Older couples in a bad marriage - particularly female spouses - face a higher risk of heart disease than those in a good marriage, finds the first nationally representative study of its kind.
Marriage counselling is focused largely on younger couples.
"But results show that marital quality is just as important at older ages, even when the couple has been married for 40 or 50 years," said lead investigator Hui Liu, a Michigan State University sociologist.
Liu analysed five years of data from about 1,200 married men and women who participated in the National Social Life, Health and Aging Project.
Respondents were aged 57-85 at the beginning of the study.
Liu set out to learn how marital quality is related to risk of heart disease over time, and whether this relationship varies by gender and age.
She found that negative martial quality has a bigger effect on heart health than positive marital quality.
"In other words, a bad marriage is more harmful to your heart health than a good marriage is beneficial," Liu noted.
The effect of marital quality on cardiovascular risk becomes much stronger at older ages.
"Over time, the stress from a bad marriage may stimulate more and more intense, cardiovascular responses because of the declining immune function and increasing frailty that typically develop in old age," Liu explained.
Marital quality has a bigger effect on women's heart health than it does on men's, "possibly because women tend to internalise negative feelings and are more likely to feel depressed and develop cardiovascular problems," the author continued.
The study, funded by the US National Institute of Aging, was published online in the Journal of Health and Social Behaviour.