically predisposed stages or steps: Although progress through the various stages may be quick or slow, everyone passes through the stages in the same order. Are there clear-cut stages of psychological development, as there are physical stages such as walking before running? The stage theories we will consider — Jean Piaget’s theory of cognitive development, and Erik Erikson’s theory of psychosocial development — propose developmental stages (summarized in Figure 3.1-1). But as we will also see, some research casts doubt on the idea that life proceeds through neatly defined age-linked stages. Figure 3.1-1 Comparing the stage theories 1 Although many modern developmental psychologists do not identify as stage theorists, the stage concept remains useful. The human brain does experience growth spurts during childhood and puberty that correspond roughly to Piaget’s stages (Thatcher et al., 1987). And stage theories contribute a developmental perspective on the whole 1057 lifespan by suggesting how people of one age think and act differently when they arrive at a later age. Stability and Change As we follow lives through time, do we find more evidence for stability or change? If reunited with a long-lost childhood friend, do we instantly realize that “it’s the same old Jordan”? Or do long-ago friends now seem like strangers? (At least one acquaintance of mine [DM’s] would choose the second option. At his 40-year college reunion, he failed to recognize a former classmate. The understandably appalled classmate was his first wife!) We experience both stability and change. Some of our characteristics, such as temperament, are very stable. Following thousands of New Zealanders and Americans over several decades, researchers have been struck by the consistency of temperament and emotionality across time (Kassing et al., 2019; Moffitt et al., 2013; Slutske et al., 2012). Inhibited 14-month-olds mostly grow up to be reserved, introverted adults (Tang et al., 2020). Out-of-control young children are later the most likely to engage in teen smoking, adult criminal behavior, or out-of-control gambling. Inattentive Canadian kindergarteners are less likely to earn high salaries in their adult careers (Vergunst et al., 2019). Moreover, children observed being repeatedly cruel to animals often become violent adults (Hensley et al., 2018). But on a happier note, the widest smilers in childhood and college photos are the ones most likely to enjoy enduring marriages (Hertenstein et al., 2009). 1058  AP® Science Practice Research Do inhibited 14-month-olds mostly grow up to be reserved, introverted adults? To answer this question, researchers would use the longitudinal method. They would assess 14-month- olds’ inhibition and then retest them again later in life. By following lives through time, longitudinal studies enable researchers to examine whether certain traits remain stable or change over time. Smiles predict marital stability In one longitudinal study of 306 U.S. college graduates, 1 in 4 with yearbook expressions like the one in photo (a) later divorced, as did only 1 in 20 with smiles like the one in photo (b) (Hertenstein et al., 2009). We cannot, however, predict all aspects of our future selves based on our early life. Our social attitudes, for example, are much less stable than our temperament, especially during the impressionable late adolescent years (Krosnick & Alwin, 1989; Rekker et al., 2015). Older children and adolescents learn new ways of coping. Although 1059 delinquent children have elevated rates of later problems, many confused and troubled children blossom into mature, successful adults (Moffitt et al., 2002; Roberts et al., 2013; Thomas & Chess, 1986). Life is a process of becoming. Today’s struggles may lay the foundation for tomorrow’s happiness. In some ways, we all change with age. Most shy, fearful toddlers begin opening up by age 4, and after adolescence most people gradually become more conscientious, stable, agreeable, and self- confident (Furnham & Cheng, 2019; Lucas & Donnellan, 2009; Van den Akker et al., 2014). Risk-prone adolescents tend, as adults, to become more cautious (Mata et al., 2016). Indeed, many irresponsible 16-year-olds have matured into 40-year-old business or cultural leaders. (If you are the former, you aren’t done yet!) But when asked how they have changed in the last decade and will change in the next decade, people — both young and old — exhibit an end of history illusion. They recognize that they have changed but presume they will change little in the future (Quoidbach et al., 2013). AP® Science Practice Developing Arguments Psychological scientists use scientifically derived evidence to support their claims. Which research findings support the idea of stability in personality across the lifespan? Life requires both stability and change. Stability provides our identity. Change gives us our hope for a brighter future, allowing us 1060 to adapt and grow from experience. As adults grow older, there is continuity of self AP® Science Practice Check Your Understanding Examine the Concept Explain the difference between longitudinal and cross-sectional studies. Developmental researchers who emphasize learning and experience are supporting ; those who emphasize biological maturation are supporting. Apply the Concept Are you the same person you were as a preschooler? As an 8-year-old? As a 12-year-old? How are you different? How are you the same? Answers to the Examine the Concept questions can be found in Appendix C at the end of the book. 1061  AP® Science Practice Exploring Research Methods & Design As you learned in this module, developmental psychologists explore both stability and change. For example, one Spanish research team tested the emotion-detecting ability of 12,198 people ages 17 to 76 (Cabello et al., 2016). When comparing the age groups, they found an inverted-U pattern: Middle-aged adults displayed the best emotion-detecting ability. Is this study cross-sectional or longitudinal? Explain your choice. Did the researchers use the experimental method in this study? Explain your answer. Describe the sample in this study. Who do you think is the intended population? Draw a scatterplot depicting these findings. Be sure to label each axis. AP® Exam Tip The AP® Exam focuses on application, not just the ability to define key terms. As you work your way through Modules 3.1–3.6, think of how the material relates to you, your relatives, and your friends. The more often you do this, the easier it will be to apply the material. Module 3.1 REVIEW 3.1-1 What three themes have engaged developmental psychologists, and what are two 1062 methods they typically use to study human development over time? Developmental psychologists study physical, cognitive, and social- emotional changes throughout the lifespan. They explore three issues: nature and nurture (the interaction between our genetic inheritance and our experiences); continuity and stages (which aspects of development are gradual and continuous, and which change relatively abruptly); and stability and change (whether our traits endure or change as we age). Developmental psychologists often use cross-sectional studies (comparing people of different ages at one point in time) and longitudinal studies (retesting the same people over a period of years). AP® Practice Multiple Choice Questions 1. Dr. Lynelle conducts research on how people’s bodies, minds, and relationships change over time. In which major area of psychology does Dr. Lynelle conduct research? a. Biological b. Developmental c. Social 1063  d. Cognitive 2. Janis gave a presentation on how genes and environment interact. Which of the following was Janis most likely discussing? a. Stability and change b. Continuity and stability c. Continuity and stages d. Nature and nurture Use the following text to answer questions 3–5: Dr. Hargen believes that development progresses slowly and steadily. Dr. Hargen followed a group of individuals from birth to age 18 to test her hypothesis. 3. Which of the following perspectives best aligns with Dr. Hargen’s belief? a. Development occurs in stages. b. Development occurs longitudinally. c. Development occurs continuously. d. Development occurs cross-sectionally. 1064 4. What type of research method did Dr. Hargen use in her study? a. Longitudinal b. Cross-sectional c. Experimental d. Survey 5. Which of the following statements best supports Dr. Hargen’s hypothesis? a. Development occurs like the transformation of a tadpole into a frog, with each stage being distinct from the last. b. Development occurs like a caterpillar changing into a butterfly, with clearly different forms emerging at different time points. c. Development occurs like a tree grows, with the branches continually growing over time. d. Development occurs like the hatching of a chicken from an egg, with the sudden appearance of new abilities. 1065 Module 3.2a Physical Development Across the Lifespan: Prenatal Development, Infancy, and Childhood Learning Targets 3.2-1 Describe the course of prenatal development, and explain how teratogens affect that development. 3.2-2 Explain some abilities of the newborn, and explain how researchers are able to explore infants’ mental abilities. 3.2-3 Explain key developmental changes in the brain and in motor skills during infancy and childhood. 3.2-4 Explain how an infant’s brain begins processing memories. Developmental psychologists study physical development, including the developing brain (and blossoming mind), from inside the womb to out in the room, and throughout childhood. Prenatal Development and the Newborn 3.2-1 What is the course of prenatal development, and how do teratogens affect that development? 