In the discussions on number of sexual partners, divorce and infidelity I'm surprised no one brings up the heritability of behavior. Behavioral genetics has what is called the 50-0-50 rule: around 50% of the variance in personality and behavior is heritable, around 50% is determined by non-shared environment.

Here is a twin study of 1600 pairs - female twins who were separated at birth and raised part. It found the expected moderate heritability for number of sexual partners and for infidelity - 41% and 38%.

This study also examined the link of infidelity with the vasopressin gene locus. Vasopressin is a hormone active in human pair bonding. However, they did not find a link between the vasopressin gene locus and infidelity. (Another example of how the "pair bonding" explanation of infidelity has not really panned out.)

Interestingly the study did not find a heritable link with attitudes to infidelity. The variance here was 30% shared environment, 70% unique unique environment. The authors noted that there was a disconnect between disapproval of infidelity and actual infidelity. Even women who cheated reported disapproval of infidelity.

This twin study indicates that among women, 41% of variation in infidelity and 38% of variation in number of sexual partners is explained by underlying genetic variation, with little or no role for the shared environment. For infidelity, shared environmental influences were shown to have no significant effect; for number of sexual partners, they explained only 13% of variation. In contrast, genes did not seem to significantly affect attitudes towards infidelity. Whether subjects considered infidelity to be ‘always wrong’ was partly explained by environmental influences shared among twins, such as parents, peers and upbringing, yet 70% was due to unique environmental influences and random error. The heritability findings seen here are in a similar range (35% to 60%) to other behavioral and complex traits reported elsewhere such as migraine, blood pressure, anxiety and depression, menarche, and age at menopause (MacGregor et al., 2000).

On a first screen, none of these regions harboured the oxytocin or vasopressin candidate genes or any obvious alternative candidates. In addition, the association analysis failed to support the hypothesis that a gene implicated in rodent sexual behavior (AVPR1A) could explain the observed variation in human sexual behavior.

Support for Evolutionary Theory

In general, the fact that psychosocial traits such as number of sexual partners and infidelity appear to behave as other common complex genetic traits in humans, in that they have a heritable component, lends support to evolutionary psychologists’ theories on the origins of human behavior (Bailey et al., 2000). If female infidelity and number of sexual partners are under considerable genetic influence as this study demonstrates, the logical conclusion is that these behaviors persist because they have been evolutionarily advantageous for women. Along with the findings of Kirk et al. (2001), the fact that a fitness trait is still heritable today suggests that recent selective pressures may still be operating.

Although this is the first study among women to demonstrate a genetic basis for infidelity, other studies have already shown aspects of sociosexuality and number of partners to be heritable (Bailey et al., 2000).

https://pubmed.ncbi.nlm.nih.gov/15607016/