A new study finds that chimpanzees and orangutans, too, often experience a mid-life crisis, suggesting the causes are inherent in primate biology and not specific to human society.

"We were just stunned" when data on the apes showed a U-shaped curve of happiness, said economist Andrew Oswald of the University of Warwick in England and a co-author of the paper, which was published on Monday in the Proceedings of the National Academy of Sciences of the USA.

The U-shaped curve of human happiness and other aspects of well-being are as thoroughly documented as the reasons for it are controversial. Since 2002 studies in some 50 countries have found that well-being is high in youth, plunges in mid-life and rises in old age. The euphoria of youth comes from unlimited hopes and good health, while the contentment and serenity of the elderly likely reflects "accumulated wisdom and the fact that when you've seen friends and family die, you value what you have," said Oswald.

The reasons for the plunge in well-being in middle age, when suicides and use of anti-depressants both peak, are murkier. In recent years researchers have emphasized sociological and economic factors, from the accountant's recognition that she will never realize her dream of starring on Broadway to the middle manager's fear of being downsized, not to mention failing marriages and financial woes.

In what Oswald, 58, calls "a burst of madness," since no such study had ever been attempted, he and his colleagues decided to see whether creatures that don't have career regrets or underwater mortgages might nevertheless suffer a well-being plunge in middle age.

They enlisted colleagues to assess the well-being of 155 chimps in Japanese zoos, 181 in U.S. and Australian zoos and 172 orangs in zoos in the United States, Canada, Australia and Singapore. Keepers, volunteers, researchers and caretakers who knew the apes well used a four-item questionnaire to assess the level of contentment in the animals, said psychologist Alex Weiss of Scotland's University of Edinburgh. One question, for instance, asked how much pleasure the animals - which ranged from infants to graybeards - get from social interactions.

All three groups of apes experienced mid-life malaise: a U-shaped contentment curve with the nadir at ages 28, 27 and 35, respectively, comparable to human ages of 45 to 50.

Why would chimps and orangs have a mid-life crisis? It could be that their societies are similar enough to the human variety that social, and not only biological, factors are at work, Oswald said. Perhaps apes feel existential despair, too, when they realize they'll never be the alpha male or female.

An evolutionary explanation is even more intriguing. "Maybe nature doesn't want us to be contented in middle age, doesn't want us sitting around contentedly with our feet up in a tree," he said. "Maybe discontent lights a fire under people, causing them to achieve more" for themselves and their family.

"By knowing our results, people might be gentler on themselves" when they experience a mid-life crisis, Oswald said. "Knowing that it's biological, they'll realize that if they can just hang on they'll likely come out the other side."

(Editing by James Dalgleish)

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