Love isn’t so much an emotion, says biological anthropologist Helen Fisher, as it is a brain system, one of three that’s related to mating and reproduction. It’s those other two systems that explain why human beings are capable of infidelity even as we so highly value love. Here Fisher explains more about cheating — why it occurs, how common it is and how a study shows it could potentially correlate to a gene.

1. Pairbonding is a hallmark of humanity. Data from the Demographic Yearbooks of the United Nations on 97 societies between 1947 and 1992 indicate that approximately 93.1% of women and 91.8% of men marry by age 49. More recent data indicates that some 85% of Americans will eventually marry.

Further reading:

2. However, monogamy is only part of the human reproductive strategy. Infidelity is also widespread. Current studies of American couples indicate that 20 to 40% of heterosexual married men and 20 to 25% of heterosexual married women will also have an extramarital affair during their lifetime.

Further reading:

3. Brain architecture may contribute to infidelity. Human beings have three primary brain systems related to love. 1) The sex drive evolved to motivate individuals to seek copulation with a range of partners; 2) romantic love evolved to motivate individuals to focus their mating energy on specific partners, thereby conserving courtship time and metabolic energy; 3) partner attachment evolved to motivate mating individuals to remain together at least long enough to rear a single child through infancy together. These three basic neural systems interact with one another and other brain systems in myriad flexible, combinatorial patterns to provide the range of motivations, emotions and behaviors necessary to orchestrate our complex human reproductive strategy. But this brain architecture makes it biologically possible to express deep feelings of attachment for one partner, while one feels intense romantic love for another individual, while one feels the sex drive for even more extra-dyadic partners.

Further reading:

4. Infidelity has been a reality across cultures. It was also common among the classical Greeks and Romans, pre-industrial Europeans, historical Japanese, Chinese and Hindus and among the traditional Inuit of the arctic, Kuikuru of the jungles of Brazil, Kofyar of Nigeria, Turu of Tanzania and many other tribal societies.

Further reading:

5. There are different types of infidelity. Researchers have broadened the definition of infidelity to include sexual infidelity (sexual exchange with no romantic involvement), romantic infidelity (romantic exchanges with no sexual involvement) and sexual and romantic involvement.

Further reading:

6. Myriad psychological, cultural and economic variables play a role in the frequency and expression of infidelity. But one thing is clear: infidelity is a worldwide phenomenon that occurs with remarkable regularity, despite near universal disapproval of this behavior.

7. Mate poaching is a pronounced trend. In a recent survey of single American men and women, 60% of men and 53% of women admitted to “mate poaching,” trying to woo an individual away from a committed relationship to begin a relationship with them instead. Mate poaching is also common in 30 other cultures.

Further reading:

8. Infidelity doesn’t necessarily signal an unhappy relationship. Regardless of the correlation between relationship dissatisfaction and adultery, among individuals engaging in infidelity in one study, 56% of men and 34% of women rated their marriage as “happy” or “very happy,” suggesting that genetics may also play a role in philandering.

Further reading:

9. Studies show the possibility of a gene that correlates to infidelity. In 2008, Walum and colleagues investigated whether the various genes affect pair-bonding behavior in humans; 552 couples were examined; all had been married or co-habiting for at least five years. Men carrying the 334 vasopressin allele in a specific region of the vasopressin system scored significantly lower on the Partner Bonding Scale, indicating less feelings of attachment to their spouse. Moreover, their scores were dose dependent: those carrying two of these genes showed the lowest scores, followed by those carrying only one allele. Men carrying the 334 gene also experienced more marital crisis (including threat of divorce) during the past year, and men with two copies of this gene were approximately twice as likely to have had a marital crisis than those who had inherited either one or no copies of this allele. Last, the partners of men with one or two copies of this gene scored significantly lower on questionnaires measuring marital satisfaction. This study did not measure infidelity directly, but it did measure several factors likely to contribute to infidelity.

Further reading:

10. Several scientists have offered theories for the evolution of human adultery. I have proposed that during prehistory, philandering males disproportionately reproduced, selecting for the biological underpinnings of the roving eye in contemporary men. Unfaithful females reaped economic resources from their extra-dyadic partnerships, as well as additional males to help with parenting duties if their primary partner died or deserted them. Moreover, if an ancestral woman bore a child with this extra-marital partner, she also increased genetic variety in her descendants. Infidelity had unconscious biological payoffs for both males and females throughout prehistory, thus perpetuating the biological underpinnings and taste for infidelity in both sexes today.

