Sexual Infidelity: Why Do People Cheat?

This article is an excerpt from the Shortform book guide to "Mating in Captivity" by Esther Perel. Shortform has the world's best summaries and analyses of books you should be reading.

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Why do people cheat in relationships? Do you think a relationship can still work after one of the partners cheated on the other?

People are unfaithful for many disparate reasons, ranging from revenge to healing. However, according to couples therapist Esther Perel, unfaithfulness doesn’t necessarily mean that a relationship has deeper problems. Lots of cheaters are happy with their relationships.

In this article, we’ll explore why people in monogamous relationships cheat, and how to handle the possibility of sexual infidelity that exists in every relationship.

Esther Perel on Infidelity

All relationships have the possibility for sexual infidelity and Perel calls this possibility the “third.” The third is a manifestation of desire for someone, real or imaginary, outside of the relationship. They can be an ex or a fantasy. The third is intrinsic to all relationships, because fidelity wouldn’t be meaningful if it was the only option.

Fidelity is so entrenched a lot of people don’t even want to talk about it⁠—the act of bringing it up suggests it’s open for negotiation. If it’s breached, the whole relationship can blow up, not only the erotic aspect. However, ignoring the third can be just as detrimental.

The Allure of an Affair

By nature, affairs have many of the qualities that generate eroticism⁠—risk, danger, jealousy, and so on. Only two people know about the affair, so their bond is strengthened by the secret. Additionally, secrecy frees you from many of the pressures of your regular relationship, such as meeting friends and family, or everyday chores. Having an affair also requires work⁠—you have to maintain the secrecy⁠—and by doing this work, you’re proving that the other person is worth it. 

It would be difficult to create this same sort of energy in a relationship, particularly one with a family, because it’s very volatile. In fact, if an affair turns into a relationship, the relationship often ends. Everything that made the affair exciting—possessiveness, jealousy, and risk—no longer exists.


Even therapists, who often challenge common beliefs, tend to agree with the general population about monogamy. Nonmonogamy, even when consensual, is never viewed as a viable possibility, only as a lack of commitment or fear. There are many common fears associated with nonmonogamy: How can your partner love you but sleep with someone else? What if they fall for them? Once they’ve crossed one line, why not cross all of them?

Peerel believes nonmonogamy can work⁠—she’s seen couples let in the third and maintain relationships that are just as committed as more traditional arrangements. Sometimes, people open relationships to strengthen them (distance and mystery are ingredients for desire, and a threat to a relationship can kickstart desire). In these relationships, fidelity is about commitment, and the relationship boundaries are emotional rather than physical. The arrangement is often very clear, with strict rules that can be renegotiated if necessary.

Even open relationships aren’t immune to betrayal, though. Since fidelity in open relationships is about trust, even though the rules are different, they’re still breakable.

Perhaps the general population’s views on nonmonogamy will change. Many things that used to be taboo, such as premarital sex and homosexuality, are now accepted. Open relationships might make the shift from side-eyed to normal too.

How to Handle the Third

As established above, the third is ever-present in all relationships. The author thinks we should look at monogamy not as the default but as one option. Ignoring the third entirely usually creates problems, so she suggests acknowledging the third (to whatever degree you and your partner are comfortable with), rather than pretending it doesn’t exist. There are three possibilities for how to deal with it:

  • Possibility #1: Control. Some couples are threatened by the idea of the third. They’re concerned not only about who their partner is physically with, but who they think about. If our partners can think about other people, they might be able to make the leap to loving other people. As a result, some couples try to control the third. They keep tabs on their partners and set up rules such as no strip clubs, or no seeing exes alone. At its best, monogamy is a free choice, and this sort of control denies that choice. When you try to banish the third, a relationship can become so constrained that one person looks for escape in the exact place you were trying to avoid⁠—outside of it.
  • Possibility #2: Acknowledge. Some couples deal with the third by acknowledging and embracing it. This recognizes each partner’s individuality, which can actually create security. When you have your own individuality within a relationship, you don’t have to go outside the relationship to find it. If it’s safe to tell the truth, there’s less reason not to. Acknowledging the third can also increase excitement. Recognizing individuality in people’s thirds, like in all things, establishes distance and allows us to see our partners as mysterious. 
    • For example, when someone flirts with Selena, it ups her ego. When someone flirts with her partner Max, she re-realizes how desirable he is and how lucky she is to be the one who gets to go home with him.
  • Possibility #3: Invite. Some people invite the third into their relationship. Many people have qualms about open relationships, sometimes even the people in them. However, inviting in the third can be a way of rekindling desire. In pursuing your thirds, you and your partner create individuality, distance, and mystery, all of which are ingredients for desire. 
    • Example #1: Arlene isn’t as interested in sex as her younger partner Jenna, so they’ve agreed Jenna can get up to whatever she likes when she’s on location for a shoot. Arlene does feel threatened by this, but she thinks asking Jenna to be celibate would be more dangerous for their relationship.
    • Example #2: Joan and Hiro have two types of sex, sex for love (with each other) and sex for fun (with others). They go to a swinger’s convention every year in Vegas and have found it strengthened their sex life and intimacy.

If You Cheat, Should You Tell?

Talk intimacy implies that we should confess our affairs, and most American couples therapists believe that confession is necessary for healing. Some people are more upset by the secret than the actual cheating. Americans conflate respect with honesty. Other cultures view respect differently. It might be kinder not to disclose an affair, because you’re saving your partner humiliation and preserving their honor. The author thinks you should consciously choose to tell or not tell, instead of going with the cultural default.

For example, Doug and Zoe have been married for several years and their sex life has declined. Zoe has many people in her life, and Doug feels that if she’s not sleeping with him, there’s nothing to distinguish him from her family or friends. He’s no longer special.

Doug has a five-year affair with Naomi. Transgression is inherently exciting, as it contains many ingredients for desire such as selfishness and risk. But the affair wasn’t just about sex⁠—Doug received attention, and Naomi confirmed that Doug is important. The affair ends when Naomi asks Doug to stop sleeping with his wife, and Doug refuses. Naomi then starts seeing someone else, which makes Doug jealous. (He is aware of the double standard.)

Doug wanted to sleep with Naomi, but never had any intention of leaving Zoe. He wasn’t having an affair to get out of the marriage. He decided not to tell Zoe because it would hurt her and could potentially end their marriage.

Sexual Infidelity: Why Do People Cheat?

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Here's what you'll find in our full Mating in Captivity summary :

  • Why it's difficult to have a good, erotic life within a long-term relationship
  • What makes up our individual sense of desire and our desire for our partners
  • Tips on how to retain desire in a committed relationship

Darya Sinusoid

Darya’s love for reading started with fantasy novels (The LOTR trilogy is still her all-time-favorite). Growing up, however, she found herself transitioning to non-fiction, psychological, and self-help books. She has a degree in Psychology and a deep passion for the subject. She likes reading research-informed books that distill the workings of the human brain/mind/consciousness and thinking of ways to apply the insights to her own life. Some of her favorites include Thinking, Fast and Slow, How We Decide, and The Wisdom of the Enneagram.

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