1066 Your life story began when two lives — and their ancestral genetic histories — merged, and your wonder-filled development began. Conception Nothing is more natural than a species reproducing itself. And nothing is more wondrous. For you, the process started inside your grandmother — as an egg formed inside a developing female inside of her. (Your biological mother was born with all the immature eggs she would ever have.) Your biological father, in contrast, began producing sperm cells nonstop at puberty — in the beginning at a rate of more than 1000 sperm during the second it takes to read this phrase. Some time after puberty, your mother’s ovary released a mature egg — a cell roughly the size of the period that ends this sentence. Like space voyagers approaching a huge planet, some 250 million deposited sperm began their frantic race upstream, approaching a cell 85,000 times their own size. The small number reaching the egg released digestive enzymes that ate away the egg’s protective coating (Figure 3.2-1a). As soon as the one winning sperm penetrated that coating and was welcomed in (Figure 3.2-1b), the egg’s surface blocked out the others. Before half a day elapsed, the egg nucleus and the sperm nucleus fused: The two became one. 1067  Figure 3.2-1 Life is sexually transmitted (a) Sperm cells surround an egg. (b) One sperm penetrates the egg’s jellylike outer coating, triggering a series of chemical events that will cause sperm and egg to fuse into a single cell. If all goes well, that cell will subdivide again and again to emerge 9 months later as a 37-trillion-cell human being (Bianconi et al., 2013). Consider it your most fortunate of moments. Among some 250 million sperm, the one needed to make you, in combination with that one particular egg, won the race. (As individual humans, we do not reproduce; we recombine.) And so it was for innumerable generations before us. If any one of our ancestors had been conceived with a different sperm or egg, or died before conceiving, or not chanced to meet their partner, or …. The mind boggles at the improbable, unbroken chain of events that produced us. Prenatal Development How many fertilized eggs, called zygotes, survive beyond the first 2 weeks? Fewer than half (Grobstein, 1979; Hall, 2004). But for us, good fortune prevailed. One cell became 2, then 4 — each just like the first 1068 — until this cell division had produced some 100 identical cells within the first week. Then the cells began to differentiate — to specialize in structure and function (“I’ll become a brain, you become intestines!”). About 10 days after conception, the germinal stage completes as the zygote attaches to the mother’s uterine wall, beginning approximately 37 weeks of the closest human relationship. Near the beginning of this maternal bodybuilding feat, the tiny clump of cells forms two parts. The inner cells become the embryo (Figure 3.2-2a). Many of the outer cells become the placenta, the life-link that transfers nutrients and oxygen from mother to embryo. Over the next 6 weeks, the embryo’s organs begin to form and function. The heart begins to beat. Figure 3.2-2 Prenatal development (a) The embryo grows and develops rapidly. At 40 days, the spine is visible and the arms and legs are beginning to grow. (b) By the start of the ninth week, when the fetal period begins, facial features, hands, and feet have formed. (c) As the fetus enters the sixteenth week, its 3 ounces could fit in the palm of your hand. 1069 By 9 weeks after conception, an embryo looks unmistakably human (Figure 3.2-2b). It is now a fetus (Latin for “offspring” or “young one”). During the sixth month, organs such as the stomach develop enough to give the fetus a good chance of surviving and thriving if born prematurely. At each prenatal stage, genetic and environmental factors affect our development. By the sixth month, the fetus is responsive to sound. Microphone readings taken inside the uterus reveal that the fetus is exposed to the sound of its mother’s muffled voice (Ecklund-Flores, 1992; Hepper, 2005). Immediately after emerging from their underwater world, newborns prefer their mother’s voice to another woman’s, or to their father’s (DeCasper et al., 1986, 1994; Lee & Kisilevsky, 2014). They also prefer hearing their mother’s language. In one study, day- old American and Swedish newborns paused more in their pacifier sucking when listening to familiar vowels from their mother’s language (Moon et al., 2013). After repeatedly hearing a fake word (tatata) in the womb, Finnish newborns’ brain waves displayed recognition when hearing the same word after birth (Partanen et al., 2013). If their mother spoke two languages during pregnancy, newborns displayed interest in both (Byers-Heinlein et al., 2010). And just after birth, the melodic ups and downs of newborns’ cries bear the tuneful signature of their mother’s native tongue (Mampe et al., 2009). Babies born to French-speaking mothers tended to produce cries with the rising intonation of French; babies born to German-speaking mothers produced cries with the falling tones of 1070 German (Mampe et al., 2009). Would you have guessed? The learning of language begins in the womb. In the 2 months before birth, fetuses demonstrate learning in other ways, as when they adapt to a vibrating, honking device placed on their mother’s abdomen (Dirix et al., 2009). Like people who adapt to the sound of trains in their neighborhood, fetuses get used to the honking. Moreover, 4 weeks later, they recall the sound (as evidenced by their mild response, compared with the reactions of those fetuses not previously exposed to such honking). Sounds are not the only environmental factors that impact fetal development. In addition to transferring nutrients and oxygen from mother to fetus, the placenta screens out many harmful substances. Even so, some slip by. Teratogens, agents such as viruses and drugs, can damage an embryo or fetus. This is one reason pregnant women are advised not to drink alcoholic beverages or use nicotine or marijuana (Kuehn, 2019; Saint Louis, 2017). A pregnant woman never smokes, vapes, or drinks alone. When alcohol enters her bloodstream and that of her fetus, it reduces activity in both their central nervous systems. Alcohol use during pregnancy may prime the woman’s offspring to like alcohol and put them at risk for heavy drinking and alcohol use disorder during their teen years. In experiments, when pregnant rats drank alcohol, their young offspring later displayed a liking for alcohol’s taste and odor (Youngentob & Glendinning, 2009; Youngentob et al., 2007). 1071  AP® Science Practice Research The alcohol experiments described here were conducted on rats. That is because it would be unethical to introduce a teratogen, such as alcohol, into humans. The American Psychological Association has ethical guidelines for animal welfare in research as well (see Module 0.5). teratogens agents, such as chemicals and viruses, that can reach the embryo or fetus during prenatal development and cause harm. Worldwide, 1 in 10 women report consuming alcohol while pregnant (Popova et al., 2019). Even light drinking, occasional binge drinking, or marijuana smoking can affect the fetal brain (CDC, 2018c; Ghazi Sherbaf et al., 2019; Marjonen et al., 2015). Persistent heavy drinking puts the fetus at risk for congenital (present at birth) disabilities, future behavior problems, and lower intelligence. For 1 in about 130 children worldwide and 1 in 30 in the United States, the effects are visible as fetal alcohol spectrum disorder (Lange et al., 2017; May et al., 2018). Its most serious form is fetal alcohol syndrome (FAS), which is marked by lifelong physical and mental abnormalities. The fetal damage may occur because alcohol has an epigenetic effect: It leaves chemical marks on DNA that switch genes abnormally on or off (Liu et al., 2009). Smoking cigarettes or marijuana during pregnancy also leaves epigenetic scars that may increase vulnerability to stress or addiction (Stroud et al., 2014; Szutorisz & Hurd, 2016). fetal alcohol syndrome (FAS) 1072 physical and cognitive function deficits in children caused by their birth mother’s heavy drinking during pregnancy. In severe cases, symptoms include a small, out-of-proportion head and distinct facial features. If a pregnant woman experiences extreme stress, the stress hormones flooding her body may indicate a survival threat to the fetus and produce an earlier delivery (Glynn & Sandman, 2011). And malnourishment, maternal illness, and genetic mutations can put a child at risk for health problems and psychiatric disorders (Glynn & Sandman, 2011; Hardie & Landale, 2013; Santavirta et al., 2018). The Competent Newborn 3.2-2 What are some newborn abilities, and how do researchers explore infants’ mental abilities? Babies come with apps preloaded. Having survived prenatal hazards, we as newborns came equipped with automatic reflex responses ideally suited for our survival. We withdraw our limbs to escape pain. If a cloth over our face interferes with our breathing, we turn our head from side to side and swipe at it. New parents are often in awe of the coordinated sequence of reflexes by which their baby gets food. When something touches their cheek, babies turn toward that touch, open their mouth, and vigorously root for a nipple. Finding one, they automatically close on it and begin sucking. (Failing to find satisfaction, the hungry baby may cry — a behavior parents find highly unpleasant, and very rewarding to relieve.) Other adaptive reflexes include the startle reflex (when arms 1073 and legs spring out, quickly followed by fist clenching and loud crying) and the surprisingly strong grasping reflex, both of which may have helped infants stay close to their caregivers. The pioneering American psychologist William James presumed that newborns experience a “blooming, buzzing confusion,” an assumption few people challenged until the 1960s. Then scientists discovered that babies can tell you a lot — if you know how to ask. To ask, you must capitalize on what babies can do — gaze, suck, and turn their heads. So, equipped with eye-tracking machines and pacifiers wired to electronic gear, researchers set out to answer parents’ age-old questions: What can my baby see, hear, smell, and think? Consider how researchers exploit habituation — decreased responding with repeated stimulation. We saw this earlier when fetuses adapted to a vibrating, honking device placed on their mother’s abdomen. The novel stimulus gets attention when first presented. With repetition, the response weakens. This seeming boredom with familiar stimuli gives us a way to ask infants what they see and remember. habituation decreasing responsiveness with repeated stimulation. As infants gain familiarity with repeated exposure to a stimulus, their interest wanes and they look away sooner. As newborns, we prefer sights and sounds that facilitate social responsiveness. We turn our head in the direction of human voices. We gaze longer at a drawing of a face-like image (Figure 3.2-3). Even 1074 late-stage fetuses look more at face-like patterns in red lights shined into the womb (Reid et al., 2017). As young infants, we also prefer to look at objects 8 to 12 inches away, which — wonder of wonders — just happens to be about the distance between a nursing infant’s eyes and its mother’s (Maurer & Maurer, 1988). Our brain’s default settings help us connect socially. Figure 3.2-3 Newborns’ preference for faces When shown these two images with the same three elements, Italian newborns spent nearly twice as many seconds looking at the face-like image (Valenza et al., 1996). Newborns — average age 53 minutes in one Canadian study — have an apparently inborn