It is the disease that makes you unhappy and not the other way round, say researchers, adding that the widespread but mistaken belief that unhappiness makes you ill came from studies that had simply confused people.
In a study comprising one million British women, researchers found that happiness itself has no direct effect on 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 10-year study of a million women," said lead author Bette Liu, now at University of New South Wales, Australia.
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 investigators found that unhappiness itself was no longer associated with increased mortality.
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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.
"Many still believe that stress or unhappiness can directly cause disease, but they are simply confusing cause and effect," added co-author professor Richard Peto from University of Oxford.
Previous reports of reduced mortality being associated with happiness had not allowed properly for the strong effect of ill health on unhappiness and on stress.
"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 noted in a paper published in the journal The Lancet.