FRIDAY, July 20 (Health Day News) -- The propensity toward divorce does not lie mainly in the genes, new research suggests.

An Australian study of twins and their grown children finds that family history plays a key role, however. Adults whose own parents had split had nearly twice the risk of going through a divorce themselves, the researchers found.

But there is no "gene" for divorce, so to speak, said lead researcher Robert M. D'Onofrio, an Indiana University psychologist. "Genetic factors that influence both generations do not [significantly] account for that increased risk," he said.

The findings are published in the August issue of the Journal of Marriage and Family.

Prior studies have found that a higher percentage of divorced people come from families split by divorce. That raised the question of whether genes, "could account for the increased risk of marital instability in offspring of divorce," D'Onofrio explained. His team is the first "to test out that possibility and, in large part, rule out the role of genetic factors," he said.

The research did not completely eliminate all genetic factors, however. According to D'Onofrio, about 66 percent of the increased risk for divorce appears to stem from the simple fact of a person's parents having been divorced. The remaining 34 percent of the risk seemed to be tied to genetic factors, as well as other factors affecting parents and children.

Also, since the study was conducted in Australia, the results cannot be generalized to the United States, D'Onofrio said. To do that, researchers will need to replicate the results in an American sample -- something his group is already working on.

The study is unique, the researcher said, because it is based on data from more than 2,300 twins, their spouses and their adult offspring. In other words, many of the younger people in the study are actually cousins who are also "genetically half-siblings," because their aunt or uncle shares their parents' genes.

So, to help separate out the effects of genetics from family environment, the Australian team compared the marital success of cousins who grew up in stable families (no divorce) against cousins who came from families split by divorce.

The study still had flaws, one expert said.

One factor that D'Onofrio and his colleagues did not look at in their study was what's known as "assortative mating" -- the tendency of people to marry people like themselves, noted British expert Dr. Stephen Stansfeld, a professor of psychiatry at Barts and The London, Queen Mary's School of Medicine and Dentistry.

According to Stansfeld, this means that people who experienced a parents' divorce as children may be romantically drawn to people with similar experiences -- potentially raising their own odds for an unsuccessful marriage.

In that sense, the study doesn't address what happens when "people from a background of unstable relationships" marry each other, Stansfeld explained.

D'Onofrio acknowledged that his team's results are limited by not taking this factor into account.

"The tendency for individuals to marry similar people may place some children at greater risk for marital separations, because the offspring are exposed to two parents with increased levels of psychopathology and other characteristics," that could have negative effects on their children, the study said.

It's also not known whether assortative mating is genetically driven and how that might affect children's genetic propensity for stable or unstable marriages, D'Onofrio explained.

Divorce can and does often undermine people's happiness, added Richard Lucas, an associate professor of psychology at Michigan State University. "Once people get divorced, people seemed to be permanently changed" and are generally less happy, he said.

"We've known for quite awhile that people with divorced parents are more likely to divorce. This study really does a nice job of looking at why that might be," Lucas said. Anything that increases understanding of all the factors involved in divorce "should help people figure out what they should be focusing their efforts on in terms of ending divorce," he added.

More information

For more on divorce's psychological impact, head to the U.S. National Institutes of Mental Health.

SOURCES: Robert M. D'Onofrio, Ph.D., assistant professor, psychology, Indiana University, Bloomington, Ind.; Richard E. Lucas, Ph.D., associate professor psychology, Michigan State University, East Lansing, Mich.; Stephen Stansfeld, M.D., professor, psychiatry, Centre for Psychiatry, Wolfson Institute of Preventive Medicine, Barts and The London, Queen Mary School of Medicine and Dentistry, University of London, U.K.; August 2007, Journal of Marriage and Family

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