One of us, Larry, conducts studies of bonding in a monogamous rodent called a prairie vole. His work has often been cited by social conservatives who have used the prairie vole as a kind of furry paragon of morality because the critters are stubbornly loyal to their mates. Socially, that is. Sexually, it’s another story. Many bonded males are happy to have sex with another female if they happen to run across one that’s in heat and willing. But they come home to their partner at end of the day.
In fact, prairie voles are miserable when they are separated from their bonded partner for very long. A stress-related chemical called corticotropin releasing factor (CRF) acts in males separated from their female mates just the way it does in drug addicts who are separated from their supply. “Divorce” a male vole from his mate and you get a very miserable vole whose CRF system has been fired like a gun, triggering yearning and depression. The chemical is helping enforce social monogamy.
The neurochemical dopamine is motivational. It drives us to act to appease a desire, such as for food or sex, and when we do, we get a reward, typically a burst of endogenous opioids. With experience, we learn just how pleasurable it can be to tickle this reward system.
So our brains are organized according to chemically-controlled circuits, each whispering to us about what it wants. When we see an attractive man or woman, reward circuits tell us how incredibly hot sex with that person would be. But oxytocin- and vasopressin-related circuits are telling us we love our partner, and CRF is helping us picture how miserable we’d be without our mate. The rational part of our brain, primarily the prefrontal cortex, is weighing these possible costs of cheating, and reminding us that the sexy person is married to our boss.
Which system shouts the loudest may depend partly on our genes. But one person’s genome is not exactly like another’s. We have variation. As we explain in our book, The Chemistry Between Us: Love, Sex, and the Science of Attraction, that variation can make a lot of difference. When a European team studied monogamous birds called great tits, they found that 13 percent of chicks resulted from extra-pair mating. The birds, both male and female, most likely to fly off to find a paramour tended have “bold” personalities. This gregarious, novelty-seeking personality has been linked to a variation in a gene that holds the recipe for a dopamine receptor called D4, or DRD4 in humans.
A version of that gene known as 7R+ has been implicated in drug addiction, impulsive behavior, risk taking, and gambling. But it’s also been found to be prevalent in people who are migrants, innovators, the ambitious—people who have key traits for success. (There has been no study so far of its prevalence in four-star generals or political leaders.) In one sample of 181 young adults, those who had at least one copy of 7R+ had 50 percent more instances of sexual infidelity than noncarriers.
In other words, sexual and social monogamy are driven by different brain circuits. The recent oxytocin study showed that exposing a man’s brain to a surge of the chemical immediately before he meets an attractive woman can drive him to keep his distance, but short of carrying a bottle of oxytocin spray in a purse for use just before every cocktail party, what else can we take away from the new study? For one, have more sex. As the study’s authors wrote, “the most obvious physiological stimulus for promoting endogenous [oxytocin] release in men would be having sex with their mate.” Larry believes that kissing, hugging, caressing, and intimate small talk can all help to keep a man’s oxytocin high, too.
But there’s a problem with this takeaway message. The longer we’re with our social partner, the less intimacy and sex we have. When new mates are first introduced, they have sex like crazy. After a time—in marmosets, for example, it’s about 80 days—they won’t be having much sex at all.
Less sex does not mean we’re less devoted. We have other powerful reasons to maintain the social relationship, not least CRF. But depending at least partly on our genetic makeup, our motivation to seek erotic reward can be more or less powerfully awakened by a new potential partner. If the circuit shouts loudly enough, we’ll risk the committed relationship, our careers, and our reputations to satisfy fleeting desire.
We’re not automatons. We are responsible for our actions. But our baked-in biases can make us susceptible to infidelity. Our brains can be a battlefield of competing interests, and sometimes desire wins. It may win more often in people like Petraeus whose bold, creative thinking we so admire can come with a bias toward behavior we don’t. That doesn’t make him special, it makes him human.