Even though adultery is punishable by death in some societies, it still occurs regularly. This tells Dr. Helen Fisher there is probably a genetic predisposition toward cheating on your partner. Of course not everyone cheats, so it’s not necessary for survival, but if we dial back ten thousand years, to a time when resources were more scarce, adultery would have helped genes survive the present generation and be passed onto the next. That gives a clear rational for men’s desire to cheat, but what about women? Fisher offers several explanations…

Helen Fisher: Everywhere you look in the world people are adulterers, even where you can get your head chopped off for it or stoned. That means that it probably has some biological predisposition. There’s all kinds of cultural reasons that people are adulterers. If you ask a person why they’re adulterers they may say, "Well I get lonely when my partner is out of town; I want to solve a sex problem; I’d like to have more sex; I’d like to get caught and patch up my marriage; I’d like to get caught and end my marriage; I’d like to supplement my marriage." But scientists are beginning to find out some biological predispositions. A predisposition doesn’t mean that you’re necessarily going to be adulterous. I mean, you can have a predisposition to alcohol and give up drinking.

And there’s some things about the brain that seem to predispose some people to adultery. One of them is a set of genes or a gene, we’re not quite sure which, in the vasopressin system. And there was a wonderful study out of Sweden. They studied a gene in the vasopressin among 552 men. Some men had no copies of the gene, some had one copy of the gene and some had two copies of the gene. And the more copies of this gene you had, the less stable your primary relationship was. They were not studying adultery but they were studying the stability of a partnership which can certainly lead to adultery that’s unstable. There’s also some genes in the immune system that seem to play a role in adultery. We tend to be drawn to people who have a different set of genes in this part of the immune system and, in fact, when the data show that when you are with a partner who is very similar to you in this part of the immune system women particularly are more likely to be adulterers and more likely to be adulterers when they’re ovulating, when they’re more likely to get pregnant.

I think we’ve evolved these three distinctly different brain systems for mating and reproduction – sex drive, feelings of intense romantic love and feelings of deep attachment. They’re often connected to each other. You can fall in love with somebody, drives up the dopamine system, triggers the testosterone system and all of a sudden they’re the sexiest person in the whole world. But they’re not always well connected. You can lie in bed at night and feel deep attachment for one person and then swing wildly into feelings of intense romantic love for somebody else and then swing wildly into feeling the sex drive for somebody who you barely ever met.

Which made me wonder whether millions of years ago there was something adaptive about having a partnership with one person and raising your babies and having extra relationships with other people. And it’s actually relatively easy to explain. Let’s dial back a million years. You’ve got a man who’s got a wife, a partnership, and two children. And he occasionally goes over the hill and sleeps with another woman and has two children, extra children, with her. He’s doubled the amount of DNA he has spread into the next generation. Those children will live and pass on whatever it is in him, some of the genetics, some of the brain circuitry to be predisposed to adultery.

But why would a woman be adulterous? A lot of people think that they’re not as adulterous but every time there’s a man sleeping around he’s generally sleeping around with a woman so you’ve got to explain women too. What would a woman have gotten if she’s had a partner a million years ago and two children. She slips over the hill and has sex with another man. Well she’ll get extra goods and resources and extra meat, extra protection. If her husband gets injured and dies one of these extra lovers might come in and help her with her children, even think some of those children are his. It’s an insurance policy. And she may even have an extra child and create more genetic variety in her lineage.

So the bottom line is for millions of years there were some reproductive payoffs not only to forming a pair bond but also to adultery, leaving each one of us with a tremendous drive to fall in love and pair up but also some susceptibility to cheating on the side.