STUDY: Why People Cheat

Main Ideas

1. Study Design

• The presenter and colleagues asked 254 people from around the world who had had affairs what motivated them.
• They coded participants’ responses for recurring themes and analyzed how often those themes appeared and differed by sex.
• They also compared participants’ views of their affair partner and primary partner across physical attractiveness, personality, parenting potential, and overall desirability.
• The comparison study was pre-registered and used open methods and data. The presenter emphasizes that it examined actual affairs rather than hypothetical scenarios and used a diverse sample rather than only American undergraduates.

2. What Participants Said Motivated Them

Motivations could overlap; a person might report more than one reason.

Women’s most commonly reported reasons:

1. Relationship dissatisfaction, the most frequent reason. Women reported this about twice as often as men.
2. Having an uninvested primary partner, reported about four times as often by women as by men.
3. Revenge, often because the primary partner had cheated; women reported this about five times as often as men.

Men’s most commonly reported reasons:

1. Relationship dissatisfaction, also their leading reason, reported by about 30% of men.
2. An attractive affair partner, reported by about 16% of men and around three times as often as by women.
3. Sexual desire, reported by about 14% of men and around five times as often as by women.

The presenter suggests that dissatisfaction may lower the perceived cost of risking a primary relationship—an “infidelity discount.” He also cautions against interpreting revenge reports too elaborately: because men are more likely to cheat, women may simply have more opportunities to retaliate.

3. Limits of Self-Report

The presenter warns that people may not have perfect insight into their motives and may tell accounts that make them look better.

• People may emphasize socially acceptable reasons, such as revenge or emotional connection.
• They may downplay less socially acceptable reasons, such as lust.
• They may also be more likely to report motives they consciously recognize than subtler influences such as physical attraction.
• For example, few women named their affair partner’s looks as a motive, but the study’s attraction measures found women were more likely to rate the affair partner as better-looking than the primary partner.
• This does not necessarily mean participants were lying; they may simply have chosen to emphasize one part of a more complex explanation.

4. Evolutionary Explanations Discussed

The presenter argues that infidelity may have evolutionary roots, while stressing that “natural” does not mean good, inevitable, or morally acceptable.

Reasons offered for thinking infidelity has deep roots include that it occurs in other socially monogamous animals, appears across cultures and history despite strong social penalties, and that humans experience romantic and sexual jealousy.

The presenter notes that evolutionary explanations describe possible pressures shaping behavior; they do not mean people consciously cheat to reproduce.

Men

The commonly proposed explanation is that additional mates could, in principle, increase a man’s number of offspring. The presenter says this may help explain why men’s reported motivations in this study were more often sexual, while noting that human mating strategies may involve more than simply maximizing the number of mates.

Women

Because pregnancy limits how many offspring a woman can have at once, the presenter focuses on mate quality rather than simply mate quantity. He discusses two hypotheses:

• Dual-mating hypothesis: An affair could provide access to a partner with desirable genetic or fertility-related traits, while the primary partner provides greater parental investment. “Good genes” here means reproductive or health-related traits, not moral superiority.
• Mate-switching hypothesis: An affair could help a woman assess and attract a replacement partner who is better overall, including as a potential co-parent.

5. Study Findings and Interpretation

The results were more consistent with the dual-mating hypothesis than with mate-switching:

• Affair partners were rated as better-looking.
• They were not rated as better overall, better in personality, or better as potential parents.
• Primary partners were rated as the better potential co-parents.
• This contrast is harder to explain as a general “new partner” effect or a halo effect from attractiveness, because affair partners were not viewed more favorably in every category.

The presenter stresses that this pattern does not explain every affair. A significant minority of women found their primary partner more physically attractive, and many people reported other motives, including dissatisfaction, revenge, or seeking a new partner. Infidelity is presented as a behavior that can serve different strategies in different people and populations.

The study found no sex difference in the main pattern of partner ratings, which the presenter describes as unexpected and interprets cautiously. He speculates that men could also show a dual pattern, though with different possible functions: affairs might relate to additional offspring, while the primary partner remains the valued co-parent.

Speakers and Sources Featured

• Speaker: Macken Murphy, the presenter and researcher.
• Named research collaborators and supervisors: Dr. Candace Blake and Dr. Caroline Phillips.
• Other named scholar cited: Steven Stewart-Williams.
• Research and examples discussed: Prior research on infidelity and sex differences; studies of extra-pair mating in gibbons, birds, and cockatiels; research on the HBA of Namibia; and the historical account of Valentina Vassilyeva.
• The presenter says the study was published in Evolution and Human Behavior.