Consider the humble prairie vole. Unlike 97 percent of species, prairie voles are faithfully monogamous. Their lives may be short — they're an easy snack for hawks and snakes — but once two prairie voles mate, they are bonded until the end.
Not so with the prairie vole's close genetic cousin, the philandering montane vole. Montane voles form weak social bonds and prefer the mating strategy of "use 'em and lose 'em." The stark differences in mating behavior between the two vole species have made them excellent subjects for decoding the genetic roots of sexual monogamy and infidelity.
Hormones and Bonding
According to a number of studies, prairie voles have more receptors in their brains for a hormone called vasopressin, which is believed to play a key role in pair bonding. Not only do the faithful prairie voles have more of these receptors than their cheating cousins, but the receptors are located in a part of the brain that's closer to the reward center.
So when prairie voles mate, their bodies produce vasopressin, which causes their brains to reward the vole couple with a flood of pleasurable emotions, sealing the social bond. The brains of montane voles, on the other hand, have far fewer vasopressin receptors and therefore make much weaker connections between pair bonding and pleasure. So it's on to the next conquest.
The location and sensitivity of hormone receptors is dictated by our genes, which naturally leads to the question, could the urge to cheat on our romantic partners be partly a product of our genes? Are some of us walking around with prairie vole brains and others are stuck with the wandering eye of a montane vole?
The real story about the roots of infidelity and monogamy is far more complicated than whether you have the "cheating gene." Human sexual behavior is the product of countless influences and interactions, from our early relationships with our parents, to social norms around sexuality, to yes, our genetic predispositions.
"We're never prisoners of our biology," says Justin Garcia, an evolutionary biologist and sex researcher at the pioneering Kinsey Institute at Indiana University. "But it does explain why some people wake up with somewhat different motivations in these areas than other people."
The influence of these different genetically based "motivations" is difficult to quantify, but a 2014 study by Australian researcher Brendan Zietsch offers some intriguing clues. Zietsch surveyed the sexual habits of nearly 7,400 twins and siblings in Finland and found that 9.8 percent of men and 6.4 percent of women had more than one sexual partner in the past year.
But the fascinating finding was that the sets of identical twins — with identical genomes — reported the same exact levels of fidelity, while fraternal twins and regular siblings didn't. That indicates that variations in genes are powerful enough to influence sexual behavior beyond other environmental factors. In fact, Zietsch put a number on it: Our genes account for roughly 63 percent of infidelity in males and 40 percent in females.
Vasopressin isn't the only hormone that's been linked to varying levels of monogamy and infidelity. Oxytocin is another hormone released during sex (and also during childbirth and nursing) that strengthens social bonds, and female voles with more oxytocin receptors are also more likely to mate for life.
Dopamine and Risky Behavior
Garcia at the Kinsey Institute conducted a landmark study of dopamine receptors and sexual straying. It's long been established that people with fewer or weaker dopamine receptors engage in riskier behavior — drug and alcohol abuse, and gambling — to get the same dopamine rush that the average person might get from eating a Snickers.
Garcia tested 181 participants, some of whom carried the weaker D4 variant of the dopamine receptor. He found that people with the D4 receptor were 50 percent more likely to report sexual infidelity. And when he looked at all participants who cheated in the study, those with the D4 receptor were far more likely to do it multiple times.
For Garcia, the genetic evidence points to a more nuanced understanding of what it means when somebody cheats in a relationship.
"The classic explanation is that they're not really in love," says Garcia. "But maybe they're more motivated by other feelings of sensation, risk and novelty."