Further reading:

And a few other books that may be of interest. Further reading on mate choice:

And for further reading on love addiction, see:

Helen Fisher is a biological anthropologist who studies the brain in love. She is the Chief Scientific Advisor for dating site and the author of five books on love, sex and relationships, including Why Him? Why Her?: How to Find and Keep Lasting Love.

Join the conversation! 24 Comments

  1. What is missing here is that pair bonding doesn’t mean you have to lie and cheat to have other romantic and/or sexual partners. Swinging and polyamory are two viable concepts that allow people to have one or more long term committed sexual loving relationships, recreational sex, and romantic, non-sexual relationships while being transparent, honest, respectful, and loving to all their partners. Millions of people are already doing this. Why arent’ you?

    • Thank you John for expressing what was on my mind also.

      • It is what it is. It should never be up to another being to judge the actions of another being

  2. good article, would have been more easily readable had it been more thoroughly proof read though.

  3. What evidence is there that supports the theory that Homo sapiens are naturally monogamous?
    On the contrary, a great deal of research from primatology, anthropology, anatomy, and psychology says otherwise.
    Adultery has been documented in EVERY monogamous human society ever studied-including those in which fornicators are routinely stoned to death. Why would so many risk their families, careers, and reputations for something that runs against human nature?
    What animal needs to be threatened with death to act in accord with its own nature?

    For more info, I recommend the book, “Sex at Dawn: How We Mate, Why We Stray, and What it Means for Modern Relationships”

  4. Is infidelity caused by having no pet dog?

    Does it matter?

    IDK and IDC.

  5. Monogamy, like vegetarianism, is a lifestyle choice and not a genetic default. And like vegetarianism, there are good reasons for it to makes sense in modern culture.

    • Live is all about choices, but the “problem” for most people is the pressure of society.

  6. For some people, monogamy is enjoyable and effortless. It’s like wanting to eat the same meal every day. Others can’t fathom that. They say, “But think of all the VARIETY you’re missing out on!” To which the reply might go, “Variety shmariety. I desire and crave this one dish. Why SHOULDN’T I eat it every day!? My palate is very specific; I don’t care for all those other dishes anyway.”

    Reciprocally, the monogamous are perplexed by those who don’t share their wiring. They say, “But what about LOYALTY? Don’t you have a favourite? A dish you could eat every day without growing tired of it?” To which the reply might be, “Having a broader palate doesn’t mean I can’t sometimes crave specific dishes. But sameness isn’t for me; I crave diversity, and the same dish every day, no matter how enjoyable or appealing, just wouldn’t cut it.”

    Both monogamy and non-monogamy are natural. But which one *feels* natural depends on the individual. :)

  7. adding this article to my blog at

  8. 3 types of infidelity? Says who? Sexual contact is infidelity. Talking, even sexual talking, while perhaps wrong or problematic, is not cheating. Sure, some people think even flirting or similar behavior is unfaithful. I have met people who think having a meal with a person of the opposite sex is cheating or just a friendly conversation. But, generally speaking, if the statistics for infidelity she quotes include anything other than sexual contact, then they are not what most people mean by infidelity.

    Also, my anecdotal experience is that while people of both sexes cheat for many reasons, it is more likely a man will cheat because of mere opportunity than a woman will and more likely a woman will cheat because of relationship problems than because of an opportunity, and that is the main difference in the respective statistics. There was an experiment done at the U. of Miami (I believe) a few years back where men and women were approached by an attractive person of the opposite sex and propositioned for sex. Men who responded affirmatively – 75%. Women – 0%. Though this statistic doesn’t tell us who would have been cheating or not, what an enormous difference between the two sexes.

    • David, would you really not consider clandestine, highly explicit chat room messages or text messages between your partner and another person to be a form of infidelity? For me personally, cyber sex definitely counts as sex. I’d view it as a cheating if I found out my partner was exchanging explicit messages/photos with another party and lying to me about it. Maybe I’m in the minority here, but I’d like to think not!

      As for the one-night-stand thing, I’d venture the reasons those women refused sex with a handsome stranger have more to do with culture than libido. Women are generally more circumspect when it comes to going home with a stranger, due to considerations that men don’t really have to worry about: fear of sexual assault, pregnancy, the stigma of being branded a slut, etc. Also, if the primary purpose of a one-night-stand is to get off, well, heterosexual intercourse doesn’t really pass muster for most women. And many women have difficulty climaxing with their partner, let alone with someone doesn’t know their way around her body! I think many straight women, if propositioned by a handsome stranger, would be far more likely to politely decline, then go home and have a little fun on their own while picturing said stranger, because this would be much safer and probably a lot more fun to boot.

