Monogamous cheaters

In my previous post I wrote about vasopressin and oxytocin, so-called “love molecules” that promote attachment and pair-bonding in voles and humans. These molecules act on receptors found in the dopamine-reward system to enhance the dopamine “pleasure” response thereby re-enforcing monogamous activities. However, socially monogamous voles, like humans, are prone to “mistakes” or “slip-ups” with males and females frequency engaging in uncommitted sexual behaviors. Indeed, using genetic testing of offspring, researchers have found that many of the species first thought to be “monogamous” like birds and gibbons actually participate in extra-pair copulations. It’s now more exciting to find a species that is actually sexually monogamous (such as the recent discovery of a monogamous frog). Evolutionarily speaking this is not surprising; we want to get it on with as many people as possible to pass on our genes and increase genetic diversity, while at the same time having enough resources to take care of our young. Therefore, it is possible that two competing systems co-evolved in humans: one that promotes monogamy (vasopressin, oxytocin and others) and another previously uncharacterized system that promotes sleeping around with as many people as possible. This predicts that our tendencies towards infidelity and sexual promiscuity could also be genetically encoded.

Is unfaithfulness really all in the genes? A new study published recently in PLoS ONE suggests that it might be, at least in part. The authors linked a certain variant of the dopamine D4 receptor gene to the propensity towards one-night stands (but not the actual number) and the number of sexual partners in those that were unfaithful (but not to unfaithfulness per se, although there was a trend towards significance). Interestingly, people with this variant have less dopamine D4 receptors in the reward centers of the brain and these receptors show less binding to dopamine, suggesting that these individuals might need more dopamine floating around to reach the same feel-good mood. It’s not a surprise then, that this variant has also been associated with a slew of other behaviors that increase dopamine release including addictions, risky behavior and novelty-seeking.

So, what does this mean? The authors are careful to point out that the dopamine D4 receptor gene should not be labeled the “cheating gene” or the “promiscuity gene” (although it already has) since having the variant does not necessarily mean that you will sleep around or be unfaithful. Many other genetic, environmental (alcohol) and cultural influences are likely to play into an individual’s decision to sleep around or cheat. In addition, the variant may be associated with a third confounding variable like being more honest about sexual behavior, more attractive, etc. Or it may simply be associated with risky behavior and novelty-seeking, increasing the likelihood of wanting uncommitted sex.

For now, I’d hold off on sending your significant other for genetic testing.