Wedded bliss for men may depend on the same gene that keeps little prairie rodents faithful to their mates, according to researchers studying the so-called monogamy gene.

Investigators have long known the gene that controls the hormone vasopressin is responsible for monogamy in prairie voles, small animals common in the grasslands of North America. Now researchers from the Karolinska Institute in Stockholm have uncovered it in men — and showed it may play a role in whether they and their spouses are happily married.

The researchers ran genetic tests on 2,186 participants in the Twin and Offspring Study in Sweden and had them fill out a survey about the quality of their marriage. Men with a genetic variation scored significantly lower on a scale of partner bonding. One in three reported a crisis in their marriage within the past year, twice the number as those without the variation.

"This really shows that in humans, even complex social behavior like our relationships have a biological root," said Larry Young, a professor of psychiatry at Emory University in Atlanta, who has done similar gene research in animals. While the findings need to be confirmed, "if it does hold up, it's a really intriguing story," he said in a telephone interview.

The results published in the Proceedings of the National Academy of Sciences held up when the researchers asked women about their relationships. Those whose husbands had one or two copies of the gene variation scored significantly lower on tests asking about their marriage quality than those without it.

The gene was previously linked to other aspects of human behavior, including autism, age at first sexual intercourse and altruism. Those findings don't confirm the significance of the gene, though they do show a consistent pattern, Young said.

"You can imagine that a gene that would cause social deficits in autism would also cause you to have trouble in your personal relationships," he said.

Paul Lichtenstein, the senior author of the paper from the Karolinska Institute, said people shouldn't over-interpret the results. The gene alone can't predict a successful relationship, and it should never be used as a litmus test before marriage.

"This doesn't explain how you succeed in a relationship, but it gives us an understanding of why people bond," said Lichtenstein, a professor of genetic epidemiology. "It gives you a predisposition, but it doesn't determine how successful you will be in marriage."

Young agreed. While people want to know before they get into a committed relationship whether their chances are dimmed by genetics, there is no way to know that, he said.

"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."