This is a story of two male rodents, one a naturally faithful husband and good father, the other a loner, a scoundrel and a cad.

It's a story of the discovery of a single gene that accounts for the difference between the two.

And it is, inevitably, a story of the temptation to inject a batch of the fidelity genes into the deadbeat dads and other dirty rats in our own species--and why, at least for the foreseeable future, biology and alimony just won't mix that way.

The story begins in the Midwest, the symbolic homeland of American family values and favored stomping ground of prairie voles, mouse-like creatures that live in burrows. Male prairie voles choose their mates for life. They are reliably monogamous, help raise the kids and spend extraordinary amounts of time cuddling and grooming their wives.

Their cousins, the montane voles, are 99 percent genetically identical to them. But montane males live reclusively in the mountains and surrounding high ground of Idaho. They venture out occasionally for sex, but before long they're back home, snacking and watching the game.

The key difference between these two closely related species involves something called vasopressin receptors, tangled proteins that grow like microscopic bushes on the surface of certain brain cells. These receptors snag molecules of the hormone vasopressin in the brain, and in doing so can start a cascade of reactions within nerve cells that ultimately add up to various behaviors.

Prairie voles, it turns out, have these receptors in certain parts of the brain, while montane voles have them in entirely different parts of the brain, according to research by Larry Young, Thomas Insel and their colleagues at Emory University, published in the journal Nature last week. Moreover, the team found, the distribution difference is attributable to just a few extra letters of genetic code in the prairie vole version of the vasopressin receptor gene.

Somehow that stretch of genetic code determines where receptors will appear in a vole embryo's developing brain. Embryos with the prairie vole version of the gene end up with receptors in parts of their brain that encourage gregariousness, monogamy and daddyhood. Montane voles, which lack the extra sequence, get the default pattern of receptors, which drives them to be the indifferent playboys they are.

In previous work, the Emory researchers had shown that if they blocked the vasopressin receptors in a male prairie vole's brain, he would start behaving like his two-timing montane counterparts. But could they do the reverse: make a bad vole turn good?

In the new work, the team took copies of the prairie vole gene and stuck them into male mouse embryos. (They used mice because scientists don't know how to put genes into voles yet.) Mice have their own brain pattern of vasopressin receptors, but the engineered mice bore receptor patterns resembling those seen in prairie vole brains. And sure enough, the researchers found, those mice grew up to be not the cold, reclusive rogues that most male mice are. They snuggled up to their sweeties and became, well, nice.

The temptation to extrapolate is irresistible. Judges have foisted libido-dampening medicines upon sex offenders. Why not inject philanderers with a bit of prairie vole DNA? Or maybe scientists could develop a test that looks for the prairie vole-equivalent gene in men, results of which could be made available to potential fiancees?

Sorry, says Young. "I don't consider this a faithfulness gene or a gene for monogamy."

In fact, even with the vole genes on board, the male mice still fell something short of romantic. The experiment tested the males' general sociability in the presence of females not in heat. Compared with standard male mice, which could not care less about a female unavailable for sex, the prairie-volish mice showed "increased affiliative response." That means some "exploration" of the female, a little nuzzling, and in some cases a few licks of her fur. That's more than can be expected from some guys, perhaps, but does not necessarily make them marriage material.

Anyway, the genetically enhanced mice failed the monogamy test. "Even though they have this increased social interest, it probably requires something more to become monogamous," Insel said.

A bigger complication for those in search of a human fidelity fix is that vasopressin does different things in different species. In people, vasopressin affects blood pressure, fluid retention and perhaps learning and memory, among other things. So far there's no evidence that vasopressin receptors in the human brain come in different versions, as they do in voles, much less that such versions affect a person's social behavior or level of spousal devotion.

"It's a mistake to try to directly extrapolate from the effects on behavior in a rodent to the effects on behavior in humans," said David Goldman, chief of the laboratory of neurogenetics at the National Institute on Alcohol Abuse and Alcoholism.

Still, the research proves an important scientific point and even has medical potential.

"It shows that depending on where a receptor is in the brain, you can get a completely different behavior and social organization," said Insel. And in the long run, he said, "it may help us get a handle on why some kids are totally asocial."

Insel is now looking at vasopressin receptor patterns in different species of monkeys and apes that exhibit different levels of gregariousness and promiscuity, and he hopes to study receptor patterns in the preserved brains of people with diseases that affect social behavior, such as autism and schizophrenia. That work may lead to a better understanding of the molecular basis of social behaviors and could speed development of therapies for those diseases.

Insel and others warned, however, that the work should not be interpreted to mean that behavior is genetically dictated. "People might get the feeling that by finding this one gene then we have solved the problem of understanding where social behavior comes from and how these social behaviors are regulated," said Geert de Vries, a neuroendocrinologist at the University of Massachusetts in Amherst.

Countless social, cultural and environmental factors--including parental influences, rituals such as marriage, and legal and economic threats such as child support--can profoundly affect human social behavior, said Emory psychologist Frans de Waal. In one famous series of experiments, de Waal showed that members of a selfish and antisocial monkey species could grow up to be more agreeable if they were raised with monkeys of another species who knew how to share.

"People want simple answers, but things are usually not simple at all," de Waal said. "It's simple to say there's a gene for fidelity or for thrill-seeking, instead of saying, 'This is a behavior that's influenced by how your parents act and how your siblings act.' That's much more complicated, but it's probably closer to the truth."