If you thought happiness contributed to longevity in women, think again. A new study for UK's Million Women Study project suggests that being content does not directly affect mortality.

"Happiness and related measures of wellbeing do not appear to have any direct effect on mortality," the researchers wrote in their paper, published online in the 9 December issue of medical journal The Lancet. "Previous reports of reduced mortality associated with happiness could be due to the increased mortality of people who are unhappy because of their poor health," they added.

The study, led by Dr Bette Liu at the University of New South Wales and researchers at the University of Oxford, was done for the UK's Million Women Study programme. The project involves more than 1m women aged 50 and over and is aimed at investigating how various reproductive and lifestyle factors affect women's health.

The present study examined over 700,000 women over a period of 10 years. They were asked how happy they felt. About 39% of the women who said they felt content most of the time and 44% of them who felt usually fine were categorised as "happy" by the researchers. The remaining 17% who said they never or rarely felt happy were rated "unhappy".

About 30,000 participants died during the study. According to researchers, the overall death rate among women who had rated themselves as unhappy was the same as that among those who rated themselves as happy.

Unhappiness doesn't cause illness

The researchers also asked the women to self-rate their health as good or poor. Women reported suffering from hypertension, diabetes, asthma, arthritis, depression, or anxiety, and several lifestyle factors, including smoking, deprivation, and body-mass index.

The study found that women in poor health felt unhappy and stressed but that there was no direct effect of unhappiness on death rates, as being unhappy doesn't really cause diseases.

"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 10-year study of a million women," lead author of the study, Bette Liu, said in a statement.

"Many still believe that stress or unhappiness can directly cause disease, but they are simply confusing cause and effect. 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," co-author professor Sir Richard Peto added.