Ill health can cause unhappiness, but unhappiness does not cause ill health, a new study claims.
The study - led by Dr. Bette Liu of the University of South Wales in Australia - is published in The Lancet.
Dr. Liu and colleagues reached their findings by conducting an analysis of 719,671 women who were part of the UK's Million Women Study.
A median age of 59 years, the women were recruited to the study between 1996-2001. Three years after enrollment, they were asked to complete a questionnaire detailing their health and feelings of stress, happiness, control and relaxation.
Around 39% of the women reported being happy most of the time, 44% said they were usually happy, while 17% said they were unhappy.
'No robust evidence' remains that unhappiness, stress increase mortality
The researchers found that women who were already in poor health at study baseline were most likely to report being unhappy, stressed, not in control and not relaxed.
Reflecting results of previous studies, women who were unhappy were more likely to smoke, be of low socioeconomic status, not live with a partner and have low physical activity.
Controlling for pre-existing differences in health and lifestyle, the team found that rates of all-cause mortality, heart disease mortality and cancer mortality over the 10-year follow-up were the same between both happy and unhappy women.
"After adjustment for these factors, no robust evidence remains that unhappiness or stress increase mortality or that being happy, relaxed, or in control reduces mortality," write the authors.
They say that previous studies associating unhappiness with increased mortality or happiness with reduced mortality have not accounted for how ill health impacts a person's happiness or feelings of stress.
Commenting on the findings, study coauthor Prof. Sir Richard Peto, of the UK's University of Oxford, says:
"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."
In an editorial linked to the study, Dr. Philipe de Souto Barreto and Prof. Yves Rolland, of the Institute of Ageing at the University Hospital of Toulouse in France, say the study offers "extremely valuable and robust information" about how happiness may influence health and mortality, and they say the association should be investigated further with randomized trials.
"Such studies should be powered to allow comparisons to be made across age ranges and between men and women," they suggest. "Cross-cultural studies could also shed light on the generalizability of interventions to promote happiness."
Last month, Medical News Today reported on a study in which researchers from Japan suggest a person's happiness may depend on the size of a specific brain region known as the precuneus.