When Elizabeth Edwards announced that her breast cancer had returned in 2007, her husband John posed for the cameras as the perfect, loving spouse. Now, thanks to embarrassing tabloid stories about his affair with a campaign aide, we know the reality was far from admirable: he might even be the father of his lover's baby. While the continuing revelations have decimated his political career, new research suggests that cheating on a cancer-stricken spouse may be far more common than we want to believe. (Click here to follow Barbara Kantrowitz and Pat Wingert).

In a study published in the Nov. 15 issue of the journal Cancer, neuro-oncologists Marc Chamberlain of the Fred Hutchinson Cancer Research Center in Seattle and Michael Glanz of the University of Utah School of Medicine followed 515 married patients with serious cases of cancer or multiple sclerosis for five years. The overall divorce or separation rate was 11.6 percent—not much different from the general population. But when couples did split, the patient was six times more likely to be a woman than a man.

The results reflected a trend both Chamberlain and Glanz had noticed in their own practices over the years, which was why they began the study: when a man got sick, his wife usually felt compelled to stick by him. But husbands were more likely to take off, even if that meant the wife suffered more. And the study found that the medical consequences were considerable. Abandoned spouses, the researchers found, were more likely to be depressed and less likely to complete prescribed treatment or enroll in new therapies. They also spent more time in the hospital and were less likely to enroll in hospice care, probably because that's a service that generally takes place at home, Chamberlain says.

It's important to note that most men do stay. Chamberlain says he currently has a patient in her late 60s who has a brain tumor that has returned after treatment. "The husband has done everything he can to keep their life as normal as possible," he says—an effort he describes as "heroic." This long-married couple also reflects another finding in the study: marriages that had lasted at least 15 years before the disease hit fared better than shorter unions. "That relationship has shown commitment by both parties," Chamberlain says. "There is emotional cement there." But the researchers also found that the older the woman patient, the more likely that her husband would leave. That could reflect women in second marriages, which are statistically more unstable, but the researchers didn't track the number of times the patients had been married or the quality of the marriage before the diagnosis.

The results are not surprising to people who study the effect of illness on marriage and divorce. "There are heaps of data that women are more likely to take on the role of caregiver than men," says Deborah Carr, a sociologist at Rutgers University. "Women see it as part of the bargain they signed on for. Men don't anticipate this happening, and it changes the marriage for them." Divorce is also more likely after a child develops a severe disease or disability, she says. "Death and major life-threatening illnesses throw a wrench into a relationship and forces people to deal with things they hadn't thought about previously."

Since the 1960s, men have doubled the time they spend helping with the kids, says Stephanie Coontz, a marriage historian at The Evergreen State College in Olympia, Wash. They're also doing more housework. But caregiving "is one of the last pieces to change," she says. Men are taking on more of the responsibility for caring for elderly parents, Coontz says, but are "more willing than women to hire someone to do a lot of the intimate caregiving."

In the cancer study, the researchers didn't ask the men why they left, but Chamberlain says the Edwardses (who are still legally married despite tabloid rumors of an imminent divorce) might represent another major problem: a troubled sex life after diagnosis. For a number of reasons, libido often vanishes when people are being treated for cancer, Chamberlain says. There's "an overwhelming sense of fatigue that often comes with therapy," he says, "and we're going to marshal our energies and channel them to do just the basic activities. Sexuality will fall victim to that." Many cancer patients are also depressed and feel that they are no longer physically desirable after they've lost a breast or have scars from brain surgery or have lost hair because of the effects of chemotherapy.

Becoming a caregiver can be equally damaging to a man's self-image and sense of self-worth, says Coontz. "There is something asexual and desexualizing about providing this kind of care," says Coontz, who is also director of research and public education for the Council on Contemporary Families. "For men, sexual attraction, on average, is a bigger part of marriage satisfaction—that, and not being criticized too much. It seems to be that illness not only triggers a decrease in a woman's sex drive but probably increases her criticalness. Even when he's providing care, he's not doing it right, so she's triggering things that are important to him."

When the situation is reversed, and the husband is the patient, women feel more pressure to stay, says Carr—even if the husband is unreasonably demanding and critical of his wife's care. In other words, women will often put up with behavior that drives men away. "They see this as their duty, and they probably feel more skilled to do it," she says. "But it's also interesting that when their partner dies, men marry again in a heartbeat, but women often don't. Their feeling is they gave care to a dying man once, and they don't want to do it again."