Sex & Relationships

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

With so many of these same cultures putting a premium on monogamy, why do huge numbers of people around the world still cheat?

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?

Fisher postulates that there are three individual systems for sex, romantic love, and attachment. These systems activate many of the same brain regions, including key areas in the basal ganglia and the frontal lobe. It’s like a kaleidoscope: same parts, different patterns.

And that kaleidoscope means it is possible to be both attached to one partner, yet sexually attracted to or even romantically in love with another.

“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 occursand 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.

There is also evidence that damage to this area can change social relationships. In 1848 a railroad worker named Phineas Gage sustained a severe injury to his left frontal lobe. In an explosion gone awry, a metal pole was launched through Gage’s eye socket and out through the top of his head. Given the extent of his injury, many were surprised that he lived. More shocking, however, were the changes this injury made to Gage’s personality. Before the injury he was considered a jovial and hardworking fellow. His physician, John Martyn Harlow, wrote the following about Gage after the accident:

He is fitful, irreverent, indulging at times in the grossest profanity

(which was not previously his custom), manifesting but little

deference for his fellows, impatient of restraint or advice when

it conflicts with his desires, at times pertinaciously obstinate, yet

capricious and vacillating, devising many plans of future operations,

which are no sooner arranged than they are abandoned in

turn for others appearing more feasible.

It was also rumored that this upstanding, moral guy became quite a skirt chaser after his accident. Though due to the state of science at the time—remember, phrenology was the brain science du jour back then— most of the evidence on Gage is anecdotal and somewhat unreliable.

Today clinicians can tell you that damage to the frontal lobe of the brain has been associated with sexual dysfunction, increased sex drive, and so-called sexually deviant behaviors.  

Yet what do we know about the role of an undamaged frontal lobe in love and sexual behavior?

According to Lucy Brown, Fisher’s frequent collaborator at the Albert Einstein College of Medicine, the frontal cortex works with the VTA, ventral pallidum, and nucleus accumbens in the complex dance of love and attachment. It is difficult to tease them apart and definitively say which area is responsible for what. The likely scenario is that all these brain areas work together but perform slightly different functions. “The frontal lobe needs the support of the brain stem. Areas important for decision making are fed by dopamine released by the VTA, and the two areas communicate back and forth,” said Brown. “When you are talking about this level of complexity, it’s never just one part of the brain.”

Kayt Sukel’s work has appeared in a myriad of publications, including the Atlantic Monthly, USA Today, the Washington Post, National Geographic Traveler, Continental,, American Baby, the Bark, the AARP Bulletin and Cerebrum. She is a partner in the renowned family travel Web site, Travel Savvy Mom (, blogs about international eating fort ( and is also a frequent contributor to the Dana Foundation’s many science publications.