Biologically, the years from ages 18 to 25 are prime time for hard physical work and safe reproduction. However, the fact that young adults can carry rocks, plow fields, or produce babies is no longer admired.

If a contemporary young couple dropped out of high school to marry and then had a new baby year after year, their neighbors would be appalled, not approving. Societies, families, and young adults themselves expect more education, later marriage, and fewer offspring than the norm a few decades ago.

By this point in your study, the previous paragraph probably raises questions in your mind, because you are well aware of cultural differences in what societies expect. Might some neighbors in some regions of the world approve of a young, fertile couple? As the following Opposing Perspectives suggests, developmentalists also ask this question.

Health from ages 18 to 25 is one aspect of development that has not changed, except maybe to improve. As has been true for thousands of years, every body system—including the digestive, respiratory, circulatory, and sexual-reproductive systems—-functions optimally at the end of adolescence (see Table 11.1).

Serious diseases rarely appear, and some childhood ailments are outgrown. As young people with chronic problems such as diabetes develop their identity, their sense of who they are and what they want in life leads some to incorporate good health habits that sustain them lifelong (Schwartz et al., 2013).

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EXTRA CAPACITY, EXTRA BURDEN To appreciate emerging adult health, it helps to understand three aspects of body functioning: organ reserve, homeostasis, and allostatic load.

Organ reserve refers to the extra power that each organ can employ when needed. Organ reserve shrinks each year of adulthood, so by old age, a strain—-shoveling snow, catching the flu, minor surgery—can overwhelm the body.

In the beginning of adulthood, however, organ reserve allows speedy recovery from physical demands. Emerging adults sometimes exercise too long, stay awake all night, or drink too much alcohol. Usually they recover quickly, unlike older adults who might be affected for days.

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 quickest in early adulthood, partly because all the organs have power in reserve for sudden demands.

The next time you read about a rash of heat-wave deaths (Australia in 2014 and India and Pakistan in 2015), note the age of the victims. Because homeostasis takes longer, the body dissipates heat less efficiently with age. Sometimes the demands temporarily overwhelm the heart, kidneys, or other organs. Even middle-aged adults are less protected from temperature changes—or any other stress on the body—than emerging adults (Larose et al., 2013).

Related to homeostasis is allostasis, a dynamic body adjustment that gradually changes overall physiology. The main difference between homeostasis and allostasis is time: Homeostasis requires an immediate response from body systems, whereas allostasis refers to long-term adjustment.

Allostasis depends on the biological circumstances of every earlier time of life, beginning at conception. The process continues, with early adulthood conditions affecting later life, as evident in a measure called allostatic load. Although organ reserve usually protects emerging adults, the effects accumulate because some of that reserve is spent to maintain health, gradually adding to the burden on overall health.

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Because of the protective effects of homeostasis and allostasis, few emerging adults reach the threshold that results in serious illness. However, already by early adulthood, childhood health affects metabolism, weight, lung capacity, and cholesterol. As allostatic load builds, the risk of chronic disease later in life increases.

EXAMPLES OF LOAD AND BALANCING Consider sleep. One night’s poor sleep makes a person tired the next day—that is homeostasis, the body’s way to maintain equilibrium. But if poor sleep quality is typical every day in youth, then appetite, mood, and activity adjust (more, down, less) to achieve homeostasis, while allostatic load rises. By mid- and late adulthood, years of inadequate sleep load down overall health (McEwen & Karatsoreos, 2015; Carroll et al., 2015).

Another obvious example is nutrition. If severe malnutrition characterizes fetal or infant development, later on the person might eat too much, becoming obese. Childhood obesity increases the risk of adult obesity—not always, as Chapter 1 explains, but often.

Of course, appetite is affected by much more than fetal development. How much a person eats on a given day is affected by many factors, as the peripheral nervous system sends messages to the brain. An empty stomach triggers hormones, stomach pains, low blood sugar, and so on, all signaling that it is time to eat. If that is occasional, the cascade of homeostatic reactions makes you suddenly realize at 6 P.M. that you haven’t eaten since breakfast. Dinner becomes a priority; your body tells you that food is needed.

