We’re happy when we’re young, and again in our final years; the problem is that rather large bit in the middle.

Researchers in Britain and America studied the relative cheerfulness of more than two million people in 80 countries. They found that our sense of wellbeing follows a U-shaped route through life, peaking at the beginning and end, while sinking in between. In Britain the time of minimum happiness occurs at about age 40 for women and 50 for men.

Andrew Oswald, of the University of Warwick, and David Blanchflower, of Dartmouth College, conclude, in a paper to be published in Social Science and Medicine, that relative wellbeing is not caused by having children, by divorce or by changes in jobs or income. Professor Oswald said: “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. One possibility is that individuals learn to adapt to their strengths and weaknesses and in mid-life quell their infeasible aspirations. Another possibility is that cheerful people live systematically longer. A third is that a kind of comparison process is at work in which people have seen similar-aged peers die and value more their own remaining years. Perhaps people somehow learn to count their blessings.

“It looks, from the data, like something happens deep inside humans. For the average person in the modern world, the dip in mental health and happiness comes on slowly, not suddenly 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 still physically fit, then on average you are as happy and mentally healthy as a 20-year-old.”

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