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A particular gene variant seems to predispose men to relationship issues.Graphic: A. Nandy / Source: Walum, et al.

There’s news for women who want a man who bonds instead of a James Bond: Scientists have identified a common genetic variation that appears to weaken a man’s ability to emotionally attach to one partner.

The study, to appear in the Proceedings of the National Academy of Sciences, is the first to try to examine whether a hormone that encourages monogamy in animals plays a similar role in male humans. Before getting ideas about a DNA-fidelity test, though, women should consider that the study wasn’t designed to determine how much — or even whether — the gene in question is responsible for monogamy in humans.

“We can’t with any accuracy predict effects on behavior,” says Hasse Walum of the Karolinska Institute in Stockholm. “A lot of different things determine how happy you will be in a relationship.”

But women can now wonder, “What about his vasopressin 1a receptor subtype?”

The hormone vasopressin affects several body systems, including cardiac and urinary function. In addition, scientists have long studied how vasopressin influences behavior in prairie voles. The mouselike animals, found in the grasslands of North America, are famous for social monogamy. Males tend to be family guys, sticking close to home and helping to raise the pups. Even related species such as meadow voles don’t bond for so much as a romantic weekend.

Over years of study, scientists have concluded that prairie vole bonding has much to do with vasopressin activity in the brains of males. Through a series of studies that manipulated vasopressin levels in the vole brain, scientists have even made the animals more, or less, faithful. Vasopressin is not a love potion, though. Nerve cells also have to be equipped with specific receptor molecules that allow the hormone to bind to the cell and activate certain internal circuitry.

The new study examined a gene that codes for a vasopressin receptor in the human brain. In addition, Walum and an international team of collaborators also had volunteers fill out questionnaires to measure their level of “pair bonding” and marital strife. About 500 couples, who had been together at least five years, answered questions such as, “How often do you kiss your mate?” Or, “Have you discussed divorce or separation with a close friend?”

In the end, one particular variation of the gene, called allele 334, was associated with lower scores on partner bonding and greater odds of marital conflict. The effect was concentrated in men. For instance, among men either with no copies or just one copy of the 334 allele, 15 to 16 percent reported a marital crisis in the past year. However, when men had two copies of the 334 allele, the odds of marital crisis doubled, to 34 percent.

“I think this is actually a real breakthrough paper,” says Steve Phelps of the University of Florida in Gainesville. “The magnitude of effect is really astonishing.” He says that few studies of behavior find large effects for single genes.

But he and others were also cautious. “I think the results are really intriguing,” says Larry Young of Emory University School of Medicine in Atlanta, who in 2005 reported in the journal Science that a variation of the same gene predicted the quality of bonding in male voles. “I still remain skeptical until this can be replicated,” he says.

And even if the association holds up in further experiments, it doesn’t mean that women would want a future husband to submit a genetic sample for allele-334 testing, Young says. Marital harmony is determined by the behavior of two complex individuals, of which genes play only a part.

“It will be labeling a lot of people in a way that will be absolutely wrong,” Young says. “There’s so much more that goes into the quality of a relationship than a single gene.”

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