But if a person begins a serious diet, ignoring hunger messages for days and weeks, rapid weight loss soon triggers new homeostatic reactions, allowing the person to function with reduced calorie intake. That makes it harder to lose more weight (Tremblay & Chaput, 2012).

Over the years, allostasis is evident. If a person overeats or starves day after day, the body adjusts: Appetite increases or decreases accordingly. But that ongoing homeostasis increases allostatic load. Obesity is one cause of diabetes, heart disease, high blood pressure, and so on—all the result of physiological adjustment (allostasis) to daily overeating (Sterling, 2012). At the opposite extreme, allostasis allows people with anorexia to feel energetic, not hungry, but the burden on their body eventually kills them.

A third example comes from exercise. After a few minutes of exercise, the heart beats faster and breathing becomes heavier—these are homeostatic responses. Because of organ reserve, such temporary stresses on the body in early adulthood are no problem. Over time, homeostasis adjusts and allows the person to exercise longer and harder. That decreases allostatic load by reducing the health risks evident in one’s blood and weight.

The opposite is also true, as found in an impressive longitudinal study, CARDIA (Coronary Artery Risk Development in Adulthood), which began with thousands of healthy 18- to 30-year-olds, many (3,154) reexamined 7 and 20 years later. Those who were the least fit at the first assessment (more than 400 of them) were four times more likely to have diabetes and high blood pressure in middle age.

In CARDIA, problems for the least fit began but were unnoticed (except in blood work) when participants were in their 20s. Organ reserve allowed them to function quite well. Nonetheless, each year their allostatic load increased, unless their daily habits changed (Camhi et al., 2013). By age 65, a disproportionate number had died.

These ideas help us understand the long-term effects of poverty, neglect, and abuse in childhood. Those problems affect all the functions of the body, impairing health in middle age even if the childhood problems stopped decades ago (Widom et al., 2015).

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FERTILITY, THEN AND NOW As already mentioned, the sexual-reproductive system is at its strongest during emerging adulthood: Orgasms are frequent, the sex drive is powerful, erotic responses are joyful, fertility is optimal, miscarriage less common, serious birth complications unusual. Historically, most people married before age 20, had their first child within two years, and often a second and third before age 25.

That has changed dramatically. The bodies of emerging adults still crave sex, but their minds know they are not ready for parenthood. Society deems teenage pregnancy a problem, not a blessing. For emerging adults, modern contraception has provided a solution: sex without pregnancy. The world’s 2010 birth rate for 15- to 25-year-olds is half of what it was in 1960 (United Nations, 2013).

Modern medicine has prevented most young-adult deaths and many teenage pregnancies. The typical emerging adult is healthier than was true in former times. However, problems are prevalent that were rare earlier.

SEX, NOT MARRIAGE Premarital sex was once forbidden, a taboo enforced with diligent chaperoning, single-sex schools, and even the threat of death. The reason made sense: Premarital sex meant premarital pregnancy, and that was bad for society because children thrive best if both parents are committed to each other as well as to the children. In many cultures, if an unmarried woman became pregnant, the father of her child was obligated to marry her (sometimes called a shotgun wedding because her father threatened to kill the man).

However, effective contraception now allows gratification of sexual impulses without the risk of parenthood. The shotgun wedding “is rapidly becoming a relic” of another era (Jayson, 2014), evidence of a cohort change. When unmarried women today become pregnant, if they continue the pregnancy, less than 10 percent of those in the United States marry before the birth.

Society has adjusted to this new reality in many ways. One example is the demise of the single-sex college: A century ago, most private colleges were all male or all female (Miller-Bernal, 2000). In the United States as of 2015, except for religious colleges, only 3 all-male and about 30 all-female colleges continue to exist. The thousands of new colleges opening in other nations are virtually all co-ed.

