Dirty Minds: How the Brain Evolved to Glorify Monogamy Yet Make Screwing Around Irresistible

Editor's note: The following is an excerpt from "Dirty Minds: How Our Brains Influence Love, Sex and Relationships," by Kayt Sukel, courtesy of Free Press. 

For centuries philosophers, theologians, anthropologists, physicians, and bored spouses all over the world have pondered the nature of monogamy. Is mating with one person over the course of a lifetime natural? If it is, then why do so many people go outside their monogamous relationships to, as my friend John so eloquently puts it, “do dirt”?

Statistics about infidelity vary. Do a Google search and you’ll find a wide range of numbers. The scientific literature has just as much variability. According to Janis Abrahms Spring and Michael Spring, authors of After the Affair: Healing the Pain and Rebuilding Trust When a Partner Has Been Unfaithful, a much-cited source in popular magazine articles, infidelity affects one in every 2.7 couples in the United States: that’s 37 percent of couples. Other self-reported polls say 22 percent of men and 14 percent of women are indulging in activities outside their marriage.

Many researchers estimate about 25 percent of all married couples or individuals in committed relationships cheat. For the purposes of this chapter, we’ll just keep things simple and stick with 22 and 14 percent.  Because these surveys are self-reported, many researchers generally assume that the numbers are significantly higher; after all, in many cultures cheating is still very much frowned upon. But if everyone was truly as faithful as he or she claimed, the incidence of sexually transmitted diseases (not to mention divorce) would be much lower.

When I asked Helen Fisher, an evolutionary biologist who studies love, about the prevalence of monogamy (or absence of it, as it were), she told me, “There is not a culture on earth where people don’t cheat. I’ve studied forty-two different cultures across the globe, and you find it in every single one.” The question remains: With so many of these same cultures putting a premium on monogamy, why is “dirt” so widespread?

“The way you feel when you are madly in love is different than what you feel after casual sex,” Fisher told me. These systems use different neurochemical systems, resulting in different emotional states and behaviors. “Yet there is bound to be some interaction happening between these different brain areas. In a sense, the brain is very well built for both monogamy and cheating.”

And how. The frontal cortex likely plays a big role in fidelity. Although all mammals have forebrains, the human frontal lobe is the largest and most complex. Beyond DNA, it is what differentiates us from our primate relatives. The frontal lobe is the seat of what neuroscientists call “executive function”—the place where planning, decision making, metacognition, and other higher cognitive processing and behavior occurs—and it is also implicated in moral judgments and religious belief systems. Considering the fact that it is linked to the basal ganglia circuitry (and often lights up in love-related neuroimaging studies) and contains the most dopamine-sensitive neurons in the entire brain, one would think it also has a say in whether we cheat. It certainly has the right setup to be a candidate for the government of monogamous behaviors, with the frontal lobe processing signals from the romantic love and sex drive systems in the basal ganglia and then acting in an inhibitory fashion when behaviors have the potential to get in the way of long-term attachments.