A study of a million UK women has shown that happiness has no direct effect on mortality nor does unhappiness and stress directly cause ill health.

Studies that claim it had simply confused cause and effect.  

Life-threatening poor health can cause unhappiness, and so unhappiness is associated with increased mortality but that is more confirmation bias than neutral analysis. Smokers also tend to be unhappier than non-smokers but accounting for previous ill health, smoking, and other lifestyle and socio-economic factors, the investigators found that unhappiness itself was no longer associated with increased mortality.

The lead author, Dr Bette Liu, now at the University of New South Wales, Australia said, "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."

The investigation was conducted within the UK Million Women Study. Three years after joining the study, women were sent a questionnaire asking them to self-rate their health, happiness, stress, feelings of control, and whether they felt relaxed. Five out of six of the women said they were generally happy, but one in six said they were generally unhappy.

As in other studies, 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.

The main analyses 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. The study is so large that it rules out unhappiness being a direct cause of any material increase in overall mortality, in women.

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.

Co-author Professor Sir Richard Peto, of the University of Oxford, Oxford, UK said: "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."

Previous reports of reduced mortality being associated with happiness, with being in control, with being relaxed, or with related measures of wellbeing had not allowed properly for the strong effect of ill health on unhappiness and on stress.

In a linked Comment, Dr Philipe de Souto Barreto and Professor Yves Rolland, Institute of Ageing, University Hospital of Toulouse, France say that the study provides extremely valuable and robust information about happiness, health, and mortality. Dr de Souto Barreto and Professor Rolland also call for randomised trials to investigate the issue further: "Such studies should be powered to allow comparisons to be made across age ranges and between men and women. Cross-cultural studies could also shed light on the generalisability of interventions to promote happiness."