It's been a long-held belief that happiness makes people live a longer, healthier life, but researchers from the U.K. have discovered that happiness does not really affect a person's health or life span. They also found that unhappiness and stress do not cause sickness nor deduct years from people's lives.

"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," lead researcher Dr. Bette Liu from the University of New South Wales in Australia said in a press release.

The researchers explained that people who are sick tend to be unhappy, which is why unhappiness has been linked to increased mortality. This does not mean, however, that the unhappiness is causing increased mortality.

To determine the association between happiness and mortality, the researchers investigated 700,000 women under The Million Women Study. The women's average age was 59. Three years after being recruited to join the study, the women were given questionnaires in which they were asked to rate themselves in five areas: happiness, health, feeling of being relaxed, stress and feeling of control.  

When the researchers analyzed the responses, they found that the participants associated unhappiness with smoking, not living with a partner, deprivation and lack of exercise. Those who suffered from poor health indicated that they were stressed, unhappy, not in control and not feeling relaxed.

The study participants were followed in the succeeding years. Within 10 years, 30,000 of them had died.

After taking into account factors like lifestyle and health, the researchers observed that there was no difference in the death rate of those who were happy and those who were unhappy. This pattern was observed for mortality in general and even for deaths caused by cancer or heart disease. Stress and unhappiness also did not influence mortality.

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

The study was published online Dec. 9 in the journal The Lancet.