      Interestingly, I remember reading a similar study that looked at how bisexual women responded to being propositioned by a female stranger. Their numbers were pretty much the same as the men’s in your study. Whaddya know: when women feel safe and there’s a good chance they’ll get off, the gender gap narrows rapidly.

      • No you are not. I too think that such action, even if it does not result in sex, already constitute infidelity.

        But it is probably a case where the person has stepped on the red line, but have not gone too far across. In any case, if it is stopped at this stage, the relationship will definitely be easier to salvage than if sexual contact has been initiated :P.

      • I think are exaggerating when they catalog a sex chat as cheating. I have heard cases in which a woman feels offended when their husband is staring at the TV when there are pretty women there. Or for example, they worry about a guy watching porn or a magazine. Me personally wont feel threatened because my girl is staring at a guy that she finds attractive. For God’s sake it is TV, it is the the internet.

        I think women try to control things too much and it doesnt make any sense. It turns almost paranoid.

      • “I think are exaggerating when they catalog a sex chat as cheating.”

        I don’t think it’s up to you to make that call if it’s not your relationship, Daniel; everyone’s different. You might be perfectly okay with your girlfriend having sex chats with another man online without telling you about it; if your relationship has a sexual “don’t ask, don’t tell” policy, that’s totally fine! (although that’s still different to infidelity, since it was agreed upon beforehand. And I was talking about a chat that was part of a real-life *affair*, not just looking at naked chicks on the internet, which I do a fair bit of myself).

        “I think women try to control things too much and it doesnt make any sense. It turns almost paranoid.”

        I’m sorry that you seem to go for paranoid, controlling women. This statement could apply equally well to men who are jealous and irrational about their wives or girlfriends even so much talking to other men, or perhaps to men who become obsessed with the false idea that their girlfriends want them to have a penis like the guys in porn. This type of controlling behavior goes both ways; I think sexist generalizations are unhelpful.

        The point of my original comment wasn’t about being “offended” or “controlling”, but an intellectual exercise to make David and others think about why our society frequently assumes that bumping genitals together is the only form of sex that exists. Kind of a limited definition, don’t you think? For example, I count Skype sessions with my partner as sex even if we’re just sitting by ourselves masturbating. We’re both interacting, we’re both getting off, therefore=sex. Who cares if we’re not in the same room? It being the internet doesn’t make our orgasms any less real! (although I wouldn’t personally count masturbating by oneself as sex; I think there has to be some kind of interaction taking place between two or more people.) And no, I also wouldn’t react jealously to my partner looking at pornography since I do this also. But if others are uncomfortable with their partner viewing explicit materials, that’s their decision to make as a couple, not mine. I think people should discuss important things like this with their partner from the get-go to make sure they know that they’re on the same page, and if not either work out a compromise or not pursue the relationship.

  9. Genetics is not destiny. I’m sure there are many people with or without certain genes are and aren’t engaging in infidelity. More important is the fact that infidelity — whatever the source — is a violation of a promise, a vow, and/or a contract with another human being. That is a betrayal, it is dishonest, and it is wrong. Now, if you go into a relationship, as some comments mentioned, structured to recognize those tendencies, that’s another thing. But, please, let’s not blame cheating on your wife on genetics. It is character.

  10. Sadly so many who express their opinions about this topic seem to rely on their judgment to justify their opinion. How any of us “feels” about the topic is only relevant in allowing us to justify our own behavior yet is too often used to pass judgment on the actions of others.

    • Duane, I think you might be referring to me since I was the one who mentioned feelings?

      I think feelings and anecdotal experiences are at least partially relevant to this discussion since infidelity touches on our values, which are informed by our feelings. This isn’t scientific, but then, neither are parts of this article; as people mentioned, it omits to mention a number of things (consensual polyarmory, homosexuality, bisexuality, the fact that you don’t need marriage to be monogamous, etc.) and reinforces some dubious others (gender stereotypes). So it’s reasonable to assume that some people are going to get into the realm of the personal (and also take issue with science being done poorly).

