Happiness is U-shaped, for we are happier at the start and end of our lives but hit a slump when we are middle-aged, British and US researchers say.

Economists from the University of Warwick, central England, and from Dartmouth College, New Hampshire, looked at data on the mental health of two million people from 80 countries.

In Britain, the probability of depression for men and women peaks at around 44 years of age, Warwick University said in a press release.

In the United States, though, there was a big difference between men and women.

Among women, unhappiness peaked at around the age of 40, whereas among men, it was about 50.

But the U-shape of happiness is constant around the world, and mid-life depression occurs regardless of marital status, changes in job or income.

The study appears in Social Science & Medicine, published by the Dutch publishing house Elsevier.

"It happens to men and women, to single and married people, to rich and poor, and to those with and without children." said co-author Andrew Oswald.

One possibility may be that people realize they won't achieve many of their aspirations at middle age, the researchers said.

Another reason could be that after seeing their fellow middle-aged peers begin to die, people begin to value their own remaining years and embrace life once more.

But the good news is that if people make it to aged 70 and are still physically fit, they are on average as happy and mentally healthy as a 20-year old.

"For the average persons in the modern world, the dip in mental health and happiness comes on slowly, not suddenly in a single year," Oswald said. "Only in their fifties do people emerge from this low period."