preference for looking toward faces (Mondloch et al., 1999). Within days after birth, our brain’s neural networks were stamped with the smell of our mother’s body. Week-old nursing babies, placed between a gauze pad from their mother’s bra and one from another nursing mother, usually turn toward the smell of their own mother’s pad (MacFarlane, 1978). What’s more, that smell preference lasts. One study capitalized on the fact that some nursing mothers in a 1075 French maternity ward used a chamomile-scented balm to prevent nipple soreness (Delaunay-El Allam et al., 2010). Twenty-one months later, their toddlers preferred playing with chamomile-scented toys! Their peers who had not sniffed the scent while breast-feeding showed no such preference. (This makes us wonder: Will these children grow up to become devoted chamomile tea drinkers?) Such studies reveal the remarkable abilities with which we enter our world. AP® Science Practice Research Research on infants can be challenging, because researchers can’t ask them directly about their taste or smell preferences. Instead, researchers assess infants’ preferences by their gazing time or head turning. Gazing time or head turning is the operational definition of infant preferences. Prepared to feed and eat Like birds and other animals, we are predisposed to respond to our offspring’s cries for food — even if we are in the middle of a 314-mile ultramarathon, as I [ND] was when my 18-month-old, Bevy, decided that only Daddy could feed her. 1076  AP® Science Practice Check Your Understanding Examine the Concept Explain what is meant by “a pregnant woman never smokes, vapes, or drinks alone.” Explain the effect of teratogens on prenatal development. Apply the Concept Are you surprised by the news of infants’ competencies? Explain. Provide an example of a teratogen. Answers to the Examine the Concept questions can be found in Appendix C at the end of the book. Physical Development in Infancy and Childhood 3.2-3 During infancy and childhood, how do the brain and motor skills develop? As a flower unfolds in accordance with its genetic instructions, so do we humans. Maturation — the orderly sequence of biological growth — decrees many of our commonalities. Babies first stand, then walk. Toddlers use nouns, then verbs. Adverse childhood experiences (ACEs), such as severe deprivation or abuse, can slow development, but genetic growth patterns come “factory-installed” — they are inborn. 1077 Maturation (nature) sets the basic course of development; experience (nurture) adjusts it. Genes and scenes interact. maturation biological growth processes that enable orderly changes in behavior, relatively uninfluenced by experience. AP® Exam Tip Note that maturation, to developmental psychologists, is a biological sequence. This is much more precise than the general notion that maturation means to become more adult-like. Brain Development In your mother’s womb, your developing brain formed nerve cells at the explosive rate of nearly a quarter million per minute. The developing brain cortex actually overproduces neurons, with the number peaking at 28 weeks (Rabinowicz et al., 1996, 1999). From infancy on, brain and mind — neural hardware and cognitive software — develop together. On the day you were born, you had most of the brain cells you would ever have. However, your nervous system was immature: After birth, the branching neural networks that eventually enabled all your abilities underwent a wild growth spurt (Figure 3.2-4). This rapid development helps explain why infant brain size increases rapidly in the early days after birth (Holland et al., 2014). 1078  Figure 3.2-4 Infant brain development In humans, the brain is immature at birth. As the child matures, the neural networks grow increasingly complex. From ages 3 to 6, the most rapid brain growth was in your frontal lobes, which enable rational planning. During those years, your brain required vast amounts of energy (Kuzawa et al., 2014). This energy-intensive process caused rapid progress in your ability to control your attention and behavior (Garon et al., 2008; Thompson- Schill et al., 2009). The brain’s association areas — those linked with thinking, memory, and language — were the last cortical areas to develop. As they did, your mental abilities surged (Chugani & Phelps, 1986; Thatcher et al., 1987). Fiber pathways supporting agility, language, and self-control proliferated into puberty. Under the influence of adrenal hormones, tens of billions of synapses formed and organized, while a use-it-or- 1079 lose-it synaptic pruning process shut down unused links (Paus et al., 1999; Thompson et al., 2000). Baby brains This electrode cap allows researchers to detect changes in brain activity triggered by different stimuli. Your genes dictate your overall brain architecture, rather like the lines of a coloring book, but experience fills in the details (Kenrick et al., 2009). So how do early experiences leave their “fingerprints” in the brain? Mark Rosenzweig, David Krech, and their colleagues (1962) opened a window on that process when they raised some young rats in solitary confinement, and others in a communal playground that simulated a natural environment. When they later analyzed the rats’ brains, those who died with the most toys had won: The rats living in the enriched environment had usually developed a heavier and thicker brain cortex (Figure 3.2-5). 1080  Figure 3.2-5 Experience affects brain development Researchers raised rats either alone in an environment without playthings, or with other rats in an environment enriched with playthings changed daily (Rosenzweig et al., 1962). In 14 of 16 repetitions of this basic experiment, rats in the enriched environment developed significantly more cerebral cortex (relative to the rest of the brain’s tissue) than did those in the impoverished environment. Rosenzweig was so surprised that he repeated the experiment several times before publishing his findings (Renner & Rosenzweig, 1987; Rosenzweig, 1984). So great were the effects that, if shown brief video clips of the rats, you could tell from their activity and curiosity whether their environment had been impoverished or enriched (Renner & Renner, 1993). After 60 days in the enriched environment, the rats’ brain weights increased 7 to 10 percent and the number of brain synapse connections mushroomed by about 20 percent (Kolb & Whishaw, 1998). In humans, too, lack of stimulation can slow brain and cognitive development (Farah, 2017). AP® Science Practice 1081  Research As explained in Module 0.4, the independent variable is the factor the experimenter manipulates to study its effect on the dependent variable (the outcome that is measured). Can you identify the independent variable in the experiment described in Figure 3.2-5? If you said “type of environment,” you are correct! Such results have motivated improvements in environments for laboratory, farm, and zoo animals — and for children in institutions. Stimulation by touch or massage also benefits both infant rats and premature babies (Field et al., 2007; Sarro et al., 2014). “Handled” infants of these species develop faster neurologically and gain weight more rapidly. Preemies who have had skin-to-skin contact with their parents sleep better, experience less stress, and show better cognitive development 10 years later (Britto et al., 2017; Feldman et al., 2014). Nature and nurture interact to sculpt our synapses. Brain maturation provides us with an abundance of neural connections. Experiences — sights and smells, touches and tastes, music and movement — activate and strengthen some neural pathways while others weaken from disuse. Like forest pathways, popular neural tracks broaden and less-traveled ones gradually disappear (Dahl et al., 2018; Gopnik et al., 2015). By puberty, this pruning process results in a massive loss of unemployed connections. 1082  Stringing the circuits starts young String musicians who started playing before age 12 have larger and more complex neural circuits controlling the note-making left-hand fingers than do string musicians whose training started later (Elbert et al., 1995). Here at the juncture of nurture and nature is the biological reality of early childhood learning. During early childhood — while excess connections are still on call — youngsters can most easily master such skills as the grammar and accent of another language. We seem to have a critical period, or sensitive period, for some skills. Lacking any exposure to spoken, written, or signed language before adolescence, a person will never master any language (see Module 3.5). Likewise, lacking visual experience during the early years, a person whose vision is later restored by cataract removal will never achieve normal perceptions (see Module 2.1b) (Gregory, 1978; Wiesel, 1982). Without that early visual stimulation, the brain cells normally assigned to vision will die or be diverted to other uses. The maturing brain’s rule: Use it or lose it. 1083 critical period an optimal period early in the life of an organism when exposure to certain stimuli or experiences produces normal development. Although normal stimulation during the early years is critical, brain development does not end with childhood. As we saw in Module 1.4, thanks to the brain’s amazing neuroplasticity, our neural tissue is ever changing and reorganizing in response to new experiences. New neurons also are born. If a monkey pushes a lever with the same finger many times a day, brain tissue controlling that finger changes to reflect the experience (Karni et al., 1998). Human brains work similarly. Whether learning to keyboard, skateboard, or navigate London’s streets, we perform with increasing skill as our brain incorporates the learning (Ambrose, 2010; Maguire et al., 2000). Motor Development The developing brain enables fine motor (involving the small muscles of the body) and gross motor (involving large muscles and whole-body movement) coordination. Skills emerge as infants exercise their maturing muscles and nervous system. With occasional exceptions, the fine and gross motor development sequence is universal. Babies grasp before they build towers of small blocks, and they roll over before they sit unsupported. These behaviors reflect not imitation but a maturing nervous system. 