( AFP )- 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 or offspring.
The study appears in Social Science & Medicine, published by the Dutch publishing house Elsevier.
"Some people suffer more than others but in our data the average effect is large," said co-author Andrew Oswald.
"It happens to men and women, to single and married people, to rich and poor, and to those with and without children. Nobody knows why we see this consistency."
The dip in mental health comes on slowly, rather than a big slide in a single year.
"Only in their 50s do most people emerge from the low period. But encouragingly, by the time you are 70, if you are physically fit then on average you are as happy and mentally healthy as a 20-year-old," said Oswald.
"Perhaps realising that such feelings are completely normal in midlife might even help individuals survive this phase better."