AMERICANS disapprove of marital infidelity. Ninety-one percent of them find it morally wrong, more than the number that reject polygamy, human cloning or suicide, according to a 2013 Gallup poll.
Yet the number of Americans who actually cheat on their partners is rather substantial: Over the past two decades, the rate of infidelity among married men has been pretty constant at around 21 percent, while the percentage of married women who admitted to cheating has mostly hovered between 10 and 15 percent, according to the General Social Survey at the University of Chicago’s independent research organization, NORC.
We are accustomed to thinking of sexual infidelity as a symptom of an unhappy relationship, a moral flaw or a sign of deteriorating social values. When I was trained as a psychiatrist we were told to look for various emotional and developmental factors — like a history of unstable relationships or a philandering parent — to explain infidelity.
But during my career, many of the questions we asked patients were found to be insufficient because for so much behavior, it turns out that genes, gene expression and hormones matter a lot.
Now that even appears to be the case for infidelity.
We have long known that men have a genetic, evolutionary impulse to cheat, because that increases the odds of having more of their offspring in the world.
But now there is intriguing new research showing that some women, too, are biologically inclined to wander, although not for clear evolutionary benefits. Women who carry certain variants of the vasopressin receptor gene are much more likely to engage in “extra pair bonding,” the scientific euphemism for sexual infidelity.
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.
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