Midlife crises strike millions of people each year, throwing them into depression, making them question their success, and prompting them to buy ridiculously expensive cars. Sociologists have proposed many possible causes of the ubiquitous midlife crisis, ranging from feelings of regret to financial hardship and unmet expectations. However, new evidence from a somewhat unlikely place—zoos and primate sanctuaries—suggests that midlife crises may have more to do with our evolutionary history than previously thought.

Since we share so much with our great ape relatives, researchers wondered if they, too, tended to have decreased well-being in the middle of their lives. The researchers recruited more than 500 captive great apes, including chimpanzees and orangutans, from North America, Australia, and Asia, to take part in the study. Caretakers, primate researchers, and volunteers that were familiar with the apes were asked several questions, such as whether the individual was in a positive or negative mood and how much pleasure they currently derive from various situations. These responses were averaged and used to determine the general happiness and well-being of each ape.

Just as in humans, well-being in the apes was U-shaped, declining steadily until reaching a low point, then increasing until death. On average, apes were rated as least happy at 31.9 years old, which is approximately middle age for an ape. Like humans, chimpanzees and orangutans appear to experience a type of psychological “crisis” in the middle of their lives, in which their well-being is at its lowest point.

The similarity suggests that there may be an evolutionary aspect to the midlife crisis. The researchers suggest (but didn't test) a few possible mechanisms for low well-being during middle age in both apes and humans. First, brain changes across our development may cause the U-shaped well-being graph in humans and their closest ape relatives. Alternatively, our ability to deal with our feelings—and possibly our regrets—may change as we age, with both our emotions and our coping mechanisms reaching low points in the middle of our lives. Finally, it’s also possible that happiness and longevity are related, and that unhappy primates may die off at higher rates than others, especially later in life.

The authors of the study recognize that social and cultural causes likely play a role in midlife crises in humans, and rightly state that these factors should continue to be investigated. But the results of the study, published in PNAS, certainly suggest that something in our shared history with great apes predisposes us to a low point in middle age.

Questionnaires such as the ones used here have been met with some criticism, but in recent years, research has shown that subjective assessments of animals can be reliable and representative if conducted under specific conditions. In this case, the questionnaires were filled out by multiple people who knew the animals well and interacted with them regularly for at least two years. Inter-rater reliability was high, indicating that different observers tended to respond similarly when talking about individual great apes.

While there are shortcomings and limitations to the study, it appears that cross-species research may play a role in helping us understand how and why happiness and well-being change as we age. Further research on great apes and other animals may help us understand why our 40s and 50s are difficult for so many people, and may hint at new ways to cope with this tumultuous time in our lives.

PNAS, 2012. DOI: 10.1073/pnas.1212592109 (About DOIs).