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In the middle of life and having a crisis? You've got mates ? primates.

Researchout Monday says that apes also experience emotional discontent atlife's midpoint and that it's probably hard-wired into both species bybiology. The "why" needs more study.

Previous studies have foundthat humans' emotional health generally follows a U-shape between 20 and70, with the mid- to late-40s marking the bottom of the big valley.

Tosee if great apes followed a similar trajectory, Andrew Oswald, aprofessor of economics at the University of Warwick in England, and hiscohorts examined data on the well-being of 508 chimpanzees and orangutansfrom zoos and research centers in the United States, Australia, Canada,Singapore and Japan. Caretakers and scientists were asked about anape's mood, the enjoyment the primate gained from socializing, successat achieving goals -- and how the humans would feel about being the apefor a week.

"In all three groups we find evidence that well-beingis lowest in chimpanzees and orangutans at an age that roughlycorresponds to midlife in humans," co-author Alex Weiss, a psychologistat Edinburgh University, told the Guardian."On average, well-being scores are lowest when animals are around 30years old." (An ape's lifespan is about half of a human's.)

"Wehoped to understand a famous scientific puzzle: why does human happinessfollow an approximate U-shape through life? We ended up showing that itcannot be because of mortgages, marital breakup, mobile phones, or anyof the other paraphernalia of modern life. Apes also have a pronouncedmidlife low, and they have none of those," Oswald said.

Althoughthey showed signs of depression, "unlike men, great apes are not knownto pursue radical and often disastrous lifestyle changes in middle age,"the Guardian says. And as the Associated Press noted, they don't buy sports cars or dump their mates" for some cute young bonobos."

Naturally, the findings have attracted skeptics.

"Thejudgment of happiness is hard enough in humans, and here we have humansjudging the happiness of apes," Frans de Waal, a primatologist at theUniversity of Emory who was not involved in the study, told LiveScience. "The study thus scores the well-being of animals through a human filter, perhaps introducing human bias."

The findings were published in the Proceedings of the National Academy of Science.

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