Biologically, the years from ages 18 to 25 are prime time for hard physical work and safe reproduction. However, the reality that young adults can carry rocks, plow fields, or haul water is no longer admired, nor is their fertility. If a contemporary young couple had a baby every year, their neighbors would be more appalled than approving. Now, societies, families, and young adults themselves expect more education, later marriage, and fewer children than was the norm a few decades ago.

Health has not changed, except maybe to improve. As always, every body -system—including the digestive, respiratory, circulatory, and sexual-reproductive systems—functions optimally at the end of adolescence. Serious diseases are not yet apparent, and some childhood ailments are outgrown.

To understand this, it is helpful to understand three aspects of body functioning that keep emerging adults healthy: organ reserve, homeostasis, and allostatic load.

Organ reserve refers to the extra power that every organ can call upon when needed. That reserve power decreases each year, but in emerging adulthood, it usually allows speedy recovery from excess demands on the body, such as exercising for hours, staying awake all night, or drinking too much alcohol.

Closely related to organ reserve is homeostasis—a balance between various body functions 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.

The next time you read about heat-wave deaths (as in England in 2013 or Australia in 2014), note the age of the victims. Because homeostasis slows down, the body dissipates heat less well with age. 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.

The effects of early life are apparent. Organ reserve usually protects emerging adults, because some reserve is used to maintain health. In a measure called allostatic load, daily, weekly, monthly, and yearly adjustments accumulate, gradually affecting the burden, or load, that weighs down health. Few emerging adults reach the threshold that results in serious illness, but health habits early in life are already evident by age 25 in metabolism, weight, lung capacity, cholesterol or so on. Even the nutrition of the mother when she was pregnant affects the fetus, the child, and then the young adult.

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For example, how much a person eats is affected by many factors. An empty stomach immediately triggers hormones, stomach pains, low blood sugar, and so on, all of which lead a person to eat again. That is homeostasis. 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 a reduced calorie intake, which makes it harder to lose more weight (Tremblay & Chaput, 2012).

Over the years, allostasis becomes crucial. If a person overeats or starves day after day, the body adjusts: Appetite increases or decreases accordingly. But continued homeostasis may take a toll on health, increasing the allostatic load. Obesity is one cause of diabetes, heart disease, high blood pressure, and so on—all the result of physiological adjustment (allostasis) to day-after-day overeating (Sterling, 2012).

Another 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, the 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 the blood and weight.

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

In CARDIA, problems began but were unnoticed (except in blood work) when participants were in their 20s. Organ reserve allowed them to function. Nonetheless, each year their allostatic load increased, unless their daily habits changed (Camhi et al., 2013). By late adulthood, the load is deadly (see Table EP.1).

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 is less common, serious birth complications are rare. Historically, most people married by 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. For many, the solution is sex without pregnancy, made possible by modern contraception. The world’s 2010 birth rate for 15- to 25-year-olds is half of what it was in 1960 (United Nations, 2013).

Remember that each age group has its own gains and losses, characteristics that can be a blessing or a burden. 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.

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Societies as well as individuals benefit because emerging adults take chances. Enrolling in college, moving to a new state or nation, getting married, having a baby—all are risky. So is starting a business, filming a documentary, entering an athletic contest, enlisting in the military, or joining the Peace Corps.

Yet 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. The low rate of disease between ages 18 and 25 is counterbalanced by a high rate of violent death (see Table EP.1).

Destructive risks that are common in emerging adulthood include unprotected sex with a new partner, driving fast without a seat belt, carrying a loaded gun, abusing drugs, and addictive gambling—all done partly for a rush of adrenaline (Cosgrave, 2010). Accidents, homicides, and suicides are the leading causes of death among young adults—killing more adults under age 30 than all diseases combined, even in nations where infectious diseases and malnutrition are rampant.

Premarital sex, while avoiding parenthood, seems a good solution to the conflict between physical and psychological maturation, but it has serious consequences. Sexually transmitted infections (STIs) are increasing, which may cause lifetime infertility and disease, including some that seem unrelated, such as cancer and tuberculosis.

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 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. Within 30 years, primarily because of the sexual activities of young adults, HIV has become a worldwide epidemic, with more female than male victims (Fan et al., 2014).

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

Indeed, tracing HIV in Africa shows that it originally was only in the capital of what now is the Democratic Republic of the Congo. Then it spread following the route of the railroads built as part of modernization: carriers of HIV took the disease to nations where it had not been (Faria et al., 2014). Travel benefits emerging adults in many ways, expanding the mind and fostering cultural understanding, but newcomers introduce diseases, and catch diseases, that were once controlled by local conditions.

Many young adults enjoy risks in recreation: They climb mountains, swim in oceans, run in pain, play past exhaustion, and so on. Skydiving, bungee jumping, pond swooping, parkour, potholing (in caves), waterfall kayaking, ziplining, and many more activities have been invented to increase excitement. Serious injury is not the goal, but risk adds to the thrill.

Competitive extreme sports (such as 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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The same impulse that is admired in extreme sports leads to behaviors that are destructive, not only for individuals but for the community. The most studied of these is drug abuse (Reith, 2005).

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. From an emerging-adult perspective, the potential for abuse and arrest is part of the allure in buying, carrying, and using an illegal drug. Illegal drug use peaks at about age 20 and declines sharply with age (see Figure EP.1). The thrill is gone. Addiction to legal drugs (not immediately dangerous) may continue in adulthood, but after age 25, most adults want to quit cigarettes and avoid alcohol abuse. That is difficult for many, but unlike emerging adults, they no longer enjoy it.

Drug abuse is more common among college students than among their peers who are not in college, partly because groups of emerging adults urge each other on. For instance, college students are most likely to engage in extreme drinking, with 25 percent of young college men reporting that they consumed 10 or more drinks in a row at least once in the previous two weeks (Johnston et al., 2009). Such extreme behavior arises from the same drive as extreme sports or other risks—with the same possible consequence: death.

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