When surveyed, 9.8 percent of men and 6.4 percent of women admitted to having more than one sexual partner in the last year. For both sexes, identical twins correlated strongly with one other in terms of unfaithfulness while fraternal twins and siblings did not. The results suggest that there is indeed a genetic component to one’s likelihood of committing infidelity — as in, nature accounts for roughly 63 percent of relationship-straying for males and 40 percent for females, with environment responsible for the rest.
“The clear finding is that an individual’s genetic makeup in general influences how likely he or she is to cheat,” said Zietsch. “We could tell that because genetically identical twins were similar in their fidelity or infidelity to partners, whereas nonidentical twins and normal siblings were not similar in this respect.”
The leading theory is that genetic variations are involved in tuning our hormones levels up or down, which affect how strongly bonded to our partners we feel. Notable studies with the monogamous prairie vole, a small rodent that mates for life, have found that vasopressin plays a key role in pair-bonding. Prairie voles possess many more of this hormone’s receptors in the brain as compared to promiscuous montane voles — a free-wheeling species with no interest in settling down. However, increasing the number of receptors in the brain of a male montane vole quickly turns him into a one-female-only type of partner.