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Why Infidelity in Relationships Is So Common

Research revisits the question: Is monogamy possible?

Key points

  • Infidelity is rampant, yet Americans demand monogamy.
  • Among other mammal species, only about 9 percent mate for life.
  • Proponents of monogamy insist open relationships do not work, but maybe it's time to rethink that.
Source: Olena Yakobchuk/Shutterstock

Most coupled Americans assume—in fact, demand—monogamy. For many, any breach of sexual exclusivity spells disaster. “He cheated. It’s over.” Even when infidelity doesn’t precipitate breakups, it often causes severe relationship damage. Therapists see a steady stream of couples trying to pick up the pieces. Coupled folks have every right to insist on monogamy, but clearly, many people find it impossible to limit themselves to just one lover for life.

Are Humans Naturally Monogamous?

Many insist that monogamy is “natural.” Actually, only around 9 percent of mammal species mate for life, and among humans, the prevalence of infidelity obliterates assertions that sexual exclusivity is innate:

  • In the Bible, polygamy was common—several wives or one official wife plus concubines. In Genesis, Jacob has two wives, Leah and Rachel, and two concubines, Bilhah and Zilpah.
  • The Ten Commandments consider infidelity is so vile a sin that not just one but two commandments prohibit it: Thou shalt not commit adultery. Thou shalt not covet thy neighbor’s wife. Don’t do it. Don’t even think about it. If the ancients had been comfortably monogamous, these commandments would have been unnecessary.
  • Mormons were publicly polygamous until 1890. Some still are.
  • Every U.S. metropolitan area and many rural locales boast sex and swing clubs—search “sex and swing clubs” anywhere. The former are typically open to all adults, the latter to couples and single women.

Advocates of strict monogamy often claim that non-monogamy just doesn’t work. For most, that may be true, but I know several happy long-term couples who have practiced occasional non-monogamy for decades:

  • One, together 15 years, is monogamous, but each year for the woman’s birthday, her man annually arranges a threesome with another man.
  • Another, married 20 years, is basically monogamous, but each month the woman spends a weekend with her secondary man.
  • A third, together 25 years, maintains monogamy at home but grants each other “hall passes,” permission to play, when either travels for business.
  • A fourth, married 30 years, meets secondary lovers once every few weeks. The woman explains, “I’m in love only with my husband, and he’s in love only with me. But we both enjoy playing on the side. It keeps our marital sex fresh and exciting. Occasionally, around town, we run into one of our secondaries. We make introductions, chat a bit. Everyone smiles. It’s fine.”

If monogamy is natural, why do so many novels, plays, movies, songs, and TV shows revolve around its violation? Some observations:

  • “Monogamy is like using a 20-watt light bulb to read. It works, but it’s not enough.” Playwright John Patrick Shanley (1950-).
  • “We drove back to the hotel and said goodbye. How hypocritical to leave the man you want to be with for a man you don't want, and then, in great excitement, have sex with the one you don’t want while pretending he’s the one you do. That’s monogamy.” Author Erica Jong (1942-), in Fear of Flying (1973).
  • “I told my wife I was seeing a psychiatrist. She told me she was seeing a psychiatrist, two plumbers, and a bartender.” Comedian Rodney Dangerfield (1921-2004).

Monogamy critic Dan Savage points out that until the twentieth century, most cultures assumed men were naturally non-monogamous. Monogamy was only for women, enforced by men to control women’s sexuality and guarantee paternity. In many cultures, that’s still the case.

Savage points out that we humans are decidedly imperfect, yet when it comes to sexual exclusivity, many demand perfection. “Isn’t it time to rethink monogamy?” he asks. “It’s like sobriety. You can be sober for years, then fall off the wagon and sober up again. If couples have been married 30 years and each step out only a few times, they’re not reprehensible. They’re actually very good at monogamy.” Savage coined the term “monogamish” to describe ostensibly monogamous couples who accept occasional lapses.

How Prevalent Is Infidelity?

Infidelity is difficult to research. Few willingly admit it. I recall a survey showing that only a tiny percentage of married folks had ever strayed. The researchers interviewed subjects in the presence of their spouses. Duh!

Admissions of non-monogamy depend on how researchers ask the question. University of Colorado scientists asked 4,800 married women about infidelity during the previous year using both face-to-face interviews and an anonymous questionnaire. In the interviews, only 1 percent admitted it, in the anonymous questionnaire, 6 percent.

Meanwhile, controversy clouds the definition of “infidelity.” Most say it’s sex with anyone other than your mate. But what about spouses who have separated but not divorced? Or couples separated by extended military deployment? Or involved in don’t-ask-don’t-tell marriages? Is infidelity defined as any sex outside of marriage? Or just secret sex? Or only sex with emotional involvement? What about sex with sex workers? Or ostensibly heterosexual folks who have gay, lesbian flings? And does cheating require intercourse? What if you simply flirt? Or kiss?

