Scientists working with a rat-like animal called a vole have found that promiscuous males can be reprogrammed into monogamous partners by introducing a single gene into a specific part of their brains.
Once they have been converted, the voles hang around the family nests and even huddle with their female partners after sex.
"A mutation in a single gene can have a profound impact on complex social behavior," said Larry Young, a neuroscientist at Emory University who reported the results in the current issue of the journal Nature.
The research helps shed light on monogamy and hints that perhaps specific genes could play a role in human relationships.
Voles, found in the wild throughout much of North America, have been particularly useful in studying monogamy, which in biology refers more to the complicated social bonds based on partnership than to absolute sexual fidelity.
One variety, the prairie vole, pairs up like humans. Males may occasionally stray from their lifelong partners, but they inevitably return to their nests.
In contrast, meadow voles, a similar but separate species, prowl their habitat for any available female and show no interest in staying in touch.
The difference, it turns out, is a receptor for the hormone vasopressin. Prairie voles have such receptors in a part of the brain known as the ventral pallidum. Meadow voles do not.
To make promiscuous male meadow voles behave like their loyal prairie cousins, the scientists used a common gene-therapy technique. They injected the animals' forebrains with a harmless virus carrying the gene responsible for expressing the receptors.
Human relationships, of course, are complicated, and culture and socialization probably matter as much as biology. Even so, Young suggested that genetic differences could help explain why some men have trouble maintaining relationships.
Gene Robinson, head of neuroscience at the University of Illinois at Urbana-Champaign, cautioned against extrapolating the results to humans. "The behavior of animals is much simpler than the behavior of humans," he said.
In any case, don't expect gene therapy for human swingers.
"This is not something that we should be playing around with," Young said.