Genetic variations correlate to cheating behavior.

Nisian Hughes/Getty Images

The Infidelity Gene Debate

In October 2011, Czech anthropologists published a study possibly explaining the Kennedy brothers' -- Robert's, John's and Ted's -- common tendency to seek sex outside of marriage. They surveyed a group of 86 cohabitating couples about their relationship satisfaction and infidelities, in addition to their knowledge of any parental unfaithfulness. The only correlation they unearthed was between fathers' and sons' cheating behavior. Specifically, a father's known history of extramarital affairs predicted a higher probability that his son would follow in his philandering footsteps [source: Havlíček et al]. Lo and behold, the Kennedy pater familias, Joseph, carried on his fair share of extramarital romances, including one with cinema icon Gloria Swanson in the late 1920s [source: Korda].

Scientists worth their salt know that such correlative relationships don't prove population-wide causations, but that 2011 Czech study nevertheless supported earlier evidence of a genetic component to infidelity. A 2008 headline-sparking genetic investigation out of Sweden identified allele 334, a genetic variation associated with male infidelity because it interferes with the brain's processing of vasopressin, a neurochemical associated with monogamous pair bonding [source: Karolinska Institute]. Men with two copies of allele 334 were twice as likely to have weathered a major relationship crisis, compared to men bearing one allele copy[source: Tsapelas, Fisher and Aron]. Their romantic partners also reported the lowest relationship satisfaction among the participant pool.

Two years later, a separate team of scientists implicated the brain's dopamine D4 receptor gene, linked to addictive behavior, as another key in the infidelity ignition [source: Garcia et al]. One type of D4 genetic variation called a 7R+ allele effectively diminishes the concentration of dopamine receptors in the brain's reward system; previous studies have likewise associated it with sensation-seeking behavior, including monetary spending, promiscuity and cigarette smoking. And, indeed, infidelity study participants possessing the 7R+ allele reported higher numbers of sexual partners than those with a standard 7R- allele. Compellingly, the 7R+ folks were no more prone to cheating than their 7R- counterparts. Once they crossed the adultery line, however, they tended to do so with a higher number of sexual partners [source: Garcia et al].

But those savvy scientists would also admonish against treating genetic predisposition as crystal ball predictions, despite splashy news stories suggesting otherwise. A genetic study published in November 2010 from Saint Thomas' Hospital in London supports this. Their analysis of 1,600 adult female twin pairs attributed 38 percent of unfaithful behavioral to inheritable genes, discrediting the notion that predisposed cheaters are completely at the mercy of vasopressin- and dopamine-mangling DNA [source: Cherkas et al]. In that case, the ultimate fate of romantic fidelity -- to cheat or not to cheat -- is left up to environmental chance and personal choice, naturally.