It is rare for couples married for 40 or 50 years to divorce, but that doesn't mean these couples are always happy. Many married couples have long-simmering conflicts and disagreements.

Working through these differences — or negotiating some lasting form of dĂ©tente — could help older couples improve their quality of life and health, a new study has found.

“Marriage counseling is focused largely on younger couples,” researcher Hui Liu, a Michigan State University sociologist, said in a statement. “But these results show that marital quality is just as important at older ages, even when the couple has been married 40 or 50 years.”

The study illustrates just how damaging a bad marriage can be to one's health — and how protective a good marriage can be.

Mental health and heart health are related. We know, for instance, that both can be directly compromised by stress. The new study illustrates just how damaging a bad marriage can be to one's health, and how protective a good marriage can be.

Other studies have found that married people are healthier than those who are not. Recent research shows that marital quality is a more significant factor in health outcomes than marital status. Marriages in which members of the couple feel mutually supported offer protection from the effects of stress, while those that are experienced negatively are a source of it.

Poor marriage quality directly affects heart health through the release of stress hormones which cause blood pressure and heart rate to rise, breathing to accelerate and blood vessels to constrict. The more frequent this stress response, the greater the damage to the cardiovascular system.

The researchers analyzed data from a longitudinal study of older adults called the National Social Life, Health and Aging Project. The women and men were between 57 and 85 years of age at the time of the first assessment. They were followed up again some years later, enabling the researchers to look at the effects of marriage quality over time.

The results confirmed earlier research which found that poor marriages pose more cardiovascular risk for women than men. Depression is often thought to be a reason for this effect, though the study reported that the link between negative marital quality and women's cardiovascular risk women existed even when depression was taken out of the picture.

The oldest group of women, those between 75 and 85, had the greatest cardiovascular risk associated with high negative marital quality. Yet this group also showed positive marital quality linked to lower cardiovascular risk. The potential heart-protective effects of a good marriage only begin to appear for women in this oldest group.

One other association examined by these researchers is the potential effect of poor health on marital quality. Having a spouse with poor cardiovascular health has a negative effect on marriage quality for women but not for men.

Marriage counseling can be helpful for elderly couples with a troubled marriage, Liu suggests.

Although it is easy to find support for the adage “old habits die hard,” there is also good reason to work on the long-standing relationships of spouses in their 70s and 80s. An improved relationship should be seen as a health change that can improve a person's experience of aging and promote longevity.

Couples’ therapy can be effective at any point in a relationship by bringing to light unconscious feelings, exploring the dynamics between the partners and helping to stop unhelpful behavior patterns, replacing them with healthier ones.

The very experience of being in therapy can be rejuvenating and a revelation for a person accustomed to the unexamined life. In other words, it may not change the relationship, but it can still help both members of a couple feel better.

A marriage where the partners experience it positively can dramatically improve an elderly person's quality of life. It is something to live for.

When old age is accompanied by multiple losses — of abilities, opportunities and of cherished friends and family — a positive marriage can remain an anchor for new experience, shared memories and feelings of well-being.

The study is published in Journal of Health and Social Behavior.