Marital infidelity, it seems, has been the province of sensational journalism, politicians, and celebrities—major, minor, and wannabes. But an article in the New York Times reports that lots and lots of us common folks are willing “sinners” as well.

As Richard A. Friedman, Professor of Psychiatry at Weill Cornell Medical College in NY, reminds us, 91% of Americans find marital infidelity morally wrong. Yet, the rate of infidelity has been pretty constant over the past two decades at around 21% for married men according to a 2013 Gallup poll. It’s between 10-15% for married women, according to the General Social Survey at the University of Chicago’s independent research organization, NORC.


Ashley Madison

You don’t have to be a whiz in statistics to notice that the numbers don’t add up. But even these numbers don’t tell the whole truth. Noel Biderman, the founder and former CEO of the infamous website, Ashley Madison, [you most likely remember it from the high profile hacking and release of confidential data that had many of its members quaking in their boots] that is dedicated to extramarital affairs, was quoted on CNBC saying,

“It hasn’t been easy to study infidelity before the Internet—cheaters don’t put their hands up. I’m sitting on a lot of big data. I sign up 35,000 people a day. I get 120 million visitors a month. There are 1.2 million communications sent on my platform every day!”

Is this an American phenomenon? “Not at all,” says Biderman. “It turns out infidelity is global! There’s no country you can point me to where it doesn’t exist. Even in places where it’s punishable by death! How many things would you risk your life for right now?”

Not only is the practice widespread (Ashley Madison boasts having “over 43,645,000 anonymous members!”), but it has deep roots in human history. Six hundred years before Moses intoned “thou shalt not covet your neighbor’s wife”, the Babylonian king Hammurabi, “the King of Righteousness,” included in his law codex a section on marital infidelity. The Code listed different punishments for men and women for this transgression. Men were allowed to have extramarital relationships with maid-servants and slaves, but philandering women were to be bound and tossed into the Euphrates along with their lovers.

But wait, doesn’t the fact that infidelity is so ancient, so common, and practiced at the risk of losing one’s life tell us that it is not just a momentary moral indiscretion, but rather something more fundamental to our humanity? In fact, it goes even beyond that because even animals do it.


Birds do it, bees do it, let’s fall in love

Studies suggest that only about 10% of the birds and mammals that seem to mate for life are actually faithful to their partners, according to studies that suggest that infidelity may be nature’s way. This is another way of saying that sexual infidelity is favored by natural selection. Animal parents may gain important benefits for the future of their species this way, research shows. A female may stray to pick up the best genes possible for her offspring while males may be driven by an impulse to father as many offsprings as possible. Some males go overboard ensuring that their DNA, and only theirs, survives. Male lions and mountain gorillas kill their female’s offspring sired by another male. And, to make doubly sure—they eat them.


My hormones made me do it

Prairie vole bonding with mate
The prairie vole

The paradigm of oxytocin, the “love hormone,” as promoting social bonding got its start with the remarkable discovery of the sexual behavior of two very similar rat-like rodents: the prairie vole and the montane vole. The male prairie vole becomes socially monogamous once it mates with a female. His cousin, the montane vole, is more the “love-em-and-leave-em” kind.

And, what’s the difference between them? Turns out that they both secrete oxytocin and the structurally related vasopressin, but the oxytocin, as well as the vasopressin-secreting cells, in the prairie vole are located in the ventral forebrain and the adjacent reward center (the nucleus accumbens). In the montane vole, the cells are located in the amygdala, an area of the brain that has to do with fear, anxiety, and rage. There, they activate an inhibitory system, reducing the normal fear and anxiety response. So our philandering montane vole apparently does it not only without any warm and fuzzy feelings, but also without any pangs of conscience.


But, what about us humans?

These, you might argue, are all animal stories. What about us and our complex behaviors? Can hormones provide a biological explanation?

When nasally administered to humans, oxytocin has been reported to reduce fear, possibly by inhibiting the amygdala, just like the montane vole. But oxytocin/vasopressin help us go beyond FDR’s exhortation that “the only thing we should fear is fear itself”; it actually enhances trust. In a risky investment game, experimental subjects given nasally administered oxytocin displayed “the highest level of trust” twice as often as the control group. Subjects who were told they were interacting with a computer showed no such reaction.

Disclosure of emotional events is a sign of trust in humans. When recounting a negative event, humans who receive intranasal oxytocin share more emotional details and stories with more emotional significance. Judging by the popularity of our reality TV shows, we must be blessed with an overabundance of the stuff.

The Peaceable Kingdom by Edward Hicks
The Peaceable Kingdom by Edward Hicks

Humans also find faces more trustworthy after receiving intranasal oxytocin. In a study, participants who received intranasal oxytocin viewed photographs of human faces with neutral expressions and found them to be more trustworthy than did those who did not receive oxytocin.

