America does love a good sex scandal, almost as much as the British do, and the Petraeus, um, affair has been an especially juicy one. It's been so complicated, in that reality-show kind of way, that we need charts depicting who's connected to whom.

As always happens when a powerful married man is revealed to have been hiking the Appalachian Trail, finding Freudian uses for cigars, or supporting his maid's child, there's been a lot of speculation about the psychology of honcho guys. What is it about the powerful?

This noodling is off-base on several counts. First, it neglects the fact that roughly one-quarter of married people — approximately equal numbers of men and women — report having had an extramarital affair. They aren't all powerful. It's true that the successful may indeed be more likely to commit adultery, but not for the reasons usually cited, such as their supposed sense of entitlement.

It also conflates social monogamy with sexual monogamy, assuming that these complicated sets of behaviors are one and the same. But Petraeus' experience shows this is not necessarily true. By all accounts, Petraeus highly values his relationship with his wife. Yet he was not sexually monogamous, and because he wasn't, he placed his social relationship with his wife, not to mention his job and reputation, at risk.

The difference between social and sexual monogamy is partly chemical, as was illustrated recently in a fascinating experiment. You may have missed it while being steeped in Jill Kelley news, but the Journal of Neuroscience released a study earlier this month about the effects of the neurochemical oxytocin on the behavior of monogamous males.

Oxytocin has gotten a lot of publicity over the past few years, not all of it entirely accurate. It's been called the "cuddle" hormone, a "love drug," even "the moral molecule." But it turns out that the effects of oxytocin depend on social context.

In the study, the scientists squirted oxytocin (or an inert spray) up the noses of male test subjects, some of whom were involved in a monogamous relationship with a woman and some of whom were not. Then they introduced the men to an attractive woman.

Rather than approach the attractive woman, as one might anticipate given oxytocin's reputation, the men in monogamous relationships who received oxytocin tended to keep her at a distance compared to men who received placebo squirts. Oxytocin had no effect on single guys.

In other words, giving a shot of oxytocin up the nose of bonded men tended to reinforce monogamy. This finding supports work by a Dutch scientist named Carsten de Dreu who has shown that oxytocin tends to increase trust toward members of a group, but not toward outsiders. In this case, the pretty woman is the outsider.

This is an interesting finding, but not so much because it tells us that oxytocin will forever prevent a man from stepping out. Rather, it shows us that chemicals in our heads can influence our social behavior just as they influence the behavior of animals, to the point that we do not have complete rational control over our actions.

A large body of evidence gathered over the past 15 years or so shows how chemical communication between neurons can bias our behavior. Oxytocin is only one of the chemicals involved. Stress hormones, dopamine, vasopressin, opioids (the brain's heroin) all have their say. And the science tells us that "monogamous" individuals — whether birds, rodents, or people — can be driven to have sex with those outside their socially exclusive pair bond.

One of us, Larry Young, 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," this 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 Young 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. This doesn't make him special, it makes him human.

Brian Alexander is a journalist and author whose books include "Rapture: How Biotech Became the New Religion" and "America Unzipped: The Search for Sex and Satisfaction." He and Larry Young are co-authors of "The Chemistry Between Us: Love, Sex, and the Science of Attraction."