1084  Physical development Sit, crawl, walk, run — the sequence of these motor development milestones is the same around the world, though babies reach them at varying ages. Genes guide motor development. In the United States, 25 percent of all babies walk by age 11 months, 50 percent within a week after their first birthday, and 90 percent by age 15 months (Frankenburg et al., 1992). Identical twins typically begin walking on nearly the same day (Wilson, 1979). Maturation — including the rapid development of the cerebellum at the back of the brain — creates our readiness to learn walking at about age 1. The same is true for other physical skills, including bowel and bladder control. Before necessary muscular and neural maturation, neither pleading nor punishment will produce successful toilet training. You can’t rush a child’s first flush. 1085 Still, nurture may amend what nature intends. In some regions of Africa, the Caribbean, and India, caregivers often massage and exercise babies, which can accelerate the process of learning to walk (Karasik et al., 2010). The recommended infant back to sleep position (putting babies to sleep on their backs to reduce crib-death risk) has been associated with somewhat later crawling but not with later walking (Davis et al., 1998; Lipsitt, 2003). Brain Maturation and Infant Memory 3.2-4 How does an infant’s developing brain begin processing memories? Can you recall your third birthday? Most of us consciously recall little from before age 4. Mice and monkeys also forget their early life, as rapid neuron growth disrupts the circuits that stored old memories (Akers et al., 2014). But as children mature, this infantile amnesia wanes, and they become increasingly capable of remembering experiences, even for a year or more (Bauer & Larkina, 2014; Morris et al., 2010). The brain areas underlying memory, such as the hippocampus and frontal lobes, continue to mature during and after adolescence (Luby et al., 2016; Murty et al., 2016). 1086 Despite consciously recalling little from our early years, our brain was processing and storing information. While finishing her doctoral work in psychology, Carolyn Rovee-Collier observed nonverbal infant memory in action. Her 2-month-old, Benjamin, could be calmed by moving a crib mobile. Weary of hitting the mobile, she strung a cloth ribbon connecting the mobile to Benjamin’s foot. Soon, he was kicking his foot to move the mobile. Thinking about her unintended home study, Rovee-Collier realized that, contrary to popular opinion in the 1960s, babies can learn and remember. To know for sure that her son wasn’t just a whiz kid, she repeated the study with other infants (Rovee-Collier, 1989, 1999). Sure enough, they, too, soon kicked more when hitched to a mobile, both on the day of the study and the day after. If, however, she hitched them to a different mobile the next day, the infants showed no learning, indicating that they remembered the original mobile and recognized the difference. Moreover, when tethered to the 1087 familiar mobile a month later, they remembered the association and again began kicking. Traces of forgotten childhood languages may also persist. One study tested English-speaking British adults who had no conscious memory of the Hindi or Zulu they had spoken as children. Yet up to age 40, they could relearn subtle sound contrasts in these languages that other English speakers could not learn (Bowers et al., 2009). Chinese adoptees living in Canada since age 1 process Chinese sounds as do fluent Chinese speakers, even if they have no conscious recollection of Chinese words (Pierce et al., 2014). We see our two- track mind at work here: What the conscious mind does not know and cannot express in words, the nervous system and unconscious mind somehow remember. AP® Science Practice Check Your Understanding Examine the Concept Explain maturation. Explain what is meant by critical or sensitive period. Apply the Concept What do you regard as your earliest memory? Now that you know about infantile amnesia, has your opinion changed about the accuracy of that memory? Answers to the Examine the Concept questions can be found in Appendix C at the end of the book. 1088  Module 3.2a REVIEW 3.2-1 What is the course of prenatal development, and how do teratogens affect that development? The life cycle begins at conception, when one sperm cell unites with an egg. In the next 6 weeks, body organs begin to form and function. Teratogens are potentially harmful agents that can pass through the placenta and harm the developing embryo or fetus, as happens with fetal alcohol syndrome (FAS). Malnourishment, maternal illness, and genetic mutations can put a child at risk for health problems and psychiatric disorders. 3.2-2 What are some newborn abilities, and how do researchers explore infants’ mental abilities? Babies are born with sensory equipment and reflexes that facilitate their survival and their social interactions with adults. For example, they quickly learn to discriminate their mother’s smell, and they prefer the sound of human voices. Researchers use techniques that test habituation to explore infants’ abilities. 3.2-3 During infancy and childhood, how do the 1089 brain and motor skills develop? The brain’s nerve cells are sculpted by heredity and experience. As a child’s brain develops, neural connections grow more numerous and complex. Experiences then trigger a pruning process, in which unused connections weaken and heavily used ones strengthen. Complex motor skills — sitting, standing, walking — develop in a predictable sequence, though the timing of that sequence is a function of individual maturation and culture. For some skills, we seem to have a critical (or sensitive) period. 3.2-4 How does an infant’s developing brain begin processing memories? We have few or no conscious memories of events occurring before about age 4. This infantile amnesia occurs in part because major brain areas have not yet matured. Despite the lack of conscious recall, our brains were still processing and storing information. This is evident in individuals who spoke different languages as young children and are able to relearn their sounds more easily as adults. AP® Practice Multiple Choice 1090 Questions 1. If a pregnant person contracts the rubella virus, their child may experience lifelong physical and cognitive function deficiencies. The rubella virus is considered to be a(n) a. reflex. b. embryo. c. teratogen. d. zygote. 2. FunToys Company is developing a new line of infant toys. Evelyn, a toy designer at FunToys, suggests using face-like images in all their new toys. Which of the following statements provides the best support for Evelyn’s idea? a. Babies need to practice looking at faces to form effective interpersonal relationships. b. Babies prefer sights and sounds that facilitate social responsiveness. c. Babies prefer looking at faces rather than abstract images. d. Babies will learn to laugh faster by looking at human faces. 1091 3. As infants gain familiarity with repeated exposure to a visual stimulus, their interest wanes and they look away sooner. The decrease in an infant’s responsiveness is called a. teratogens. b. habituation. c. stability. d. conception. 4. As the infant’s brain develops, some neural pathways will decay if not used. This use-it-or-lose-it process is known as a. motor development. b. synaptic pruning. c. infantile amnesia. d. maturation. 5. Thuy-Linh’s example of maturation accurately indicated that a. we stand before we walk. b. we like familiar people. c. we learn the language accents of our peers. d. we prefer face-like images. 1092 6. Dr. Lim wants to conduct a study exploring the effects of color on visual preferences of newborns. Which of the following would be an appropriate operational definition of the dependent variable in her study? a. Visual preferences b. Color contrast c. Habituation to color d. Being a newborn 7. Which of the following is a reflex that helps infants stay close to their caregivers? a. Maturation b. Blinking c. Habituation d. Grasping 8. Researchers found that week-old nursing babies, placed between a gauze pad from their mother’s bra and one from another nursing mother, usually turn toward the smell of their own mother’s pad. Which of the following statements is true of this study? a. The researchers can draw causal conclusions because they used an experimental design. 1093 b. The researchers cannot draw causal conclusions because they used an experimental design. c. The researchers can draw causal conclusions because they used a non-experimental design. d. The researchers cannot draw causal conclusions because they used a non-experimental design. 1094 Module 3.2b Physical Development Across the Lifespan: Adolescence and Adulthood Learning Targets 3.2-5 Define adolescence, and explain how the physical changes during this period affect developing teens. 3.2-6 Explain the physical changes that occur during middle and late adulthood. Many psychologists once believed that childhood sets our traits. Today’s developmental psychologists see development as lifelong. As this lifespan perspective emerged, psychologists began to look at how maturation and experience shape us not only in infancy and childhood, but also — after we are no longer handed the children’s menu — in adolescence and adulthood. Physical Development in Adolescence 3.2-5 How is adolescence defined, and how do physical changes affect developing teens? Adolescence — the years spent morphing from child to adult — starts with the physical beginnings of sexual maturity and ends with the social achievement of independent adult status. Thus, in cultures where postpubertal teens are self-supporting, such as among 1095 Aboriginal Australians, adolescence hardly exists (Senior et al., 2021). And in Western cultures, where sexual maturation occurs earlier and independence later, adolescence is lengthening (Sawyer et al., 2018; Worthman & Trang, 2018). adolescence the transition period from childhood to adulthood, extending from puberty to independence. In industrialized countries, what are the teen years like? In Leo Tolstoy’s Anna Karenina, the teen years were “that blissful time when childhood is just coming to an end, and out of that vast circle, happy and gay, a path takes shape.” But another teenager, Anne Frank, writing in her diary while hiding from the Nazis, described tumultuous teen emotions: My treatment varies so much. One day Anne is so sensible and is allowed to know everything; and the next day I hear that Anne is just a silly little goat who doesn’t know anything at all and imagines that she’s learned a wonderful lot from books…. Oh, so many things bubble up inside me as I lie in bed, having to put up with people I’m fed up with, who always misinterpret my intentions. G. Stanley Hall (1904), one of the first psychologists to describe adolescence, believed that the tension between biological maturity and social dependence creates a period of “storm and stress.” It’s a time of diminishing parental control (Lionetti et al., 2019). It’s also a time when teens crave social acceptance, but often feel socially disconnected. Three