So, is oxytocin the biological basis of Isaiah’s 11:6 vision of

“The wolf will live with the lamb, the leopard will lie down with the goat, the calf and the lion and the yearling together; and a little child will lead them?”

Although this quote is quite inspirational, it poses severe difficulties with our understanding of evolution and its tool, natural selection.

How could such indiscriminate action of the “love hormone” be adaptive? What about a competing tribe usurping your grazing grounds? Are you going to sing Kumbaya with them?

A paper in Science examines this conundrum. In three experiments using double-blind placebo-controlled designs, male participants self-administered oxytocin or placebo and made decisions with financial consequences to themselves, their in-group, and a competing out-group. Results showed that oxytocin drives a “tend and defend” response in that it promoted in-group trust and cooperation and defensive, but not offensive, aggression toward competing out-groups. So, oxytocin doesn’t make us gullible. It makes us “love,” but not indiscriminately—only our in-group.


Is oxytocin all there is?

We would be naïve to think so. Not only does our veneer of culture and social mores “interfere” with a purely hormonal deterministic mechanism, even the biology is bound to be much more complex. How do we explain the different degrees of hormonal effects? There’s got to be more complexity to such basic mechanisms of natural selection as sexual attraction, love, and yes, infidelity. Indeed, it is more complex. Here is one example.

Dopamine, the feel-good neurotransmitter, is associated with a variety of behavioral actions, including enhancement of oxytocin and vasopressin activity. It raises the question: Could the unsubtle action of oxytocin/vasopressin be fine-tuned by the dopamine receptor? This is not a wild guess. There are several subtypes of this receptor, but one of them, DR4, is of particular interest because it mediates a variety of behaviors associated with sensation-seeking. The gene contains a region in which 48 base pairs repeat themselves between 2 and 11 times. Receptors that contain 7 or more repeats (dubbed the 7R polymorphism) show reduced number of receptors in the reward structures of the brain, as well as reduced dopamine binding affinity. Humans that possess this long version of the gene display behaviors associated with  ADHD, alcoholism, financial risk-taking, impulsivity, and sexual behavior. So, it should come as no surprise that subjects who carried the 7R dopamine receptor subtype were 50% more likely to report sexual infidelity.


Is biology destiny?

So, are carriers of the 7R version of the dopamine receptor simply “bad seeds,” doomed to moral and social failure? Or, more generally, are we slaves to our genes? Not at all. Unlike other animals, we do have morals, the philosophy of ethics, and a conscience. Some of us are burdened with some pretty bad genes. All it means is we’ve got to work harder to overcome them. In the final analysis, it is in our hands.

Updated from the original post published 6/1/15.

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.


  1. So, I told my wife that if I hadn’t had the affair none of us would be here and the species would disappear.
    My divorce lawyer said that was probably why I needed a divorce lawyer.

  2. I was sexualy molested, smanked, rediculed, belittled, and till now as a grown woman I’m told I will never have nothing. Ive aways been a good person no drugs, gangs, I just wworked hard to be something. I’m proof that not all people cheat because of abuse, that’s just making more excuses for cheaters. I’m loyal and I’m the one who gets burnt.

  3. My wife was into cheating ,having sex with strangers and all that….
    She was also sexual ly abused as a 9 year old child..
    How many sexual ly abused children beco me unfaithful due to unresolved incidents? ??

  4. Wow! Finally a doctor who doesn’t fall for the evolutionary inspired excuse of “Your genes made you do it” Our genes do not free us from morality, but it is so easy to use that as an excuse when we are tempted to do wrong.

    Thank you for taking an unpopular stand!

  5. I don’t see why we have to waste money with pointless research, it doesn’t matter if your female, black white yellow or any other colour if you can do the job and do it well then you will get the job,fact , so stop bleating about not get the job or some other post as your obviously not as good as the person who did get the job or post sexual discrimination and race discrimination is crap just get a life and stop moaning

  6. You can poop in one hand and fill the other with trust and you’ll see which one weighs the most! Truth is ALL men are cheating pigs. All women in a relationship should carry a rolling pin in one hand, and install this Android on their husbands phone with the other. There’s no longer any reason to “trust”, you don’t even need to spy on their smartphone now. Get married, find the truth, leave the pig and start living for yourself!

    • Theres almost as much women doing the same thing, and sometimes even worse.

      If you leave your feminist circle, you will see women are now discriminating men equally or maybe even more. And even so, only men face the consequences.

      Slow down and open your eyes.

      • Through Adam and Eve They were two persons but described as one just as two is married they also become one as written in the bible meaning God created us to be faithful creatures to bring life into this world creating a perfect family.

  7. Historically and anthropologically men are driven to protect and to breed. It is why we survived as humans. Sure it’s hormonal driven but it is who we are in our true nature. It is not normal for men to be monogamous. Why do half the marriages end in divorce. Of the marriages that survive a large percentage of the those folks have affairs. It is in our true nature to spread our genes around. It is who we are.


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