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Brendan P. Zietsch, a psychologist at the University of Queensland, Australia, has tried to determine whether some people are just more inclined toward infidelity. In a study of nearly 7,400 Finnish twins and their siblings who had all been in a relationship for at least one year, Dr. Zietsch looked at the link between promiscuity and specific variants of vasopressin and oxytocin receptor genes. Vasopressin is a hormone that has powerful effects on social behaviors like trust, empathy and sexual bonding in humans and other animals. So it makes sense that mutations in the vasopressin receptor gene — which can alter its function — could affect human sexual behavior.

He found that 9.8 percent of men and 6.4 percent of women reported that they had two or more sexual partners in the previous year. His study, published last year in Evolution and Human Behavior, found a significant association between five different variants of the vasopressin gene and infidelity in women only and no relationship between the oxytocin genes and sexual behavior for either sex. That was impressive: Forty percent of the variation in promiscuous behavior in women could be attributed to genes. That is surprising since, as Dr. Zietsch points out, there are so many other factors that are necessary for promiscuous encounters, like circumstance and the availability of a willing and able partner. Although this is the largest and best study on this, it’s not clear why there was no relationship between the vasopressin gene and promiscuous behavior in men.

Other studies confirm that oxytocin and vasopressin are linked to partner bonding, which bears on the question of promiscuity since emotional bonding is, in a sense, the inverse of promiscuity. Hasse Walum at the Karolinska Institute in Stockholm found that in women, but not in men, there is a significant association between one variant of the oxytocin receptor gene and marital discord and lack of affection for one’s partner. In contrast, there was a significant correlation in men between a specific variant of the vasopressin receptor gene and lower marital quality reported by their spouses.

Data in animals confirm that these two hormones are significant players when it comes to sexual behavior. An intriguing clue came from the pioneering work of Dr. Thomas R. Insel, now the director of the National Institute of Mental Health, who studied the effects of vasopressin and oxytocin in a little rodent called the vole. It turns out that there are two closely related species of voles: montane voles, which are sexually promiscuous, and prairie voles, which are sexually monogamous and raise their extended families in burrows.

After sex, prairie voles quickly develop a selective and enduring preference for their mate, while mating for montane voles is more of a one-night stand.

Dr. Insel described is that the strikingly different sexual behavior of these two species of voles reflects the action of vasopressin in their brains. The vasopressin receptors in the montane and prairie voles are in completely different brain regions so that when these receptors are stimulated by vasopressin, there are very different When vasopressin is injected directly into the brain of the monogamous male prairie vole, it triggers pair bonding; in contrast, blocking the vasopressin receptors inhibits monogamy, but does nothing to stop sexual activity. In other words, vasopressin promotes social bonding, and blocking the activity of this hormone encourages social promiscuity.

In the monogamous prairie voles, the vasopressin receptors are close to the brain’s reward center, but in the philandering montane voles, these same receptors are mostly found in the amygdala, a brain region that is critical to processing anxiety and fear.

So mating for the prairie voles activates the pleasurable reward pathway, which reinforces mating and promotes attachment and thus monogamy. For the promiscuous montane voles, sex has little effect on attachment; any vole will do.

It is even possible experimentally to take a home-wrecking montane vole and make him behave like a family-oriented prairie vole. Using a virus as a delivery vehicle to transmit the vasopressin receptor gene, it’s easy to artificially boost the number of vasopressin receptors in the brain’s reward center, and make a male vole behave monogamously. The story for female voles is similar except that it is oxytocin, not vasopressin, that triggers monogamous behavior.

We don’t yet know from human studies whether the vasopressin receptor genes that are linked with infidelity actually make the brain less responsive to these hormones, but given the animal data, it is plausible.