LONDON - Middle age is truly miserable, according to a study using data from 80 countries showing that depression is most common among men and women in their forties.

The British and U.S. researchers found that happiness for people ranging from Albania to Zimbabwe follows a U-shaped curve where life begins cheerful before turning tough during middle age and then returning to the joys of youth in the golden years.

Previous studies have shown that psychologicalwell-being remained flat throughout life but the new findings to be published in the journal Social Science & Medicine suggest we are in for a topsy-turvy emotional ride.

"In a remarkably regular way throughout the world people slide down a U-shaped level of happiness and mental health throughout their lives," Andrew Oswald at Britain's Warwick University, who co-led the study, said on Tuesday.

The researchers analyzed data on depression, anxiety levels and general mental health and well-being taken from some 2 million people in 80 countries.

U-SHAPED PATTERN

For men and women the probability of depression slowly builds and then peaks when people are in their forties -- a similar pattern found in 72 countries ranging from Albania to Zimbabwe, the researchers said.

About eight nations -- mostly in the developing world -- did not follow the U-shaped pattern for happiness levels, Oswald and his colleague David Blanchflower of Dartmouth College in the United States wrote.

"It happens to men and women, to single and married people, to rich and poor, and to those with and without children," Oswald said. "Nobody knows why we see this consistency."

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."