A forty something middle manager quits his day job, buys a sports car and abandons his wife for a younger woman. Diagnosis? Midlife crisis. Lester Burnham in American Beauty, Walter White in Breaking Bad—examples of these desperate characters abound in popular culture, and the concept is entrenched in our collective psyche. But are people of a certain age really more likely to launch a total life reboot?

It would seem so, according to scientists. Some hallmarks of midlife—increased self-reflection, aging, career and family changes—can seed deep dissatisfaction. And studies indicate that our sense of well-being naturally wanes during this stretch of adulthood. But research also shows that many common beliefs about the quintessential fortysomething meltdown are untrue. In fact, malaise can commence at almost any age, men are not more susceptible to dissatisfaction than women, and midlife crises are far from inevitable.

At some point between age 40 and 60, most people will face significant stress in one form or another, but not everyone reacts by chasing after fading youth or, worse, succumbs to depression or substance abuse. Genuine midlife breakdowns appear to be less common than many think, affecting only 10 percent of the population, according to a review in the early 1990s, or 26 percent, according to a 2000 study. For some of that group, these upheavals lead not to ruin but to a welcome second act. And the reasons certain people do fall victim may have more to do with personality and cultural expectations than age.

A life half-empty

If the midlife crisis seems to defy precise definition, consider that midlife itself is a nebulous concept. In 2011 researchers at Florida State University analyzed a questionnaire given to several thousand Americans in the 1990s and found that most participants defined midlife as running from age 44 to 59. Ten years later the same participants described midlife as age 46 to 62. The older the respondents—women in particular—the later they envisioned the debut of their dotage.

Our notion of midlife may be a moving target, but once it starts, our perspective on time shifts. Instead of counting the years from our birth, we begin guessing at how many years we have left. Psychologist Laura Carstensen, founding director of the Center on Longevity at Stanford University, has shown that this subjective sense of a life half-empty influences our goals. The fewer years we think we have, the less expansive our plans become. Instead some individuals focus on family and friends—just as young and old alike seek familiar comforts when epidemics or terrorist attacks remind us of how finite life is.

Other people, though, begin to take stock of their lives and revisit unrealized dreams. “We do not retrieve our youthful goals from 20 years before and check off, one by one, what we have achieved,” explains Alexandra Freund, a professor of applied psychology at the University of Zurich.* She has found that older people typically try to maintain what they have now to avoid future losses and that they focus less on their careers. During this transition, our sense that the demands of work exceed our ability to cope can increase. Occupational psychologist Amanda Griffiths and her colleagues at the University of Nottingham in England have reported that this perceived job stress peaks between the ages of 50 and 55.

Such changes in focus and aspiration may render us less satisfied with life. In 2008 labor economists David G. Blanchflower of Dartmouth College and Andrew J. Oswald of the University of Warwick in England reviewed data about well-being collected from half a million people of various ages in 72 countries. They concluded that the happiness level throughout an individual's life span frequently follows a U-shaped curve, bottoming out in the early to mid-40s. In some locations, this emotional nadir did not appear in the raw data but emerged once Blanchflower and Oswald reanalyzed the numbers to consider the potentially confounding contributions of marital status, income, education and other factors on contentment.

Complicating the picture, other researchers have noted the presence of peaks and plateaus of happiness within the midlife period. In 2012 economist John Haisken-DeNew of the University of Melbourne reviewed life satisfaction among Germans using data from the German Socio-Economic Panel, a 30-year-old longitudinal study that surveys some 30,000 people annually. The length of the study allowed Haisken-DeNew to account for individual differences in optimism. For instance, a happiness score of 6 out of 10 might represent an all-time high for a confirmed misanthrope but a crash for a starry-eyed romantic. Haisken-DeNew observed that happiness levels drop continuously, if slightly, throughout adulthood until theearly 60s, after which they increase until age 75 and then plummet.

Wrinkles in time

Still, the subject of much debate is whether the cause of midlife strife lies in our creaturely selves or in our stars. Some researchers argue that life events are most important. In a study published in 2000 sociologist Elaine Wethington of Cornell University conducted a telephone survey of 724 American adults between the ages of 28 and 78 and found that more than a quarter of her respondents—men and women almost equally—admitted to having had a midlife crisis. The majority attributed these spells to upsetting life events such as losing a job or parent and not to age, leading Wethington to conclude that midlife crises are not a natural part of aging.

