"In a large population, people who have this particular variant in general will have more troubles in their marriages," Young said. "But you could never genotype one person and predict what their marriage is going to be like. There are many other factors."

All of the study participants were in long-term relationships, and most had children who were teenagers or young adults. Some never married, however, and men with two copies of the gene variation were more likely to fall into that category.

It's still not clear how the vasopressin gene affects bonding in humans. When voles interact, the gene activates reward and reinforcement areas of the brain that are also involved in addiction, Young said. In humans, the brain chemical primarily regulates water retention in the body. Vasopressin has been linked to aggression and blood pressure.

The findings don't let women off the hook, the researchers said. While their genes didn't have the same effect on marriages, it may be because researchers were looking in the wrong place, Lichtenstein said.

"There is probably something similar in female genes that influence the marriage," he said in a telephone interview. "We are also looking at other things, such as oxytocin, a hormone released when breast feeding, for example."

The study addresses the basic scientific question that everyone is interested in, basically why people bond and mate for life, Lichtenstein said. The results show there is some biology behind it, he said.

"In the future it may be possible to manipulate these systems in a way to enhance social skills and social interactions in people with psychiatric disorders, where there is a deficit in social behavior," Young said. "It won't immediately lead to a cure for all the troubled marriages."