Will a bad marriage ruin my health? Married people are usually healthier than the unmarried - the research on relationships is pretty clear on that. But those in miserable relationships may be the sickest of all. A US study published last week in the Journal of Health and Social Behavior found that marriages full of rows and criticism increased the risk of heart disease, and the effect was greater than the protective effect from "good" marriages. The researchers used data from 1,198 married people, who were part of the National Social Life, Health and Aging Project in America. They compared known risk factors for heart attacks and stroke and a history of heart disease with the quality of people's relationships. If you are not married and think this study is not about you - you might be wrong. Although this study did only look at married people, other recent research suggests that cohabiting also improves health - although the effects in both states can wear off after the honeymoon period of moving in together. What all the research suggests is that relationships of any sort have to be happy to make you healthier than single people. The study of married couples found that people in stressful relationships (measured by standard quality scales that ask about closeness, emotional satisfaction, how much time people wanted to spend with their partners, and how often they could open up to their spouse about their worries) had a greater risk of heart disease - especially if they were older and female. Stress not only increases blood pressure but also messes with your hormones sufficiently to reduce immunity, increase depression and make you fat. So should we ditch our relationships if they make us unhappy or try harder to make them work? Some relationships are too broken to be saved. But people may not instinctively understand how important they are to the health of their partner. Christine Proulx, author of a study on long-term relationships published last year in the Journal of Family Psychology, found that people who have happy marriages are more likely to rate their health as better as they get older than those who are single, widowed or divorced. She argues that couples should be more aware of how their treatment of each other will affect the health of both. If you want to do something to stay healthy as you get older, you should work as hard on your marriage as you do in the gym. Top tips include being kind to each other, avoiding contempt and hostility and above all seeing the positive things your partner is doing. Do not be mean. It isn't nice and it could be the death of not only the relationship but also both of you as well.