The road to happiness is U-shaped.
New research this week has found that happiness over the course of a lifetime follows a universal curve in which the greatest bliss occurs at the beginning and end of life, while misery dominates middle age.
The pattern was consistent around the globe, according to the report, which examined social survey data on 2 million people in 80 countries, including the United States.
The study, conducted by economists Andrew Oswald of the University of Warwick in England and David Blanchflower of Dartmouth College in New Hampshire, set out to look at the relationship between age and happiness.
Researchers controlled for other factors that affect happiness, such as divorce, job loss and income.
The researchers, whose study will be published in the journal Social Science & Medicine, found that in the United States happiness reached its lowest point around age 40 in women and age 50 in men. In Britain, unhappiness was greatest in men and women at age 44.
Oswald was perplexed by the results. He said it was possible that in midlife people learn to accept their strengths and weaknesses and abandon unrealistic aspirations.
Another possibility, he said, is that cheerful people live longer, driving the curve higher. Another explanation is that older people learn to count their blessings as their peers die, Oswald said.
"It's a mystery," he said. "There seems to be something inside human beings that is unexplained by life events."
Richard A. Easterlin, a USC economist who also studies happiness, said that midlife misery is not inevitable. In fact, his research shows that when such factors as income and marital status are included in the calculations, midlife is the happiest stage of life. Finances and family life tend to improve as midlife approaches, he said, and after that things gradually get worse.
The latest study, which looks only at age, doesn't answer the question that concerns people most, Easterlin said. "People want to know their prospects, what is likely based on their life circumstances," he said.