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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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 Match.com and the author of five books on love, sex and relationships, including Why Him? Why Her?: How to Find and Keep Lasting Love.