When Mr Jonathan Rauch fell into the doldrums in his 40s, he had no idea why. He had a successful career, a solid relationship, good health and sound finances.

Then he learnt about the happiness curve and it all became clear.

Academics have found increasing evidence that happiness through adulthood is U-shaped - life satisfaction falls in our 20s and 30s, then hits a trough in our late 40s before increasing until our 80s.

Forget the saying that life begins at 40 - it's 50 we should be looking towards.

Mr Rauch, a senior fellow at United States think-tank Brookings Institution, was so relieved to have found an explanation for the gloom that hit him and, he believed, many others in middle age that he became evangelical about spreading the word.

He has written a book - The Happiness Curve: Why Life Gets Better After 50 - which includes personal stories, the latest data and illuminating interviews with economists, psychologists and neuroscientists.

"The most surprising thing is that age tends to work in favour of happiness, other things being equal," he said. "The strangest thing is that midlife slump is often about nothing."

Hold off on splashing out on that flashy sports car or embarking on an affair though. It is not the same as a midlife crisis, which according to the stereotype, demands an urgent, rash response.

The slump isn't caused by anything, according to Mr Rauch. It is a natural transition, simply due to the passing of time.

"It's a self-eating spiral of discontent," he said. "It's not because there's something wrong with your life, or your marriage, or your mind, or your mental health."

Not everyone will experience a sunnier outlook in their 50s and beyond, Mr Rauch noted, because factors such as divorce, unemployment or illness can counter this. But, other things being equal, the U-curve holds.

Mr Rauch, an author and journalist, added: "Those most likely to notice the arrow of time are the people without a lot of other change or difficulty in their life."

A 2008 study by economists David Blanchflower and Andrew Oswald found the U-curve - with the nadir, on average, at age 46 - in 55 of 80 countries, and they cited more than 20 other papers finding the U.

It tends to show up in wealthier countries where people live longer, healthier lives. Life satisfaction statistics for the United Kingdom in 2014-2015 show happiness declining from youth through middle age, hitting a low at 50 and rising to a peak at 70.

Not all economists and psychologists agree. Economists Paul Frijters and Tony Beatton factored in the possibility that those who become happier in the studies are the same people who are more content when they start out. This can help them achieve greater career or relationship success, which leads to more happiness. Correcting for this effect, the U-shape disappears.

In general, research shows that older people feel less stress and regret, dwell less on negative information and are better able to regulate their emotions. Nor is status competition as important.

Karla, 54, is on the upswing of the curve. She says she is savouring her friendships more, feeling more organised and efficient, and doing more volunteering work. "Now I feel grateful for the now. On a day-to-day basis I probably do the same things, but I feel different."

THE GUARDIAN