London: Being unhappy or stressed does not increase the risk of ill-health and happy people are no more likely to live longer, a major new study has found.

The study found that happiness itself has no direct effect on mortality, and that the widespread but mistaken belief that unhappiness and stress directly cause ill health came from studies that had simply confused cause and effect.

Life-threatening poor health can cause unhappiness, and for this reason unhappiness is associated with increased mortality. In addition, smokers tend to be unhappier than non-smokers.

However, after taking account of previous ill health, smoking, and other lifestyle and socio-economic factors, the researchers at the UK million women study found that unhappiness itself was no longer associated with increased mortality.

"Illness makes you unhappy, but unhappiness itself doesn't make you ill. We found no direct effect of unhappiness or stress on mortality, even in a ten-year study of a million women," said Bette Liu, now at the University of New South Wales, Australia, who led the study.

The main analysis of the study included 700,000 women, average age 59 years, and over the next 10 years these women were followed by electronic record linkage for mortality, during which time 30,000 of the women died.

After allowing for any differences already present in health and lifestyle, the overall death rate among those who were unhappy was the same as the death rate among those who were generally happy.

This was true for overall mortality, for cancer mortality, and for heart disease mortality, and it was true for stress as well as for unhappiness.

Unhappiness was associated with deprivation, smoking, lack of exercise, and not living with a partner.

The strongest associations, however, were that the women who were already in poor health tended to say that they were unhappy, stressed, not in control, and not relaxed.

"Many still believe that stress or unhappiness can directly cause disease, but they are simply confusing cause and effect," said Sir Richard Peto, of the University of Oxford.

"Of course people who are ill tend to be unhappier than those who are well, but the UK Million Women Study shows that happiness and unhappiness do not themselves have any direct effect on death rates," he added.

The findings were published in The Lancet journal.