in four U.S. friendships started in seventh grade dissolve by the end of eighth grade (Hartl et al., 2015). Such social disconnection hits adolescents hard — increasing their risk for substance abuse and depressive symptoms (Hussong et al., 2019). Indeed, after age 30, many who grow up in independence-fostering 1096 Western cultures look back on their teenage years as a time they would not want to relive — a time when their peers’ social approval was imperative, their sense of direction in life was in flux, and their feeling of alienation from their parents was deepest (Arnett, 1999; Macfarlane, 1964). But for others, adolescence is a time of vitality without the cares of adulthood — a time of rewarding friendships, heightened idealism, and a growing sense of life’s exciting possibilities. Cultural Awareness Notice the mentions of industrialized countries and Western culture, as well as Aboriginal Australian culture in the discussion of adolescence. Culture plays an important role in how we experience this stage of life. Can you think of a way culture influences the experience of teens? Viewing the experience of others purely through our own cultural lens can lead to misunderstanding. Adolescence begins with puberty, the time when we mature sexually. Puberty follows a surge of hormones, which may intensify moods and which trigger a series of bodily changes discussed in Module 3.3. puberty the period of sexual maturation, during which a person usually becomes capable of reproducing. The Timing of Puberty Just as in the earlier life stages, the sequence of physical changes in puberty (for example, breast buds and visible pubic hair before 1097 menarche — the first menstrual period) is far more predictable than their timing. Some girls start their growth spurt at age 9, some boys as late as age 16. Early maturation can be a challenge. Early maturing adolescents are at increased risk for mental health problems (Hamlat et al., 2019; Lee et al., 2020; Ullsperger & Nikolas, 2017). This vulnerability is greatest for teen girls and boys with emotionally reactive temperaments. Also, if a girl’s physical development outpaces her emotional maturity and her friends’ development, she may associate with older adolescents, suffer teasing or sexual harassment, and ruminate more (Alloy et al., 2016; Weingarden & Renshaw, 2012). The Teenage Brain The adolescent brain is a work in progress. Until puberty, brain cells increase their connections, like trees growing more roots and branches. Then, during adolescence, comes a selective pruning of unused neurons and connections (Blakemore, 2008). What we don’t use, we lose. As teens mature, their prefrontal cortex (in the forward part of the frontal lobes) also continues to develop. The continuing growth of myelin, the fatty tissue that forms around axons and speeds neurotransmission, enables better communication with other brain regions (Whitaker et al., 2016). These developments bring improved judgment, impulse control, and long-term planning. A landmark study following 11,000 youth from late childhood to early adulthood 1098 is examining influences on teens’ brain development, such as drugs, screen time, and sleep (NIMH, 2019; Wadman, 2018). Maturation of the prefrontal cortex nevertheless lags behind that of the emotional limbic system. Puberty’s hormonal surge and limbic system development help explain teens’ occasional impulsiveness, risky behaviors, and emotional storms — slamming doors and turning up the music (Smith, 2018; Steinberg & Icenogle, 2019). No wonder younger teens (whose unfinished prefrontal cortex isn’t yet fully equipped for making long-term plans and curbing impulses) may succumb to the lure of risky behaviors. Teens actually don’t underestimate the risks of vaping, fast driving, and unprotected sex. Their brains are just biased toward immediate rewards, which helps explain why teens worldwide struggle with self-control (Hansen et al., 2019; Steinberg et al., 2018). The teenage brain is like a car with a forceful accelerator and underdeveloped brakes (Figure 3.2-6). 1099 Figure 3.2-6 Impulse control lags reward seeking Surveys of more than 7000 American 12-to 24-year-olds reveal that sensation-seeking peaks in the mid-teens, with impulse control developing more slowly as their prefrontal cortex matures. (National Longitudinal Study of Youth and Children and Young Adults survey data presented by Steinberg, 2013.) AP® Science Practice Data Interpretation Consider Figure 3.2-6. 1100  Identify the variables represented in the graph. Are the data presented in this graph qualitative or quantitative? Explain your choice. Which age group shows the most impulse control? Which age group shows the most sensation-seeking? So, when Junior drives recklessly and struggles academically, should his parents reassure themselves that “he can’t help it; his prefrontal cortex isn’t yet fully developed”? They can take hope: Brain changes underlie teens’ new self-consciousness about what others are thinking as well as their valuing of risky rewards (Barkley-Levenson & Galván, 2014; Somerville et al., 2013). And the brain with which Junior begins his teens differs from the brain with which he will end his teens. Unless he slows his brain development with heavy drinking — leaving him prone to impulsivity and addiction — his prefrontal cortex will continue maturing until about age 25 (Crews et al., 2007; Giedd, 2015). It will also become better connected with the limbic system, enabling better emotion regulation (Cohen et al., 2016; Steinberg, 2012). 1101 In 2004, the American Psychological Association (APA) joined seven other medical and mental health associations in filing U.S. Supreme Court briefs arguing against the death penalty for 16-and 17-year- olds. The briefs documented the teen brain’s immaturity “in areas that bear upon adolescent decision making.” Brain scans of young teens reveal that prefrontal cortex immaturity is most evident among juvenile offenders and drug users (Shannon et al., 2011; Whelan et al., 2012). Thus, teens are “less guilty by reason of adolescence,” suggested psychologist Laurence Steinberg and law professor Elizabeth Scott (2003; Steinberg et al., 2009). In 2005, by a 5-to-4 margin, the Court concurred, declaring juvenile death penalties unconstitutional. In 2012, the APA offered similar arguments against mandatory sentencing of juveniles to life without parole (Banville, 2012; Steinberg, 2013). Once again, the Court, by a narrow 5-to-4 vote, concurred. AP® Science Practice Developing Arguments Psychologists assess claims using scientifically derived evidence. With this in mind, identify the reasoning behind the APA’s argument against the death penalty for 16-and 17-year-olds. Physical Development in Adulthood 3.2-6 What physical changes occur during middle and late adulthood? 1102 The unfolding of our lives continues across the lifespan. It is, however, more difficult to generalize about adulthood stages than about life’s early years. If you know that James is a 1-year-old and Jamal is a 10-year-old, you could say a great deal about each child. Not so with adults who differ by a similar number of years. The boss may be 30 or 60; the marathon runner may be 20 or 50; the 19-year- old may be a parent who supports a child or a child who receives an allowance. Yet our life courses are in some ways similar. Physically, cognitively, and especially socially, we differ at age 50 from our 25- year-old selves. In the discussion that follows, we recognize these differences and use these terms: emerging adulthood (in prosperous communities, from age 18 to mid-twenties), early adulthood (roughly twenties and thirties), middle adulthood (to age 65), and late adulthood (the years after 65). Within each of these stages, people vary widely in their physical, psychological, and social development. Like the declining daylight after the summer solstice, our physical abilities — muscular strength, reaction time, mobility, flexibility, and cardiac output — all begin an almost imperceptible decline in our mid-twenties. Athletes are often the first to notice. Baseball players peak at about age 27 — with 60 percent of Most Valuable Player awardees since 1985 being within 2 years of that age (Silver, 2012). But most of us — especially those of us whose daily lives do not require top physical performance — hardly perceive the early signs of decline. 1103 Physical Changes in Middle Adulthood Athletes over 40 know all too well that physical decline gradually accelerates. During early and middle adulthood, physical vigor has less to do with age than with a person’s health and exercise habits. Many physically fit 50-year-olds run 4 miles with ease, while sedentary 25-year-olds find themselves huffing and puffing up two flights of stairs. Adult abilities vary widely George Blair was, at age 92, the world’s oldest barefoot water skier. He is shown here in 2002 when he first set the record, at age 87. (He died at age 98.) Aging also brings a gradual decline in fertility, especially for women. For a 35-to 39-year-old woman, the chance of getting pregnant after a single act of intercourse is half that of a woman 19 to 26 years old 1104 (Dunson et al., 2002). Women experience menopause as their menstrual cycles end, usually within a few years of age 50. Worldwide, early menopause increases women’s risk for depression (Georgakis et al., 2016; Zeng et al., 2019). Men experience a gradual decline in sperm count, testosterone level, and speed of erection and ejaculation. With declining strength and changing appearance, people may experience distress. Seeing their first gray hairs, they may just want to dye. menopause the time of natural cessation of menstruation; also refers to the biological changes a woman experiences as her ability to reproduce declines. Sexual activity remains satisfying, though less frequent, after middle age. This was true of 70 percent of Canadians surveyed (ages 40 to 64) and 75 percent of Finns (ages 65 to 74) (Kontula & Haavio-Mannila, 2009; Wright, 2006). And in a sexuality survey, it was not until age 75 or older that most women and nearly half of men reported little 1105 sexual desire (DeLamater, 2012; DeLamater & Sill, 2005). As Alex Comfort (2002, p. 226) jested, “The things that stop you having sex with age are exactly the same as those that stop you riding a bicycle (bad health, thinking it looks silly, no bicycle).” AP® Science Practice Research Notice that the findings cited here came from Canadian, Finnish, and U.S. samples. Researchers conduct similar surveys in various cultures to see if the findings generalize to other older adult populations. Physical Changes in Late Adulthood Is old age “more to be feared than death” (Juvenal, The Satires)? Or is life “most delightful when it is on the downward slope” (Seneca, Epistulae ad Lucilium)? What is it like to grow old? Life Expectancy From 1950 to 2015, worldwide life expectancy at birth increased from 50 to 73 years (Dicker et al., 2018) (Figure 3.2-7). What a gift — two decades more of life! In China, the United States, the United Kingdom, Canada, and Australia (to name some countries where students read this book), life expectancy has risen to 77, 79, 81, 82, and 83 years, respectively (World Bank, 2022). This increasing life expectancy (humanity’s greatest achievement, say some) combines with decreasing birthrates: Older adults are a growing population 1106 segment, creating an increasing demand for hearing aids, retirement villages, and nursing homes. Today, 13 percent of people worldwide are 60 or older. The United Nations (2017) projects that proportion will more than double by 2100. Figure 3.2-7 Life expectancy since 1770, by world regions With improved sanitation, modern medicine, and the introduction of antibiotics, infant mortality declined during the twentieth century and life expectancy increased (Roser, 2019). Throughout the lifespan, males are more prone to dying. Although 126 male embryos begin life for every 100 females, the sex ratio is already down to 105 males for every 100 females at birth (Ritchie & Roser, 2019). During the first year, male infants’ death rates exceed females’ by one-fourth. And worldwide, women outlive men by 4.7 years (WHO, 2016b). By age 100, women outnumber men 5 to 1. 