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.