Yet research into the biology of middle age suggests that at least part of our vulnerability is built in. In 2012 psychologist Alexander Weiss and his colleagues at the University of Edinburgh reviewed accounts from zookeepers, volunteers, researchers and caretakers, who reported that middle-aged chimpanzees and orangutans showed definite signs of disgruntlement, compared with younger and older apes. These observations were subjective; the animal keepers knew the ages of the animals and may have been interpreting their behavior based on expectations. Nevertheless, Weiss's team concluded that the biology we share with our fellow great apes could underpin our midlife doldrums.

Gray hair and wrinkles aside, many adjustments associated with aging can cause psychological distress. The rates of cancer and cardiovascular disease, among other illnesses, increase, along with the risk of depression. A 2012 report from the Central Research Institute of Ambulatory Health Care in Berlin revealed that depression cases climb almost linearly until one's late 50s and peak again at around 85 for women and 90 for men. In the U.S., statistics from the Centers for Disease Control and Prevention reveal that the highest rates of depression among men and women fall between the ages of 40 and 59.

Physically, too, the brain begins to degrade more quickly after age 40. In 2010 neuroscientist Antonio Giorgio of the University of Siena in Italy, then working with colleagues at the University of Oxford, tested 66 subjects between the ages of 23 and 81 using MRI and diffusion-tensor imaging. Their results were consistent with earlier studies: The brain's white matter volume increased continuously up to the early 40s and then decreased rather rapidly. The volume of gray matter declined steadily over the entire period in most brain regions.

Older brains can usually compensate for these deficits, offsetting lost firepower with greater experience and knowledge. But the effects of hormonal attrition are more unsettling. When male andropause sets in after about age 40, testosterone levels start to decline, and both sexual interest and performance can suffer. For women, the decline of estrogen levels in the 40s leads to menopause, usually in the 50s, and a host of sometimes upsetting symptoms, including insomnia, memory problems and depression.

To every season

But whatever its source, midlife stress does not foredoom us to a life out of control, especially in our relationships. A 2011 Kinsey Institute study of more than 1,000 couples in Germany, Spain, the U.S., Japan and Brazil found that middle-aged men and women rate their relationships and sex lives higher the longer they have been married and that people entering middle age with a long-term partner have a good chance of staying together, citing earlier estimates that more than half of marriages in the U.S. and 92 percent in Spain will last more than 20 years. 

The empty-nest syndrome appears to be a myth, too. Pasqualina Perrig-Chiello, a professor of developmental psychology at the University of Bern in Switzerland, found in a 2001 study of 260 middle-aged subjects that mothers frequently view their children's departure optimistically. Fathers more often have mixed feelings, perhaps wishing that they had spent more time with their children. In a 2009 study, sociologist Barbara A. Mitchell of Simon Fraser University in British Columbia asked more than 300 parents from different cultural backgrounds about their children's departure from home. Only a minority—younger parents with health problems and fewer children—reported emotional suffering. Overall, most parents reported positive feelings, such as pride at having been successful in raising their children so that they could move out.

Perhaps the biggest misconception of all is that the outlook at 40 is grim. The John D. and Catherine T. MacArthur Foundation Research Network on Successful Midlife Development, a Harvard University–based interdisciplinary project run by 13 scholars, surveyed more than 7,000 people in the U.S. between the ages of 25 and 74 on aspects of middle age. The results, which have spawned multiple books and scores of research papers, reveal midlife to be largely a period of calm and stability: most relationships hold together, most people stay healthy and many enjoy financial security.

And when Zurich's Freund asked older people what age they would most like to be again, the majority chose their mid-40s. In some cultures, such as Japan, India, Kenya and Samoa, the concept of the midlife crisis is entirely imported. Maybe knowing that our misgivings about midlife are usually exaggerated—and temporary—can make the passage to late maturity just a bit more manageable.

*Erratum (4/29/15): This sentence was edited after posting to correct a misquote that appeared in the print version.