"The slut gene" or "Why reporters should read science articles to the very end"

This week, I’m going to start with a quality science headline:

“Like to sleep around? Blame your genes”

Really?

The story behind this headline comes from a study of human sexual behaviour. Different people have different sex drives and different sexual behaviours, and we don’t know why, so American researchers set out to solve the mystery.

The researchers enlisted 181 male and female young adults and asked each one for a detailed history of sexual behaviour and relationships (awkward!) and a sample of spit. The spit was used to analyze the participant’s DNA and to look for a specific version of a gene called DRD4 (subsequently dubbed “the slut gene” by the media). The results of this study show that participants who have the specific version of this gene are more promiscuous (researchers actually used the words “one-night stand”) and report more instances of sexual infidelity. Well, there you have it. Free will is overridden by our genes.

How does this work? Your brain’s reward system is called the dopamine system (DRD4 stands for Dopamine Receptor D4), and among other things, it takes care of your motivation for sensation-seeking behaviours like having sex. This happens through the flow of dopamine molecules, which act as a message transmitter in your brain. For a brain cell to receive a message conveyed through dopamine, it needs a dopamine receptor like the D4. The gene that encodes this receptor (DRD4) comes in two forms: one that binds dopamine tightly and one that binds dopamine not as tightly. If you have the version of the gene that encodes the receptor which doesn’t bind dopamine tightly, you need more dopamine to achieve the same end-result in your brain (the feeling of reward), hence the string of one-night stands.

So are cheating and one-night stands excused because our genes made us do it? At the risk of becoming unpopular, I have to say the answer to that is no. The relationship between the special version of the DRD4 gene and promiscuity is not deterministic: having the gene doesn’t automatically lead to one-night stands. Many people in the study had the gene and didn’t cheat. The gene-sexual behaviour relationship is what we call probabilistic: having the gene only increases the probability that you would exhibit a given behaviour. What’s more, our environment can change how different genes are expressed, and it is possible to modify our behaviour. No excuses!

There are also two caveats to note in this study. The first is that not all the results were statistically significant. For example, 50% of people with the special version of the gene reported being unfaithful, compared with 22% for the participants with the normal version. While this may seem like a big difference, it was not significant because they are not looking at a big enough sample of people to ensure this couldn’t happen just by chance. The second problem is that the relationship between the gene and sexual behaviour could be due to a confounder, which is a variable that has not been studied that could explain the results. For example, if having the special gene makes you more honest about your sexual history, then these results would be due to a truth-telling tendency, not a sleeping around tendency.

My favorite part of the entire research article is at the very end when the researchers write:

“…we emphasize that it would be prudent to avoid premature and facile characterizations of the DRD4 VNTR polymorphism as “the promiscuity gene” or “the cheating gene.”

It’s funny, but it’s also a bit sad. They saw the “slut gene” stories coming from a mile away.

Reference: Associations between dopamine D4 receptor gene variation with both infidelity and sexual promiscuity. (2010) Garcia JR et al. PLoS ONE 5(11):e1412.