Study after study shows that married people live longer than their unmarried peers. They're healthier too, especially the men. But why? Is marriage a side effect of good health or the cause of it? And does it really matter if the couple is married or do happy cohabitors reap the health benefits too?

A major survey conducted by Harvard researchers found that married men are healthier and live longer than their single buddies. But is it that marriage causes good health or are healthier men more likely to marry? Researchers found that unhealthy men actually "marry earlier, are less likely to divorce, and are more likely to remarry following divorce or bereavement than healthy men," according to their report. So it's not that healthy men are more apt to marry, it's that marriage is playing a protective role in a man's overall health.

Marriage appears to have a beneficial role in a number of health issues — from cardiovascular disease and cancer to Alzheimer's disease and depression. A study of more than 3.5 million Americans found that married folks — both men and women — have healthier hearts than their divorced, widowed or single peers. That makes sense. After all, "a partner can encourage you to move more, watch what you eat, see a health professional on a regular basis, things like that," said Dr. Jeffrey Berger, a cardiologist at NYU Langone Medical Center and co-author of the study. Overall, married men and women had a 5 percent lower risk of cardiovascular disease than their single or divorced peers and a 3 percent lower risk than those who were widowed.

Studies have also shown that married people are more likely to receive cancer treatment in the early stages of their diseases, thus boosting their chances for survival. And in comparisons to patients who underwent cancer treatment, married people had higher survival rates than those who were separated or divorced. An analysis of more than 800,000 adult cancer patients conducted by the Cancer Prevention Institute of California found that single men with cancer had a death rate 27 percent higher than their married male peers, while for single women with cancer the death rate was 19 percent higher than those who were married.

Marriage provides mental health perks as well. Depression rates are lower for married people than they are for those who are not married. The same goes for Alzheimer's disease. Married people also tend to see improved cognitive function, improved blood sugar levels, and better outcomes when hospitalized, according to this review on the effects of marriage on health conducted by the U.S. Department of Health and Human Services.

Happy senior couple Research shows that married people have lower risks of many major illnesses including cardiovascular disease, cancer, Alzheimer's disease, and depression. (Photo: Monkey Business Images/Shutterstock)

Why is marriage so good for your health? Behavioral factors play a big role. As Berger suggested, married people — especially married men — take better care of themselves than they would if they were single. They eat better, they exercise more, they are more likely to get checkups and they are less likely to smoke, drink heavily or engage in other risky behaviors.

It also makes sense that the presence of a partner would reduce the likelihood of depression, social isolation and stress — mental health perks that can translate to a stronger immune system and better cardiovascular health.

As for long-term relationships that didn't start with an 'I Do,' the research is less conclusive. "The general consensus is that, yes, cohabiting has positive effects but not to the same degree as marriage," said Dr. Christopher Fagundes, a psychologist and researcher at Ohio State University, in an interview with WedMD.

One key ingredient to the health benefits of marriage, or any type of long-term relationship, seems to be the happiness of the relationship itself. While men are more likely to reap the health benefits of a happy marriage, women are more vulnerable to the negative health effects of an unhappy marriage. Women in unhappy relationships have higher rates of obesity, depression, and high blood pressure, all conditions that can significantly shorten your lifespan.

The bottom line is that marriage can indeed help you live longer, as long as the relationship is a healthy one. So if you're happily married, it's just one more reason to give your spouse a big hug. But if you're not, you can still reap some of the health benefits enjoyed by married folks by taking good care of your own health (even if you have to nag yourself to get to the doctor for checkups and eat healthier foods,) and building up your support network of friends and family, people you can lean on when life gets stressful.