While research shows that married people often enjoy better health than singles do, several studies have suggested that an unhappy marriage can take a major health toll.

Some have found a higher rate of heart disease among people who are dissatisfied with marriage.

This latest study, published in the American Journal of Health and Social Behaviour, suggests that marital strain may be particularly damaging to the health of older adults.

Lead researcher Dr Debra Umberson said it was the first study she knew of to look at the health effects of marital problems on age.

Umberson, who chairs the sociology department at the University of Texas-Austin, said there were several reasons why older adults could be more vulnerable to marital woes.

Similar to the case with smoking, she said chronic stress could have a cumulative effect on health over the years. Added to that was the fact that older adults were more susceptible to these effects through age-related declines in immune function and a higher rate of health problems such as heart disease.

Umberson and colleagues based their findings on data from a survey of US adults begun in 1986. They focused on 1049 people who were interviewed at three points over eight years and remained married throughout that time.

Study participants answered questions on marital quality - including whether their spouse made them feel "loved and cared for" and listened to their concerns - and whether they had frequent disagreements and conflicts.

They were also asked to rate their physical health from poor to excellent.

The study found that people who reported more marital strain also reported a steeper decline in their health over time.

But when the researchers separated study participants into three age groups - those aged 30, 50 and 70 at the study's start - only the oldest group showed negative health. Similarly, only older adults showed health benefits from a happy marriage.