1107  World record for longevity? French woman Jeanne Calment, the oldest human in history with authenticated age, died in 1998 at age 122 (Robine & Allard, 1999). At age 100, she was still riding a bike. But few of us live to 100. Disease strikes. The body ages. Its cells stop reproducing. It becomes frail and vulnerable to tiny insults — hot weather, a fall, a mild infection — that at age 20 would have been trivial. Chronic anger and depression increase our risk of premature death. In contrast, low stress and good health habits enable longevity, as does a positive spirit. Researchers have even observed an intriguing death-deferral phenomenon (Shimizu & Pelham, 2008). Across one 15- year period, 2000 to 3000 more Americans died on the 2 days after Christmas than on Christmas and the 2 days before. The death rate 1108 also increases when people reach their birthdays, and when they survive until after other milestones, as was true after the first day of the new millennium. Sensory Abilities, Strength, and Stamina Although physical decline begins in early adulthood, we are not usually acutely aware of it until later in life, when the stairs get steeper, other people seem to mumble more, and the print gets smaller. As humorist Dave Barry (1998) mused regarding his own aging, “For some reason, possibly to save ink, [restaurants have] started printing their menus in letters the height of bacteria.” Muscle strength, reaction time, and stamina likewise diminish in late adulthood. But even diminished vigor is sufficient for normal activities. With age, visual sharpness diminishes, as does distance perception and adaptation to light-level changes. The eye’s pupil shrinks and its lens becomes less transparent, reducing the amount of light reaching the retina. A 65-year-old retina receives only about one- third as much light as its 20-year-old counterpart (Kline & Schieber, 1985). Thus, to see as well as a 20-year-old when reading or driving, a 65-year-old needs three times as much light — a reason for buying cars with untinted windshields. This also explains why older people sometimes ask younger people, “Don’t you need better light for reading?” The senses of smell, hearing, and touch also diminish. In Wales, teens’ loitering around a convenience store has been 1109 discouraged by a device that emits an aversive high-pitched sound almost no one over 30 can hear (Lyall, 2005). Health As people age, they care less about what their bodies look like and more about how their bodies function. For those growing older, there is both bad and good news about health. The bad news: The body’s disease-fighting immune system weakens, making older adults more susceptible to life-threatening ailments such as cancer, pneumonia, and Covid. The good news: Thanks partly to a lifetime’s accumulation of antibodies, people over 65 suffer fewer short-term ailments, such as common flu and cold viruses. One study found they were half as likely as 20-year-olds and one-fifth as likely as preschoolers to suffer upper respiratory infections each year (National Center for Health Statistics, 1990). The Aging Brain Up to the teen years, we process information with greater and greater speed (Fry & Hale, 1996; Kail, 1991). But compared with you, older people take a bit more time to react, to solve perceptual puzzles, and even to remember names (Bashore et al., 1997; Verhaeghen & Salthouse, 1997). At video games, most 70-year-olds are no match for a 20-year-old. This processing lag can also have deadly consequences (Aichele et al., 2016). As Figure 3.2-8 indicates, fatal accident rates per mile driven increase sharply after age 75. By age 85, they exceed the 16-year-old level. Older drivers appear to 1110 focus well on the road ahead, but attend less to vehicles approaching from the side (Pollatsek et al., 2012). Figure 3.2-8 Age and driver fatalities Slowing reactions contribute to increased accident risk among individuals aged 75 and older, and older adults’ greater fragility increases their risk of death when accidents happen (NHTSA, 2000). Would you favor driver exams based on performance, not age, to screen out those persons whose slow reactions or sensory impairments indicate a higher accident risk? Brain regions important to memory begin to atrophy during aging (Fraser et al., 2015; Ritchie et al., 2015). The blood-brain barrier also breaks down beginning in the hippocampus, which furthers cognitive decline (Montagne et al., 2015). No wonder older adults feel even older after taking a memory test: It’s like “aging 5 years in 5 minutes,” joked one research team (Hughes et al., 2013). In early adulthood, a small, gradual net loss of brain cells begins, contributing by age 80 to a brain-weight reduction of 5 percent or so. Earlier, we noted that the late-maturing prefrontal cortex, which 1111 helps us override our undesirable urges, helps account for teen impulsivity. Late in life, some of that impulsiveness often returns as that same prefrontal cortex begins to atrophy, seemingly explaining older people’s occasional blunt questions (“Have you put on weight?”) or inappropriate comments (von Hippel, 2007, 2015). But good news: The aging brain maintains some neuroplasticity, which partly compensates for what it loses by recruiting and reorganizing neural networks (Park & McDonough, 2013). During memory tasks, for example, the left frontal lobes are especially active in young adult brains, while older adult brains use both the left and right frontal lobes. Exercise and Aging And more good news: Exercise slows aging, as shown in studies of identical twin pairs in which only one twin exercised (Iso-Markku et al., 2016; Rottensteiner et al., 2015). Midlife and older adults who do more exercising and less sitting around tend to be mentally quick older adults (Kramer & Colcombe, 2018; Won et al., 2019). Physical exercise can even slow the progression of Alzheimer’s disease (Kivipelto & Håkansson, 2017; Loprinzi et al., 2015; Smith et al., 2014). 1112 Exercise also appears to stimulate neurogenesis — the development of new brain cells — and neural connections, thanks perhaps to increased oxygen and nutrient flow (Erickson et al., 2010; Pereira et al., 2007). Sedentary older adults randomly assigned to aerobic exercise programs exhibited enhanced memory, sharpened judgment, and reduced risk of severe cognitive decline (Northey et al., 2018; Raji et al., 2016; Smith, 2016). In the aging brain, exercise reduces brain shrinkage (Gow et al., 2012). And it increases the cellular mitochondria, which help power both muscles and brain cells (Steiner et al., 2011). We are more likely to rust from disuse than to wear out from overuse. Fit bodies support fit minds. AP® Science Practice 1113  Check Your Understanding Examine the Concept Explain what is meant by adolescence. Explain the physical changes in the brain during adolescence. Apply the Concept Imagining the future, how do you think you might change? How might you stay the same? In what ways do you most want to grow as a person? Based on what you learned in this module, what logical conclusions can you draw about aging and exercise? Answers to the Examine the Concept questions can be found in Appendix C at the end of the book. Module 3.2b REVIEW 3.2-5 How is adolescence defined, and how do physical changes affect developing teens? Adolescence is the transition period from childhood to adulthood, extending from puberty to social independence. Early maturation can be a challenge for developing adolescents. The brain’s prefrontal cortex matures and myelin growth increases during adolescence, enabling improved judgment, impulse control, and long-term planning. 1114 3.2-6 What physical changes occur during middle and late adulthood? Muscular strength, reaction time, mobility, flexibility, sensory abilities, and cardiac output begin to decline almost imperceptibly in the mid-twenties; this downward trajectory accelerates through middle and late adulthood, varying considerably with personal health and exercise habits. Women’s period of fertility ends with menopause around age 50; men experience a more gradual decline. In late adulthood, the immune system weakens, increasing susceptibility to life-threatening illnesses. Exercise can slow aging; it enhances physical health as well as boosts memory, improves judgment, and reduces the risk of severe cognitive decline. AP® Practice Multiple Choice Questions 1. When Kaev reached sexual maturity, becoming capable of reproducing, he entered a. puberty. b. menopause. c. fertility. 1115  d. early adulthood. 