Married couples are more likely to untie the knot when a wife gets sick, compared to when a wife remains healthy, according to a new study. But who initiates the divorce is a mystery the researchers at Iowa State University and Purdue did not resolve.

The study, published in the Journal of Health and Social Behavior, found a 6 percent higher likelihood of divorce for couples with an ill wife. A husband's illness does not increase the likelihood of divorce.

The researchers tracked the onset of serious illness — cancer, heart problems, lung disease and/or stroke — in 2,701 marriages using the Health and Retirement Study (1992–2010) to see illness' impact on either divorce or widowhood.

They found that the wife's illness onset was associated with increased risk of divorce, but not the husband's illness. Both were associated with increased risk of widowhood. "These findings suggest the importance of health as a determinant of marital dissolution in later life via both biological and gendered social pathways," they wrote.

In a release on the study, Amelia Karraker, lead author and assistant professor of human development and family studies at Iowa State, offered some theories on why illness may stress a marriage. She said a healthy spouse may end up providing both care to the ill spouse and managing all the household details.

"There is a difference between feeling too sick to make dinner and needing someone to actually feed you," she said. "That's something that can really change the dynamics within a marriage. If your spouse is too sick to work, we know that financial strain is a major predictor of divorce in and of itself."

She also noted that wives are "generally less satisfied" with care from husbands. They have not been socialized to be caregivers as much as women have. So it's possible, she said, that women are seeking the divorce, rather than the men. The data doesn't answer the question.

In the study, 32 percent ended in divorce, 24 percent in widowhood. They looked at the marriages across nearly two decades. The couples all had one or more spouses age 51 or older when the study began.

Divorce was more common for younger spouses, then widowhood increased as the respondents got older.

Caregiving and divorce have long been associated, but often it's taking care of a parent or other relative that strains the spousal bond.

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On, Mary A. Languirand, clinical psychologist and co-author of When Someone You Love Needs Nursing Home, Assisted Living, or In-Home Care, warns that couples in caregiving situations may need to see a therapist — and they certainly need to talk openly to each other about what's going on.

"Caregiving can be extremely stressful, so much so that full-time caregivers are actually at increased risk for depression, health problems and substance abuse. It can cause relationship conflict as well (especially when one member of a relationship feels neglected)," she said. "Keep in mind that any disruption in a longstanding family pattern — a disruption of the 'family system'—can be difficult for everyone, including the care receiver."

Karraker initially conducted the research while at the University of Michigan and presented it at the Population Association of America’s annual meeting last May. She said her interest was sparked by criticism John Edwards and Newt Gingrich received when they divorced their sick wives.

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