Some feminists contend that, academically, women’s colleges are better, but in terms of protecting women from pregnancy, contraception has replaced segregation. Worldwide, more than 99 percent of college students enroll in co-educational institutions.

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NEW DISEASES Fatal diseases are rare during emerging adulthood, and premarital sex without pregnancy seems a good solution to the conflict between physical and psychological maturation. However, that solution creates a new problem.

The rate of sexually transmitted infections (STIs) is rising. Organ reserve and medical treatment almost always prevent death in emerging adulthood, but STIs cause infertility and later adult deaths that seem unrelated, such as from cancer and tuberculosis.

Transportation advances have unanticipated consequences. During the same years that contraception improved, travel became far easier and cheaper. In earlier times, prostitution was local, which kept STIs local as well. Now, with globalization, an STI caught in one place quickly spreads. Sex trafficking adds to the problem: Women and girls from one nation who are sent to another sometimes bring infections and sometimes catch diseases they never would have contracted at home.

This proliferation is particularly tragic with HIV/AIDS, which may have existed occasionally and locally for a hundred years. However, within 30 years, primarily because of travel and the sexual activities of young adults, HIV has become a worldwide epidemic, with more female than male victims (Fan et al., 2014).

Young adults are the prime STI vectors (those who spread disease) as well as the most common new victims. Genetic research finds that the subtypes and recombinations of HIV—once quite localized—are now present in many regions (Tatem et al., 2012).

Indeed, tracing HIV shows that it originally was confined to one city, Kinshasa, in the Democratic Republic of the Congo. Then it spread following the route of the railroads built as part of colonial modernization: Carriers took the disease to other nations of Africa, and later to other continents, where it had not been (Faria et al., 2014).

Now jets and bullet trains allow almost every young adult to wander far from home. Travel expands the mind and develops cultural understanding, but it also spreads diseases.

Remember that each age group has its own gains and losses, characteristics that can be an asset or a liability. This is apparent with risk taking. Some emerging adults bravely, or foolishly, take risks—a behavior that is gender- and age-related, as well as genetic and hormonal. Those who are genetically impulsive and male and emerging adults are most likely to be brave and foolish.

In one study, 10- to 30-year-olds judged “how good or bad an idea is it to . . .” do various risky items (from riding a bicycle down the stairs to taking pills at a party) (Shulman & Cauffman, 2014). The participants had only two seconds to make a snap judgment on a sliding scale from 0 to 100. For instance, the bicycle riding could be rated at 70 (somewhat bad idea) and the pills at 99 (very bad idea). There also were eight items that were not risky at all, such as eating a sandwich.

Risky items were rated more favorably (closer to a good idea) every year from age 10 to 20 and then less favorably (closer to a bad idea) every year from age 20 to 30. Experience increased (the average 15- to 17-year-old had done four of the items; the average 20- to 25-year-old had done seven), but that did not affect how good or bad each item was thought to be. Maturation, not experience, affected the assessment of risk.

The emerging adult’s willingness to take chances sometimes is beneficial. Enrolling in college, moving to a new state or nation, getting married, having a baby—all risky. So is starting a business, filming a documentary, entering an athletic contest, enlisting in the military, or joining the Peace Corps. Emerging adults do all these more than older adults, and societies benefit.

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DANGEROUS RISKS However, risk taking is often destructive. Although their bodies are strong and their reactions quick, emerging adults nonetheless have more serious accidents than do people of any other age (see Figure 11.1). The low rate of disease between ages 18 and 25 is counterbalanced by a high rate of violent death.

Risks that are more common in emerging adulthood than any other time include unprotected sex with a new partner, driving without a seat belt, carrying a loaded gun, abusing drugs, and addictive gambling—all done partly for a rush of adrenaline (Cosgrave, 2010). In the United States, the peak age for serious crime is 19, for unintended pregnancy, 18–19, for automobile driver death, 21 (Shulman & Cauffman, 2014).

Fatal accidents, homicide, and suicide result in more deaths in emerging adulthood than all other causes combined, even in nations with high rates of infectious diseases and malnutrition. The contrast between sudden, violent deaths and slower, disease-related deaths is most stark in nations with good medical systems.

