Unhappy couples sometimes say their spouse makes them sick. As it turns out, they may be right. 

Bad marriages can be detrimental to one's health -- especially, new research shows, as aggravated couples age. 

The study, financially supported by the federal National Institute on Aging, reports that marital strain is a key source of stress, which can affect one's immune system. But unhappiness also affects people's opinion of how healthy they are. 

The findings are not necessarily a call for divorce. Indeed, much research indicates that marriage is good for people, helping them live longer, happier and more prosperous lives. Instead, the study should be considered "yet another reason to identify marital difficulties and seek to improve marital quality" because "your very health may depend on it," writes Debra Umberson of the University of Texas-Austin, the study's lead researcher. 

Conflict makes people feel sick 

In the paper, titled "You Make Me Sick: Marital Quality and Health Over the Life Course," Umberson and her fellow researchers analyzed 1,049 married folks in three waves of interviews. 

In determining quality of marriage, the researchers asked, on a five-point scale, "How satisfied are you with your marriage?" They also asked whether the respondent's spouse was willing to listen to problems, and how much their spouse makes them feel loved. 

The married participants were also asked how often they feel "bothered or upset by your marriage" and how often "the two of you typically have unpleasant disagreements or conflicts." 

The answers were compared with the participants' responses reflecting how they describe their health. 

The researchers acknowledge that it is possible, in some couplings, the scenario works backward: poor health affects how people feel about their marriages. The stress of caring for an ill person or the orneriness of a sick spouse can turn once-happy marriages miserable. But the researchers say more often it's a case of personal conflict making people feel sick. 

This is particularly the case as people get older -- "marital difficulties appear to matter more for our health as we age," the researchers say. 

Why? For one, spouses become more important psychologically as people age because of the loss of other key figures in their life, the researchers say. 

Worse than divorce? 

Umberson notes earlier research that has found evidence linking good social relationships to health and long life. 

"The married do exhibit better health than the unmarried but it is not the case that any marriage is better than no marriage at all when it comes to health benefits. The quality of relationships is also linked to health," she wrote. 

Previous studies have found that individuals in "low quality" marriages exhibit "an even greater health risk than do divorced individuals," Umberson said. 

Scientists have known that marital difficulties are often a source of chronic stress, but bickering "may have long-term consequences for overall health status," she adds. 

The study appeared in this month's Journal of Health and Social Behavior, a publication of the American Sociological Association.