How do you spot the signs of a mid-life crisis in a chimpanzee or an orangutan? While you won't see an ape going out and buying a flashy new car, a new study has found that middle-aged primates are just as susceptible to midlife doldrums as humans.
There is accumulating scientific evidence that human happiness tends to follow a U-shape throughout life, with the lowest point fixed in middle age. This holds true in both developing nations and First World countries, and is virtually unaffected by other demographic characteristics. The midlife crisis period is associated with an increased risk for suicide and antidepressant consumption.
While there may be social, cultural and psychological forces unique to humans that produce this U-shape, other scientists have been interested in exploring possible evolutionary factors underlying this pattern.
An international team of researchers led by University of Edinburgh primate scientist Alexander Weiss asked zookeepers, primate handlers, and volunteers to assess the well-being of the apes in their care. 336 chimpanzees and 172 orangutans were assessed using a questionnaire based on similar forms used to measure human well-being.
In a paper published on Monday in the Proceedings of the National Academy of Sciences, Weiss and his colleagues said that a similar U-shaped well-being curved emerged from the data gathered on apes, suggesting there may be some shared biological reason for the mid-life crisis.
“These results suggest that a persuasive explanation for the human U-shape should also account for the similarity of this trend in our evolutionary cousins, the great apes,” the authors wrote.
Until now, most theories of the midlife crisis have focused on the unique qualities of the human condition. Researchers have hypothesized that it may be caused by the readjustment of aspirations – the goals once pursued now seem increasingly impossible, and are gradually given up. Another theory suggests that the curve is linked to financial hardship, with elderly people with more resources less likely to feel the sting than middle-aged adults who are still earning their daily bread. (If this theory's true, then a lot of those mid-life crisis cars are being bought on credit.) Another possible explanation floated is that aging may help humans experience less regret.
Weiss and his colleagues offer their own possible explanations for mechanisms to explain the U-shape that overlap between humans and apes. Research shows that happiness is associated with longer life in humans and in at least one species of great apes, so it's possible that the apes that don't conform to the U-shape are just dying off sooner, leaving the contented apes to represent the elderly population.
Another possibility, the researchers say, is that there are age-related changes in the brains of humans and great apes that interact with well-being.
Elderly adults of all three species may also be more likely to seek out companions and groups that spark positive emotions, or shift to more attainable goals as they age.
Natural selection may also have favored individuals with higher senses of well-being in both infancy and old age.
“These individuals, being satisﬁed at stages of their life where they have fewer resources to improve their lot, would be less likely to encounter situations that could be harmful to them or their kin,” the authors wrote.
SOURCE: Weiss et al. “Evidence for a midlife crisis in great apes consistent with the U-shape in human well-being.” PNAS published online 19 November 2012.