Happiness does not heal, but happiness protects against falling ill. As a result, happy people live longer. The size of the effect on longevity is comparable to that of smoking or not.

This is concluded from an analysis of 30 follow-up studies published in a recent issue of the Journal of Happiness Studies.

There have been more reports of happy people living longer, but for long it was unclear whether happiness causes longevity, since it can also be that good health add both to happiness and longevity.

Scientists assess causality using long-term follow-up studies, taking initial health into account. The results of such studies seemed contradictory; several studies found the expected causal effect of happiness on longevity, but other studies found no effect and some observed even earlier death among the happy. The analysis of 30 follow-up studies showed that the difference is in the people under investigation.

Happiness does not lengthen the life of seriously ill people, but it does prolong the life of healthy people. Happiness appears to protect against falling ill. One of the mechanisms behind that effect seems to be that chronic unhappiness causes stress, which on its turn reduces immune response.

Another possible mechanism is that happiness adds to the chance of adopting a healthy life style.

An implication of this finding is that public health can also be promoted by policies that aim at greater happiness for a greater number.