Andrew Oswald of the University of Warwick in England and David Blanchflower of Dartmouth College in Hanover, N.H., found a consistent international pattern that happiness levels followed a U-shaped curve, with happiness higher toward the start and end of life and leaving most miserable in middle age.

Using a sample of 1 million people from Britain, the researchers discovered that for both men and women the probability of depression peaks around 44 years of age. However, in the United States women were most miserable at age 40 and age 50 for men. Only in their 50s do most people emerge from this low period, the study said.

However, by the time people are age 70 and if physically active, on average, people are as happy and mentally healthy as those in their 20s, the researchers found.

The study, scheduled for publication in Social Science & Medicine, found the happiness, depression U-shaped curve in 72 countries from Albania to Zimbabwe among men and women, single, married, rich, poor and with and without children.