A huge research literature has investigated infidelity. Some highlights:

  • While one mate at a time is the norm, throughout history, 84 percent of known human societies have permitted men more than one ongoing sexual relationship.
  • Since Kinsey’s studies in the late 1940s, credible estimates of heterosexual Americans’ lifetime infidelity have been all over the map— for men, 12 to 72 percent, for women, 7 to 54 percent.
  • Three-quarters of American adults call extramarital sex “always wrong,” yet a majority of Americans who have been unfaithful call their own justified.
  • Infidelity is associated with: previous cheating; relationship boredom, dissatisfaction, and duration; expectations of imminent break-ups; and low-frequency, poor-quality partner sex. Among men, risk also increases when partners are pregnant or there are infants in the house.
  • Among spouses who have been unfaithful, half of men (56 percent) and one-third of women (34 percent) call their marriages “happy.”
  • Infidelity is associated with several personality traits: loneliness, extroversion, anxiety, depression, moodiness, narcissism, openness to new experiences, frequent use of alcohol, a history of child sexual abuse, and knowledge that one or both of one’s parents had been unfaithful. Traits associated with strict monogamy include conscientiousness and regular religious observance.
  • With regard to education, the curve is U-shaped. Those with the least and most education share the greatest likelihood of infidelity.
  • Working outside the home doesn’t make much difference. Half of cheaters, both male and female, meet their paramours through work, half in other ways.

Researchers at Rutgers and SUNY Stony Brook reviewed 148 studies from around the globe and concluded, “Despite near-universal disapproval, infidelity is a worldwide phenomenon that occurs with remarkable regularity.”

Infidelity is so prevalent that some researchers suggest it may be genetic and provide an evolutionary survival advantage. The evolutionary mission of life is to reproduce. The best way for men to do this is to mate with as many women as possible. Over the eons, as early primates evolved into humans, males who mated with the most females were more likely to father offspring who may well have carried genes that tilted them toward philandering.

Is There an Evolutionary Reason for Non-Monogamy?

Meanwhile, the best way for women to send their genes into the future is to raise children to sexual maturity. That’s a challenging task made easier with the help of a faithful man. But researchers speculate that women and their offspring gain a survival advantage by having “back-up” men who can provide resources if their primary mates die or leave. Women may also use infidelity to “trade up” to mates with more resources. Unfaithful women may well have had more children—passing along genes that tilted their offspring toward continued infidelity.

The Rutgers-Stony Brook researchers concluded, “Throughout prehistory, infidelity had payoffs for both men and women, thus perpetuating its genetic underpinnings and today’s taste for infidelity.”

Thou shalt not commit adultery. But evolution may well have primed us to stray. Civilization is only 10,000 years old, in evolutionary terms, new. More than we’d like to admit, we still may be beasts driven by animal instincts.

Despite tons of research, infidelity’s true prevalence remains a mystery. All we know is that it occurs so frequently that when we hear about it ensnaring couples we know, we’re always saddened but not always surprised.

Facebook image: Olena Yakobchuk/Shutterstock


Ryan, C. Sex at Dawn: How We Mate, Why We Stray, and What it Means. HarperCollins, NY, 2014.

Dan Savage on monogamy:

Allen, E.S. et al. “Intrapersonal, Interpersonal, and Contextual Factors in Engaging in and Responding to Extramarital Involvement,” Clinical Psychology: Science and Practice (2005) 12:101.

Atkins, al. “Understanding Infidelity: Correlates in a National Random Sample,” Journal of Family Psychology (2001) 15:735.

Drigotas, S.M. and W. Barta. “The Cheating Heart: Scientific Explorations of Infidelity,” Current Directions in Psychological Science (2001) 10:177.

Fisher, H.E. Why We Love: The Nature and Chemistry of Romantic Love. Henry Holt, NY, 2004.

Previti, D. and R.P. Amato. “Is Infidelity a Cause or a Consequence of Poor Marital Quality?” Journal of Social and Personal Relationships (2004) 21:217.

Thompson, A.P. “Extramarital Sex: A Review of the Research Literature,” Journal of Sex Research (1983) 19:1.

Treas, J. and D. Giesen. “Sexual Infidelity Among Married and Cohabiting Americans,” Journal of Marriage and the Family (2000) 62:48.

Whisman, M. et al. “Predicting Sexual Infidelity in a Population-Based Sample of Married Individuals,” Journal of Family Psychology (2007) 21:320.

Whisman, M.A. and D.K. Snyder. “Sexual Infidelity in a National Sample of American Women: Differences in Prevalence and Correlates as a Function of Method of Assessment,” Journal of Family Psychology (2007) 21:147.

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