The benefits of a healthy marriage have been carefully studied for decades. Statistically, people who are happily married live longer than do their single counterparts. They have lower rates of heart failure, cancer and other diseases and develop tighter networks of emotional support.

According to one Harvard University study, married women are 20 percent less likely than are single women to die of a variety of causes, including heart disease, suicide and cirrhosis of the liver. Married men enjoy an even greater benefit — they're two to three times less likely to die of such causes than are single men. Statistics have also shown married people are less likely to be victims of domestic violence, sexual assault and other violent crimes.

The upsides of healthy marriages — those which enjoy strong commitment and open lines of communications — span both mental and emotional well-being. One study states definitively that the happiness of married people is significantly greater than that of the unmarried and remains true throughout the entire life cycle.

While the benefits are clear, the reason married couples live healthier lives is more elusive. Many experts postulate that the benefits of a healthy marriage have to do with cohabitation, financial stability and networks of social support. But the prevailing explanation has to do with stress management.

Hallmarks of a healthy marriage: All roads lead to stress reduction

The detrimental effect of stress on an individual's health is well known. Cardiovascular, hormonal and immune pathways are important to a person's well-being, and stress can negatively affect these systems.

Experts reason that married couples enjoy better health partly because they're better equipped as a team to handle and defray stress than are their single counterparts.

For example, in a healthy marriage, two people share the task of mowing the lawn, bringing in income or rearing children. With two people, you have twice as many resources to address daily demands. Conversely, a single head of household is more likely to face too many demands with not enough resources — the very definition of stress.

Marriage-related stress reducers: Basic themes

Many aspects of a healthy marriage contribute to stress reduction, such as:

• More money. By pooling their incomes, married couples amass greater wealth over a lifetime than do single people. In addition, husbands and wives may have individual areas of expertise that can save them money running the household. For example, a spouse handy about the home might save the family money on home repairs, while another who's good at managing finances may preclude hiring an accountant.

  Further, there are economies of scale in cohabitation. A married couple can live more cheaply than can a single person by sharing housing costs, utilities, groceries and health insurance.

  In these ways, marriage may improve health by improving financial stability.

• Expanded support network. A healthy marriage brings together two teams of friends and family, thereby multiplying the support network upon which a couple can rely to tackle life's ups and downs. This can translate into not only physical but also mental health benefits, such as a lower probability of depression.

• Improved behaviors. People make different choices and adopt different behaviors once they're married. Healthy activities generally increase, and risky behaviors typically decrease, partly due to a sense of responsibility to a spouse. The results of these changes have a positive impact on your health.

  For example, a Centers for Disease Control and Prevention (CDC) study found in 2004 that married adults are about half as likely to be smokers as are single, divorced or separated adults. They're also less likely to be heavy drinkers or engage in behavior that leads to sexually transmitted diseases.

The lone exception: Weight control

The one negative health indicator for married versus single people comes in the area of weight. Studies show that married adults, particularly men, weigh more and have higher rates of obesity than do single adults. People who have never been married are the least likely to be obese.

Committed but unmarried couples don't show the same benefit

Of course, unmarried couples in a loving relationship may enjoy similar health gains. However, the CDC study indicates the relationship to health is in fact very different, in that unmarried couples don't reap as high a level of benefit as do those in a healthy marriage.

Because marriage entails a legal vow to stay together for life — often in front of family, friends and communities — the married couple and those who surround them are more likely to recognize and support that bond. Conversely, an unmarried couple doesn't receive the same social sanction and may develop a weaker network of support.

What about an unhealthy marriage?

Just as healthy marriages provide a host of benefits, unhealthy ones can have negative health consequences — such as a higher degree of depressive symptoms — as they can be an enormous source of stress.

A study of newlywed couples conducted at Ohio State University found that hostile and negative behavior was associated with a decline in immune system response. This can spur a number of health consequences, such as slower wound healing and greater susceptibility to infectious diseases.