Most theories for the midlife dip involve money, promotions, marriage and other social facets of modern life, [Andrew] Oswald [an economist and behavioral scientist] said. To see if there might be another explanation, he and colleagues compiled data that had been collected on more than 500 orangutans and chimpanzees living in about 60 zoos around the world.

For each animal, zookeepers, researchers or caretakers answered four questions about the well-being of their primate friends, including whether the apes seemed to be in good or bad moods. The humans also ranked how happy they thought they 'd be if they were to become the animal for a week. They had spent time with the animals for at least two years and knew them well.

Apes live to be about 50 or 55 years old and, just like in people, results showed a drop in happiness that reached its lowest point about halfway through the animals ' lives, the researchers report today in the journal Proceedings of the National Academy of Sciences.

The magnitude of the dip was on par with the dips in happiness that people experience in their middle age, Oswald said. He compared the difference between the apes ' highs and lows to the loss in well-being that people report with marital separation.

The new findings help rule out some theories for midlife slumps in humans, said Arthur Stone, a psychologist in the psychiatry department at Stony Brook University in New York. For example, a whole generation of people can end up feeling less happy at a certain time in their lives simply because of some external historical situation. But that is unlikely to happen in societies of apes.

Instead, it might be chemical or physical changes in our bodies that influence how our feelings morph throughout our lifetimes.