2. Dr. Kamara conducted a study on the transition from childhood to adulthood Dr. Kamara is interested in what variable? a. Social behavior b. Puberty c. Menopause d. Adolescence 3. Compared to when he was 17 years old, Kwame, who is now 25 years old, has noticed that he is better able to resist the urge to play video games all evening. What development in adolescence has allowed for his greater impulse control? a. Hormonal surges b. Hindbrain changes c. Prefrontal cortex maturation d. Limbic system development 4. Dr. Longoria wants to conduct a study to examine the development of puberty across 30 different countries. Based on past research on brain development in puberty, which of the following would be the best hypothesis Dr. Longoria could make? 1116  a. In Western countries, limbic system development precedes prefrontal cortex development, while the opposite is true in Eastern countries. b. In Eastern countries, puberty’s sequence begins with decreased fertility, which should be followed by menopause, while the opposite is true in Western countries. c. In all countries, puberty follows the same sequence regardless of the country’s geographic location. d. In all countries, puberty begins with increased fertility and ends in menopause. 5. Ankit has reached menopause. What is true for her? a. She, like men, likely experienced menopause around the age of 50. b. She likely experienced menopause around 50 years of age, but men experience menopause around 65 years of age. c. She likely experienced menopause around the age of 50, but men don’t experience menopause. d. Men experience menopause around the age of 65, but she likely experienced menopause in her twenties. 1117 6. Dr. Soq studies the tendency for people to die after major milestones, such as a birthday or a holiday. What does Dr. Soq study? a. Menopause and adolescence b. Death-deferral phenomenon c. Fertility and menopause d. Life expectancy 7. Dr. Raley conducted a study on a group of 1000 people who were between 65 and 85 years old and were experiencing difficulties remembering newly presented information. Which of the following is the population of interest in this study? a. The 1000 individuals in the study who were in middle adulthood b. All individuals in late adulthood c. The 1000 individuals in the study who were in late adulthood d. All individuals in middle adulthood 1118 Module 3.3a Gender and Sexual Orientation: Gender Development Learning Targets 3.3-1 Explain how the meaning of gender differs from the meaning of sex. 3.3-2 Explain some ways in which males and females tend to be alike and tend to differ. 3.3-3 Explain how sex hormones influence prenatal and adolescent sexual development. 3.3-4 Explain some cultural influences on gender roles. 3.3-5 Explain how we form our gender identity. 3.3-1 How does the meaning of gender differ from the meaning of sex? Cultures change, and ideas about gender change, too. After enrolling at Harvard Law School as one of nine women among 500+ men, Ruth Bader Ginsburg was asked by the dean, “Why are you at Harvard Law School, taking the place of a man?” After law school, a Supreme Court justice rejected her clerkship application because he wasn’t ready to hire a woman (Lewis, 1993). When Ginsburg was recommended for a law firm job, the managing partner responded flatly, “We don’t hire women.” Ginsburg devoted much of her career to changing laws that discriminated against women. In 1993, at age 60, she became the second woman to serve on the U.S. Supreme Court, becoming a champion of gender equality and an icon to younger generations. 1119 “Women will have achieved true equality,” Ginsburg said, “when men share with them the responsibility of bringing up the next generation” (Sullivan, 2001). Clearly, social and cultural factors influence our gender expectations. But how do nature and nurture interact to define gender and form our unique gender identities? How are males and females alike, and how and why do they differ? While exploring these issues, we’ll also gain insight into the psychology and biology of sexual attraction and intimacy. And as part of the journey, we’ll see how evolutionary psychologists explain our sexuality. Let’s start by asking: How does gender develop? As we saw in Module 2.2a, we humans share an irresistible urge to organize our worlds into simple categories. Among the ways we classify people — as tall or short, dull or smart, cheerful or churlish — one stands out. It’s what everyone first wanted to know about you: “Boy or girl?” Your parents may have tried to offer clues with pink or blue clothing; their answer described your birth-assigned sex. For most people, biological traits help define their assigned gender, their culture’s expectations about what it means to be a man or a woman. sex in psychology, the biologically influenced characteristics by which people define male, female, and intersex. gender in psychology, the attitudes, feelings, and behaviors that a given culture associates with a person’s biological sex. (See also gender identity.) 1120 Simply said, your body defines your sex; your mind defines your gender. But your mind’s understanding of gender arises from the interplay between your biology and your experiences (Eagly & Wood, 2013). Before we consider that interplay, let’s look at three ways that males and females differ and how they are alike. Similarities and Differences 3.3-2 What are some of the ways males and females tend to be alike and to differ? Whether male, female, or intersex, most of us receive 23 chromosomes from our mother and 23 from our father. Of those 46 chromosomes, 45 are unisex — the same for everyone. Our similar biology helped our evolutionary ancestors face similar adaptive challenges. For example, survival for men and women involved traveling long distances (migration and outrunning threats), which today is reflected biologically in men’s and women’s similar finishing times for ultralong-distance races. Everyone needed to survive, reproduce, and avoid predators, so we are in most ways alike. Your gender gives no clue to your vocabulary, happiness, or ability to see, learn, and remember, reports gender researcher Janet Shibley Hyde and her colleagues (2019). Whatever our gender, we are, on average, comparably creative and intelligent and feel similar emotions and longings (Hyde, 2014; Lauer et al., 2019; Reilly et al., 2019). intersex possessing male and female biological sexual characteristics at birth. 1121  AP® Science Practice Research Janet Shibley Hyde based her claims about gender similarities on meta-analyses that she and her team conducted. A meta-analysis, a non-experimental method, is a statistical procedure for summarizing the results of multiple studies to reach an overall conclusion. For example, a researcher might combine the results of 46 studies with hundreds of participants each, thus making her conclusions more generalizable to broader populations. SPOTLIGHT ON: Janet Shibley Hyde But in some areas, female and male traits do differ, and differences command attention. Some oft-noted differences (such as the gender difference in self-esteem) are actually quite modest (Zell et al., 2015). Others are more striking. The average female enters puberty about 2 years earlier than the average male, and her life expectancy is 4 years longer. She expresses most emotions more freely, smiling and crying more, and, in social media updates, more often expresses “love” and being “sooo excited!!!” (Fischer & LaFrance, 2015; Schwartz et al., 2013). She is better at spelling and reading (Reilly et al., 2019). Ms. Average can detect fainter odors. She also has twice the risk of developing depression and anxiety, and 10 times the risk of developing an eating disorder. By comparison, the average male is 4 times more likely to die by suicide, to abuse tobacco products, and to develop alcohol use disorder. Mr. Average also has greater size and strength, but is more likely to be diagnosed with autism spectrum disorder, color-deficient vision, and attention-deficit/hyperactivity disorder (ADHD). And as an adult, he is more at risk for antisocial 1122 personality disorder. Female and male each have their own heightened strengths and risks. Gender similarities and differences appear throughout this book, but here let’s take a closer look at three gender differences. Although individuals vary widely, the average male and female differ in aggression, social power, and social connectedness. AP® Science Practice Data The statement “Although individuals vary widely, the average male and female differ in aggression, social power, and social connectedness” illustrates an important point about psychological science. Researchers do not draw general conclusions from individuals. Instead, they compare group averages. If the difference between the groups is large enough to suggest an actual difference between the populations, the results are said to be statistically significant. (You can revisit Module 0.6 to review these important statistical concepts.) Aggression To a psychologist, aggression is any physical or verbal behavior intended to hurt someone physically or emotionally (Bushman & Huesmann, 2010). Pause to picture in your mind an aggressive person. aggression any physical or verbal behavior intended to harm someone physically or emotionally. 1123 Is the person a man? Likely yes. Men generally admit to more aggression, especially extreme physical violence (Yount et al., 2017). Nearly half of 14-to 19-year-old U.S. boys feel pressure to be “willing to punch someone if provoked” (PLAN USA, 2018). In romantic relationships between women and men, minor acts of physical aggression, such as slaps, are roughly equal, but the most violent acts are mostly committed by men (Archer, 2000; Tremblay et al., 2018). In laboratory experiments, men have been more willing to blast people with what they believed was intense and prolonged noise (Bushman et al., 2007). And outside the laboratory, men worldwide commit more violent crime, including 90 percent of murders (UNODC, 2019). Men also take the lead in hunting, fighting, warring, and supporting war (Liddle et al., 2012; Wood & Eagly, 2002, 2007). Here’s another question: Picture in your mind someone harming others by passing along hurtful gossip, by disclosing private information, by shutting someone out, or by online bullying. Was the person a woman? Perhaps. Those behaviors are acts of relational aggression, which women have been slightly more likely than men to commit (Archer, 2004, 2007, 2009). relational aggression an act of aggression (physical or verbal) intended to harm a person’s relationship or social standing. 1124 Social Power Imagine you’ve walked into a job interview and are taking your first look at the two interviewers. The unsmiling person on the left oozes self-confidence and independence, maintaining steady eye contact. The person on the right gives you a warm, welcoming smile, but makes less eye contact and seems to expect the other interviewer to take the lead. Which interviewer is male? If you said the person on the left, you’re not alone. Around the world, from Nigeria to New Zealand, people have perceived gender differences in power (Williams & Best, 1990). Even in 2020, a United Nations global survey found almost half of humanity believe men to be superior political leaders, and more than 40 percent believe them to be better business executives (UNDP, 2020). (For more on this, see Developing Arguments: Gender Bias in the Workplace.) AP® Science Practice Developing Arguments Gender Bias in the Workplace 1125 Developing Arguments Questions 1. Explain the reasoning that differences in perception lead to gender bias. 2. Using scientifically derived evidence, explain how family-care responsibilities contribute to workplace gender bias. 1. Okimoto & Brescoll, 2010. 