The Surprisingly Strong Link Between Genetics and Infidelity

Engin Akyurt/Unsplash

My friend Monica* recently found out that three of her relatives, all on the same side of the family, had been unfaithful to their spouses. She wondered whether infidelity ran in families and whether her genetic makeup could predispose her to be unfaithful to her own spouse (she has never been unfaithful to her spouse but had been unfaithful to some of her previous partners before her marriage).

When researchers investigate whether behaviors are genetically linked in humans, they often study individuals with varying degrees of genetic relationships such as monozygotic (MZ or identical twins) and dizygotic twins (DZ or fraternal twins), who not only share genes but also tend to share a common environment. MZ twins share 100% of their genes and are genetically identical, DZ twins share about 50% of their genes on average, just like non-twin siblings. Both types of twins who are raised together are assumed to share a common family environment, although the environment may be more similar for identical rather than non-identical twins, especially when non-identical twins are of different genders.

One of the first studies to examine whether human infidelity was genetically linked was performed by Cherkas and colleagues in 2004. These researchers studied more than 1,600 female twin pairs and their responses to an anonymous survey assessing infidelity, lifetime number of sexual partners, and attitudes towards infidelity.

In this study, infidelity was defined as having sex with someone other than a husband or partner while married to or living with that partner. Cherkas and colleagues found that in MZ twins, 21% of individuals reported being unfaithful to their partners while in DZ twins 23% reported unfaithfulness. These estimates are consistent with general population estimates of infidelity.

Concordance rates are defined as “agreement” between the behavior of the twins. If both twins are faithful to their spouses or both twins are unfaithful to their spouses, those twin pairs show concordance. When MZ twins exhibit higher concordance rates than DZ twins on traits or behaviors, researchers can conclude that those traits or behaviors are genetically influenced.

In Cherkas et al.’s research, concordance rates were significantly higher for MZ (46%) than DZ (32%) twins, suggesting that “MZ co-twins are approximately one-and-a-half times more likely to be unfaithful if their co-twin has been unfaithful as compared with DZ co-twins.” When adjusting for factors such as number of sexual partners and age, the authors estimated that 41% of the variation in infidelity in this sample was due to genetic factors, a “heritability estimate.” By contrast, the shared environment in which twins were raised did not contribute to twins’ concordance rates.

Furthermore, in this sample, number of sexual partners had a heritability estimate of 38%, suggesting that 38% of the variation in number of sex partners was attributable to genetics. For number of sexual partners, the shared environment also explained 13% of variation. Attitudes toward infidelity had a heritability estimate of 0%, all of the variation in attitudes towards infidelity appeared to be due to shared or unique environmental factors and none of the variation appeared related to genetics.

Cherkas and colleagues noted that the heritability estimates for infidelity as well as number of sexual partners were similar to estimates for genetic influences on blood pressure, migraine headaches, and depression.

Similar research was conducted by Zietsch and colleagues in 2015 with a sample of more than 7,000 Finnish twins. Although these individuals reported being in committed relationships for at least one year, 9.8% of men and 6.4% of women reported having two or more sex partners during that same year. In this sample, concordance rates were once again higher for MZ twins than DZ twins, suggesting genetic contributions to unfaithfulness or “extrapair mating.”

Furthermore, Zietsch et al. calculated heritability estimates of 63% for men and 40% for women, suggesting that for men as much as 63% of the variation in infidelity was due to genetic factors. While the estimate for women was very close to the earlier estimate calculated by Cherkas et al., the estimate for men was much stronger than the heritability estimate for women, potentially suggesting a stronger genetic basis for infidelity in men vs. women.

Infidelity Essential Reads

How Betrayals and Affairs Begin With Small Excuses

Cheating Is Complicated

Although Cherkas and colleagues tried to assess which genes might be associated with unfaithful behavior, their analysis did not yield results linked to particular genes. Interestingly, although the heritability estimate was higher for men, Zietsch et al. found no links to specific genes for men and only tenuous links for women. Cherkas et al. speculated that genes related to risk-taking or sensation-seeking may also be related to the likelihood of infidelity.

A study of unrelated students by Garcia et al. (2010) found that variation on the dopamine D4 receptor gene was associated with a greater likelihood of having a one-night stand as well as a greater likelihood of being unfaithful to a committed partner for both men and women. Although individuals with this genetic variation also showed stronger “desire” responses to stimuli like food and alcohol in previous research, Garcia et al. caution that this genetic variation is not “deterministic,” meaning that some but not all individuals with this genetic variation show these responses and behaviors. Garcia and colleagues as well as Cherkas et al. warn that genetic associations with infidelity should not be considered as evidence for a “cheating gene.”

The research reviewed above suggests that infidelity has a strong genetic link. All of the authors cited above posit that infidelity has a genetic basis because in our evolutionary history, it was advantageous to pursue alternate partners in order to potentially increase the number of offspring (especially for men), to provide diverse genetics for offspring, or to access additional resources for offspring.

Although these studies suggest a strong genetic basis for unfaithful behavior, they do not suggest that our behavior is entirely determined by our genes, rather they suggest that a portion of the variation in unfaithful behavior may be attributed to genetic influences.

*This name has been changed.

Facebook image: StockPhotoDirectors/Shutterstock