It had been thought that being unhappy was bad for health – particularly for the heart.
But the decade-long analysis, published in the UK medical journal the Lancet, said previous studies had just confused cause and effect.
A series of previous studies had shown that how happy people are, strongly predicts how long they are going to live.
Ideas included detrimental changes in stress hormones or the immune system resulting in a higher risk of death.
But the research team in the UK and Australia said those studies failed to deal with reverse causality – namely, that people who are ill are not very happy.
Participants in the Million Women Study were asked to regularly rate their health, happiness and levels of stress.
The results showed that whether people were “never”, “usually” or “mostly” happy had no impact on their odds of dying during the duration of the study once other factors such as health or whether they smoked were taken into account.
Dr Bette Liu, one of the researchers at the University of New South Wales in 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 10-year study of a million women.”
Source: The Lancet
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