An editorial in this month's Student BMJ says that married people have mortality rates (adjusted for age) that are 10 to 15% lower than the population as a whole. So why is this?

What are the health benefits?

Surveys of smug marrieds show they are happier, healthier and less depressed than those who are unmarried. A paper by Chris Wilson at the University of East Anglia suggests the health benefits may be equivalent to giving up smoking, while other studies show that married people are less likely to have heart disease, cancer, liver disease, chronic lung disease and even pneumonia and flu. Marriage has more health benefits for men than women (some studies suggest up to five times as much). It is worth noting that some studies can't distinguish whether there are real health benefits, or whether it is healthier people who marry in the first place.

Why should marriage make you healthier?

Married people tend to be better off than unmarried people so they should be happier – financial stress is a cause of mental ill health. Married people are also less likely to drink heavily, smoke, take drugs or do other risky things. They may also monitor each other's health and encourage each other to see the doctor if they feel ill. And this may be surprising, but married people have more sex than other groups and sex makes you happier and healthier.

Does cohabiting count?

It does, but not to the same extent – probably because surveys count living together as meaning anything from trying to save rent by sharing a flat to a couple committed to a lifelong relationship. Cohabitors tend to drink more than married people, and to have lower-quality relationships and a higher incidence of depression.

What about being in a bad relationship?

Only good marriages improve the health of women, but men's mortality rates are lower even in bad ones. Men who are widowed, divorced or single have higher rates of dying from heart disease and lung cancer than married men. Unhappy marriages make people unhealthy because of the distress they cause. A study of 292 women hospitalised for heart disease found that those with unhappy marriages (as opposed to those who were either unmarried or happily married) were three times more likely to have a repeat episode. As you would expect, marital rows cause increases in blood pressure, but that seems to affect women more than men.