For the video version of this post, click here. We've been doing these 150 second analyses for about 6 months now, so I feel I can ask you this:  Are you happy? Really happy?

Well it turns out it doesn't matter.

Plenty of observational data has suggested that higher levels of happiness are associated with greater longevity. This feels right, in some sense, but this data doesn't come from randomized trials. I'm not really sure how you'd randomize someone to be happier anyway – maybe something with puppies.

The issue is that sicker people probably don't feel as happy, so unless you account for that, how can you really say that happiness leads to longer life?

Researchers, writing in The Lancet, attempted to tackle this issue by going big. Really big. They examined around 700,000 women who participated in the Million Women Study in the United Kingdom.  These women, who were all aged 50-69, were asked simply how often they felt happy: never, rarely, sometimes, usually, or most of the time. They were then followed for around 15 years to examine cause-specific mortality.

Happier women were generally older, got more exercise, and avoided smoking.  They were also more likely to be Scottish and tended to drink more alcohol, so keep that in mind the next time you visit Loch Lomond.

As you might expect, lower levels of self-reported happiness were associated with higher mortality. Women who were happy most of the time had a mortality rate of around 4% in follow-up, compared to 5% among women who were generally unhappy.

This difference disappeared, though, when the authors adjusted for self-rated health. The conclusion? Happiness doesn’t matter.

But here's the thing: self-rated health is subjective, just like happiness is. When the researchers adjusted for objective health issues: depression, anxiety, hypertension, diabetes, being unhappy still led to an increased risk of death.

Let's also remember that being unhappy may lead to certain unhealthy behaviors – like smoking cigarettes. Adjusting for factors that lie along the causal pathway from exposure to outcome is an epidemiologic no-no. Finally, it strikes me as odd that we'd consider categorizing something as ineffable as happiness with a single survey question.

What I take from this study is the following:  Feeling unhappy is a real risk factor for mortality. So is feeling sick.  But I'm not ready to conclude that happiness is just a bystander, exerting no real effect on outcomes. We won't know for sure without that puppy-based clinical trial. But until then I'll leave you with the words of a wise woman who died before her time: Such is the force of happiness, the least - can lift a ton, assisted by its stimulus.