In the United States, of the 15- to 24-year-olds who died in 2013, about 71 percent came to a sudden, violent end. The leading cause of disease-related deaths was cancer, at about 6 percent. Homicide alone caused 4,481 deaths of 10- to 24-year olds—with rates highest in the early 20s—more than double the rate of cancer mortality (MMWR, February 27, 2015).

RISKY SPORTS Many young adults enjoy challenging recreation: They climb mountains, swim in oceans, run in pain, play past exhaustion, and so on. This has always been true. Marriage and parenthood slow down risk taking, so when adult responsibilities began earlier, risks were less common. But the impulse to seek danger has always been apparent in young men (think of the soldiers of the previous centuries).

Now recreational risks are more popular, including new ones advertised for hundreds of millions of emerging adults. Among such activities are skydiving, bungee jumping, pond swooping, parkour, potholing (in caves), waterfall kayaking, and ziplining. Serious injury is not the goal, but the possibility adds to the thrill.

Competitive extreme sports (such as freestyle motocross—motorcycle jumping off a ramp into “big air,” doing tricks during the fall, and hoping to land upright) are new sports for some emerging adults, who find golf, bowling, and so on too tame (Breivik, 2010).

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DRUG ABUSE The same impulses that are admired by the young in extreme sports also lead to actions that are clearly destructive, not only for individuals but for the community. The most studied of these are sexual risks and drug abuse.

Both promiscuity and drug use increase in emerging adulthood, which suggests that one might lead to the other. Many colleges now restrict alcohol on campus, believing this will prevent or reduce sexual assault, which tends to occur only when both parties have been drinking or drugging (Zinzow & Thompson, 2015). However, although they often co-occur, instead of alcohol leading to assault, the origin may be the impulsive and foolish risk taking that are common personality traits in these years. In Chapter 1’s terms: They correlate because of a third variable.

By definition, drug abuse occurs whenever a drug (legal or illegal, prescribed or not) is used in a harmful way, damaging a person’s physical, cognitive, or psychosocial well-being. The interaction between age and drug abuse helps illustrate the nature of the emerging adult, who may be attracted rather than repulsed by the potential for harm and arrest in buying, carrying, and using an illegal drug.

Illegal drug use peaks at about age 20 and declines sharply with age (see Figure 11.2). Addiction to legal drugs (no arrest imminent) may continue in adulthood, but after age 25, most who drink and smoke to excess want to quit.

Quitting an addictive drug is difficult at any age, but as the years go on, adults no longer enjoy the taste, the thrill, or the sensation as they once did. They use drugs to relieve anxiety, or out of habit, not primarily for pleasure. For that reason, abstinence becomes more common, a source of pride not embarrassment, after emerging adulthood (Heyman, 2013).

Drug abuse is more frequent among college students than among emerging adults who are not in college, partly because groups of emerging adults urge each other on. For instance, some hazing rituals for college fraternities involve excessive consumption of alcohol. Such forced drinking is a symptom of a general problem: One study found that 25 percent of young college men consumed 10 or more drinks in a row at least once in the previous two weeks (Johnston et al., 2009).

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In the United States, the problem is not confined to men. A nationwide study found that 24 percent of women aged 18–25 had binged on alcohol in the past month (for women a binge is defined as four or more drinks on one occasion). That emerging adult rate was higher than the rate for either younger or older women (MMWR, January 11, 2013). Moreover, those 24 percent averaged four binge episodes per month and six drinks per occasion, both more than older female bingers. Rates rose with income.

Many colleges try to reduce alcohol consumption and enforce age restrictions—now 21 in the United States—without success. As one review states, “more students now drink to get drunk, choose hard liquor over beer, and front-load, or drink in advance of social events. For many, the goal is to black out” (McMurtrie, 2014, p. A23). The impulse to abuse alcohol arises from the same drive as extreme sports or other risks—with the same possible consequence: death.