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.