Development of the Body PDF

Development of the Body The universals of this stage are biological. Early adulthood is prime time for high energy, hard physical labor, and safe reproduction. The rapid and sometimes unsettling changes of adolescence are over, and age-related declines in senses and muscles are not yet apparent. As...

Development of the Body The universals of this stage are biological. Early adulthood is prime time for high energy, hard physical labor, and safe reproduction. The rapid and sometimes unsettling changes of adolescence are over, and age-related declines in senses and muscles are not yet apparent. As has been true for thousands of years, every body system — digestive, respiratory, circulatory, musculoskeletal, and sexual-reproductive — functions optimally. Maximum height is usually reached by age 16 for girls and 18 for boys, with final touches in size and shape complete before age 25. By then, everyone is full grown: Women have adult breasts and hips, and men have full shoulder width and upper-arm strength (Whitbourne & Whitbourne, 2014). The last body part to mature is the brain. Not until the mid-20s is the prefrontal cortex fully developed, with the entire brain connected and myelinated (Colver & Dovey-Pearce, 2018; Hochberg & Konner, 2020). People can think and act like an adult. What a Body Can Do Here at age 27, Tobin Heath leaps to celebrate her goal at the soccer World Cup Final in Vancouver, her most recent of seven years of star performances. All young adults can have moments when their bodies and minds crescendo to new heights. A Universal Stage? The social context matters. The fact that young adulthood is the best time for carrying rocks, plowing fields, and birthing babies is no longer appreciated. If a young couple quit high school to marry, build a house, till a field, and give birth year after year (as my grandparents did), no one would approve. Contemporary 20-year-olds are physically strong but not ready for adult responsibilities. A college professor in Missouri, Jeffrey Arnett, recognized that his students were not yet adults: They postponed the usual markers of adulthood (steady jobs, lifelong partners, parenthood) and instead sought more education. He “proposed a new conception of development for the period from the late teens through the twenties, with a focus on ages 18–25,” calling that period emerging adulthood (Arnett, 2000). That term is not universally accepted. Some scholars still call these years “late adolescence” or “early adulthood.” However, almost everyone agrees that something has changed in the expectations and opportunities of young adults. The deeper debate is not about the label but about whether this stage is exclusive to a small sector of the world. Might emerging adulthood be a luxury, evident only in WEIRD (Western, Educated, Industrialized, Rich, Democratic) nations? Might the experience of young adulthood be quite different in less affluent nations? Might researchers ignore the reality of these years in most nations of the world? A leading researcher wrote: “The WEIRD group represents maximally 5% of the world’s population, but probably more than 90% of the researchers and scientists producing the knowledge that is represented in our textbooks work with participants from that particular context” (Keller, quoted in Armstrong, 2018). Indeed, emerging adulthood is most clearly defined and evident in the fraction of 1 percent who are college students in the United States. However, the data suggest that emerging adulthood is now everywhere. Between 2015 and 2022, according to Google Scholar, more than 75,000 scholarly articles mentioning “emerging adulthood” have been published by researchers in every continent. The specifics vary by culture — sometimes this stage ends at age 21 or 30 instead of 25 — but everywhere older adolescents postpone full adulthood if they can. One statistic illustrates that this stage has swept the globe: In the entire world, young women have half as many children as they did, choosing more education and fewer babies. In many less affluent nations, the average 25-year-old had four or five children in 1970; she has two children now (United Nations, 2019) (see Figure 11.1). In wealthier nations, most young women have no children. FIGURE 11.1 In Every Nation As you see, births continue to increase, because earlier baby booms now result in more adults of childbearing age than ever before. But those adults have far fewer children, especially during emerging adulthood now a worldwide stage. Health and Sickness In a large U.S. survey, 96.1 percent of 18- to 24-year-olds rated their health as good, very good, or excellent. Only 3.9 percent rated it fair or poor, a significant improvement over the past two decades, and better than older adults (National Center for Health Statistics, 2018). They may be fooling themselves. A study of biological aging found that some people grow old three times faster than others, with about half of that difference evident by age 26 (Belsky et al., 2015). By the mid-30s, some bodies are like the average 20-year-old, others like 50-year-olds. CUMULATIVE LOAD To appreciate this trajectory of adult health, it helps to understand organ reserve, homeostasis, and allostatic load. Organ reserve is the extra power that each organ employs when needed, allowing many 20-year-olds to stay awake all night, or exercise to exhaustion, or take drugs that disrupt the brain, and still function the next day. Because of organ reserve, any strain on the body (pregnancy, running a marathon, lifting weights) is easier at age 20 than at age 40 or 80, which explains the relationship between age and death (see Table 11.1). TABLE 11.1 U.S. Deaths from the Top Two Causes (Heart Disease and Cancer) Age Group Annual Rate per 100,000 15–24 6 25–34 16 35–44 60 45–54 176 55–64 470 65–74 961 75–84 2,119 85+ 5,494 Data from National Center for Health Statistics, 2018. This factor is relevant when COVID-19 death rates are compared internationally. People over age 75 are about a hundred times more likely to die of the virus than young adults are, because their organ reserve has been depleted. Thus, the high death rate in Italy, for example, is partly because Italy has fewer young adults than, say, Nigeria (Sudharsanan et al., 2020). Closely related to organ reserve is homeostasis — a balance between various body reactions that keeps every physical function in sync with every other. For example, if the air temperature rises, people sweat, move slowly, and thirst for cold drinks — three aspects of body functioning that cool them. Homeostasis is almost immediate in early adulthood. Heat-wave deaths are increasing worldwide, not only because of climate change but also because more people live to be very old, when homeostasis is slow (Guo et al., 2017). Young bodies can quickly summon their organ reserve to survive temperature extremes. With COVID-19, because homeostasis lags in older people, the immune system may overreact to a fever before the body can restore balance. That imbalance can be deadly (Moderbacher et al., 2020). Also related to organ reserve is allostasis, a dynamic body adjustment that gradually changes body function. The main difference between homeostasis and allostasis is time: Homeostasis is the immediate response whereas allostasis refers to long-term adjustment of body systems. It draws on organ reserve, depleting it over the decades. HABITS: SLEEP, ACTIVITY, DIET Allostasis depends on the biological adjustments of every earlier time of life, beginning at conception. Accumulated harm from sleep deprivation, inactivity, unhealthy eating, and so on shortens life by 10 years or more (Y. Li et al., 2020). Given the importance of those health habits, we describe each of them in early adulthood. One night’s short sleep makes a person tired the next day — that is homeostasis, the body’s way to maintain equilibrium (see Figure 11.2). But if poor sleep is typical every day, then appetite, mood, and activity adjust (more, down, less) to achieve homeostasis. Meanwhile, allostatic load rises and organ reserve declines. FIGURE 11.2 Don’t Set the Alarm? Every emerging adult sometimes sleeps too little and is tired the next day — that is homeostasis. But years of poor sleep habits reduce years of life — a bad bargain. That is allostatic load. By mid- and late adulthood, years of low-quality sleep decrease overall health (Carroll et al., 2014; McEwen & Karatsoreos, 2015). Sleep quality is determined by many social and psychological factors that are experienced by many emerging adults, including disruptive noise, light, and anxiety. Like sleep, physical activity protects against serious illness lifelong; inactivity takes a toll that begins early on. Exercise reduces blood pressure and strengthens the heart and lungs, reducing depression, osteoporosis, diabetes, arthritis, dementia (now called major neurocognitive disorder), and some cancers. Health benefits are substantial for men and women, old and young, former sports stars and those who never joined an athletic team. Habits are pivotal. If a young adult develops a regular exercise regime, over time, homeostasis adjusts and allows the person to exercise longer and harder. That decreases allostatic load by reducing health risks, evident in blood analysis and body fat. One longitudinal study, CARDIA (Coronary Artery Risk Development in Young Adults) began with thousands of healthy 18- to 30-year-olds. They were queried about their exercise habits. Blood and body fat markers, as well as strength and endurance, were measured.