      I’m not really seeing the judgement here; pretty much everyone on this thread seems to be okay with consensual polyfidelity. What could probably be seen as judgemental is peoples’ reaction to the deception involved in an actual affair (is that such a bad thing though? Having an open relationship where both partners are consenting harms no-one; an affair, on the other hand, almost always involves elaborate lies and is a pretty fast way to destroy trust if you’re found out. This is a value judgement, yes, but it’s also, well, a thing that happens! And as someone mentioned in the Facebook comments, it also removes the other person’s right to informed consent.)

      Most of us aren’t evolutionary psychologists, and so lack the experience to discuss this article in a more objective way. I’m opened to being enlightened though. What are your thoughts?

  11. What is most interesting and unfortunate is that the author fails to acknowledge consensual or ethical non-monogamy, instead portraying all forms of non-monogamous behaviour as cheating and infidelity. Certainly most non-monogamy as it’s practised currently falls into these categories, but not all. Ethical non-monogamy is on the rise. By portraying all forms of non-monogamy as cheating she perpetuates some very unhealthy and unhelpful cultural hang-ups. Many are on display in these comments.

    • mmm Ethical non-monogamy? Basically you are just saying polygamy, whatever form it takes?

      The problem with that though, is that the proponent for polygamy often forget that human often yearns for certainty and closeness in any relationship, not to mention such a personal and closed relationship between a man and woman. In all of the forms that I have seen, it only work among groups of people who don’t really love or care that much about each other, or at least a significant number of people (usually the chief wife) who have to suppress and harden their feelings in order for that system to work. Will there be a case where everyone in the relationship can spread their attention equally and fairly among each and everyone in the relationship? Not very likely at all.

      And also, they often forget the practical issues about division of labours and the fruits of labours and more … which just gets exponentially complicated in such a relationship that it often fail, unless it is just a convenient and “friends for benefit” types of relationships.

      • Of course many people desire the certainty and closeness of monogamy, it is still the most common relationship mode. But polyamory can take many forms and does not have to conform to the communal style that you describe. The most common formula that I have seen involves two people that consider each other their “primary partner” and are open to sexual or romantic encounters with people outside of the relationship. The arrangement rarely allows for the interloper to become an equal partner. That leads to break-ups.

        I, personally, have experimented with relationship dynamics only to arrive at a place where I find monogamy most worth my time. But I would not discourage others from writing their own love story and testing the bounds placed on us by societal norms.

  12. No .. really? Well if it wasn’t an unhappy relationship, it is going to be one hell of a stormy and turbulent relationship soon.

    This is a classic case of Scientist gone rogue .. where they start valuing their theories and figures so much that they start to see everything as figures .. and anything not in their equation, or cannot be reduced to a factor in their equation are discarded. That is nothing wrong in itself as science is an eternal pursuit of truth (though often it seems more like pursuit of the next pot of gold nowadays) and hypothesis, formula and equations are part and parcel of the process.

    But the trouble starts when she fail to recognised that those factors that cannot be included in her equation are in fact not necessary insignificant and that it is instead an indication of the limit of her current hypothesis/theory/law.

    Anyway, in this case, I will fault her conclusion by the fact that she based it on the current state of affair in USA and for failing to take into account the historical data and cultures that buck the trends.

    Moreover, she is definitely making many unsubstantiated assumptions about how the infidelity “genes” comes to be dominant … while a basic study of human behaviours and simple analysis of cost and benefit will tell you that someone who has a roving eye is NOT likely to be around to help the female take care of the child, else he will have to take care of SO MANY children. In fact, the female may end up having to care for more child than any single person. And even if it is a “tribal” society in the prehistoric time, the female is not likely to get any support from the tribe, unless she is the matriarch of a matriarchy.

    And really, in the end, she failed to explain and come up with a plausible model of how a society based on broad based infidelity can even self regulate and survive.

  13. I think what is really new is monogamy. It is but natural and advantageous from a biological point of view not to be strictly monogamous.

    The reason we have monogamy is because we need to create some social order to keep a working society. Males wanted to make sure the offspring was theirs for real, so they kept females under close watch. Now we have very reliable methods to check paternity and of course women won their rights and are autonomous. So these is changing also.

    Before it was ok for males to have multiple partners as long as they could afford it. Now women are asking for monogamy, in order to ensure that the resources are directed to their own children. However, you see many cases in which women ignore cheating because they are well provided for and as long as it is not public.

  14. This is very interesting, but I would like to see the inclusion of homosexuality.

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