2. IPU, 2021. 3. Eagly et al., 2020. 4. Colarelli et al., 2006. 5. LMIC, 2020. 6. Willett et al., 2015. 7. Witteman et al., 2019. 8. CEA, 2014; Parker & Wang, 2013; Pew, 2015. 9. Nikolova & Lamberton, 2016; Pinker, 2008. 10. Gino et al., 2015; Schwartz & Rubel- Lifschitz, 2009. 11. Eagly & Carli, 2007; van Engen & Willemsen, 2004. 12. Aries, 1987; Wood, 1126  1987. 13. Leaper & Ayres, 2007; Major et al., 1990; Schumann & Ross, 2010. 14. Harvard Business School, 2019. 15. AAMC, 2018. Now picture a heterosexual couple negotiating a car purchase. “If you won’t lower your price, we’re leaving,” says one of them to the salesperson. Which member of the couple — male or female — made the demand? If you said the male, you’re again in good company. People tend to associate negotiation with males, and men often have an advantage in negotiation outcomes (Mazei et al., 2015). Making history In 2019, West Point’s 34 Black female cadets, many shown here, were part of its most diverse graduating class in history. 1127 When asked a difficult question — “Do you have any idea why the sky is blue?” — men are more likely than women to hazard answers than to admit they don’t know, a phenomenon researchers have called the male answer syndrome. Men are also more prone to “mansplaining” — explaining something to women in a condescending and sometimes inaccurate manner (Giuliano et al., 1998; Tramontana, 2020). Social Connectedness Whatever our gender, we all have a need to belong, though we may satisfy this need in different ways (Baumeister, 2010). In the 1980s, many developmental psychologists believed that all children struggle to create a separate, independent identity. Research by Carol Gilligan and her colleagues (1982, 1990), however, suggested that this struggle describes Western individualistic males more than relationship- oriented females. Gilligan believed females tend to differ from males both in being less concerned with viewing themselves as separate 1128 individuals and in being more concerned with “making connections.” Indeed, males tend to be independent. Even as children, males typically form large play groups that brim with activity and competition, with little intimate discussion (Rose & Rudolph, 2006). As adults, men usually enjoy side-by-side activities, and their conversations often focus on problem solving (Baumeister, 2010; Tannen, 1990). Brain scans show no striking structural sex differences (Ritchie et al., 2018; Wierenga et al., 2019). “Human brains cannot be categorized into two distinct classes — male brain/female brain,” reported neuroscientist Daphna Joel and her colleagues (2015). Brain scans do, however, suggest that a female’s brain, more than a male’s, is usually wired in a way that enables social relationships (Kiesow et al., 2020). This helps explain why females tend to be more interdependent. Across nearly a thousand studies, women have been found to be “more communal than men” (Hsu et al., 2021). In childhood, girls usually play in small groups, often with one friend. They compete less and imitate social relationships more (Maccoby, 1990; Roberts, 1991). Teen girls spend more time with friends and less time alone (Wong & Csikszentmihalyi, 1991). In late adolescence, they spend more time on social media, and average more daily text messages than boys (Pew, 2015a; Yang et al., 2018). Girls’ and women’s friendships are more intimate, featuring more conversation that explores relationships (Maccoby, 2002). In one analysis of 10 million posts to Facebook (which more women use), women’s status updates 1129 were as assertive as men’s, but used warmer words; men more often swore or expressed anger (Gramlich, 2018; Park et al., 2016). An analysis of more than 700 million Facebook words found women also used more family-related words, whereas men used more work- related words (Schwartz et al., 2013). Free-for-all, or tend and befriend? Gender differences in the way we interact with others begin to appear at a very young age. When searching for understanding from someone who will share their worries and hurts, people usually turn to women. Women and men have reported that their friendships with women are more intimate, enjoyable, and nurturing (Kuttler et al., 1999; Rubin, 1985; Sapadin, 1988). When stressed, women are also more likely than men to turn to others for support. They are said to tend and befriend (Tamres et al., 2002; Taylor, 2002). Gender differences in both social connectedness and power are greatest in adolescence and early adulthood — the prime years for dating and mating (Hoff et al., 2018; Hsu et al., 2021). By their teen 1130 years, girls appear less assertive and more insecure, and boys seem more dominant and less expressive (Chaplin, 2015). In adulthood, attitude and behavior differences often peak with parenthood. Mothers especially may express more traditionally female attitudes and behaviors (Ferriman et al., 2009; Katz-Wise et al., 2010). By age 50, most gender differences subside, especially among parents. Men become less domineering and more empathic, and women — especially those with paid employment — become more assertive and self-confident (Kasen et al., 2006; Maccoby, 1998). Worldwide, fewer women than men work for pay. But, like men, women tend to be more satisfied with their lives when gainfully employed (Ryan, 2016). So, although women and men are more alike than different, there are some behavior differences between the average woman and man, some of which also occur in nonhuman primates (Lonsdorf, 2017). Are such differences dictated by their biology? Shaped by their cultures and other experiences? Read on. AP® Science Practice Check Your Understanding Examine the Concept Explain what is meant by gender. Explain physical and relational aggression in terms of gender differences. Apply the Concept Think of two people you’ve known for a long time: one male and one female. Have you observed differences in their behavior that might, based on what you’ve learned here, be attributable to their gender? Have these differences changed with age? 1131  Answers to the Examine the Concept questions can be found in Appendix C at the end of the book. The Nature of Gender 3.3-3 How do sex hormones influence prenatal and adolescent sexual development? In most physical ways — regulating heat with sweat, preferring energy-rich foods, growing calluses where the skin meets friction — we are all alike. Although biology does not dictate gender, it can influence our sexual development in two ways: Genetically — We have differing sex chromosomes. Physiologically — We have differing concentrations of sex hormones, which trigger other anatomical differences. These two influences began to form you long before you were born. Prenatal Sexual Development Six weeks after you were conceived, you looked much the same as any other tiny embryo. Then, as your genes kicked in, your biological sexual characteristics — determined by your twenty-third pair of chromosomes (the two sex chromosomes) — became more apparent. Your mother’s contribution to that chromosome pair was an X chromosome. From your father, you received the 1 chromosome out of the usual 46 that is not unisex — either another X chromosome, 1132 making you female, or a Y chromosome, making you male. (Occasionally, there are other sexual development variations, as we will see shortly.) X chromosome the sex chromosome found in females and males. Females typically have two X chromosomes; males typically have one. An X chromosome from each parent produces a female child. Y chromosome the sex chromosome typically found only in males. When paired with an X chromosome from the mother, it produces a male child. About 7 weeks after conception, a single gene on the Y chromosome throws a master switch, which triggers the testes to develop and to produce testosterone, the main androgen (male hormone) that promotes male sex organ development. Females also have testosterone, but less of it; the main female sex hormones are the estrogens, such as estradiol. testosterone the most important male sex hormone. Males and females have it, but the additional testosterone in males stimulates the growth of the male sex organs during the fetal period, and the development of male sex characteristics during puberty. estrogens sex hormones, such as estradiol, that contribute to female sex characteristics and are secreted in greater amounts by females than by males. Later, during the fourth and fifth prenatal months, sex hormones bathe the fetal brain and influence its wiring. Different patterns for males and females develop under the influence of the male’s greater testosterone and the female’s estrogens (Hines, 2004; Udry, 2000). If, however, females are prenatally exposed to unusually high levels of 1133 male hormones, they tend to grow up with more male-typical interests (Endendijk et al., 2016). Adolescent Sexual Development A flood of hormones triggers another period of dramatic physical change during adolescence, when we enter puberty. In this 2-year period of rapid sexual maturation, pronounced female-male differences emerge. A variety of changes begin at about age 10 in girls and at about age 12 in boys, though the subtle beginnings of puberty, such as budding breasts or enlarging testes, appear earlier (Biro et al., 2012; Herman-Giddens et al., 2012). A year or two before visible physical changes, we often feel the first stirrings of sexual attraction (McClintock & Herdt, 1996). Girls’ slightly earlier entry into puberty can at first propel them to greater height than boys of the same age (Figure 3.3-1). But boys catch up when they begin puberty, and by age 14, they are usually taller than girls. During these growth spurts, the primary sex characteristics — the reproductive organs and external genitalia — develop dramatically. So do the nonreproductive secondary sex characteristics. Pubic and underarm hair emerges. Girls develop breasts and larger hips. Boys’ facial hair begins growing and their voices deepen (Figure 3.3-2). (Pubertal boys may not at first like their sparse beard — but then it grows on them.) primary sex characteristics the body structures (ovaries, testes, and external genitalia) that make sexual reproduction possible. 1134 secondary sex character