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You’re Not a Slut, You’re a Mutant

By Dr. Pisaster | Pajiba Dirty Talk | December 7, 2010 | Comments ()


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It's obvious that there's a lot of variability in human's sex lives. Some people require emotional attachment to feel comfortable having sex while others will sleep with anyone that's willing. Some people are perfectly content with a single partner while others crave variety. The question, from a scientific perspective, is why? Social factors can't fully explain such things, since we don't all have the same needs and desires as our peers. The other most likely explanation is nature, or in other words, genetics. Studies on twins, for example, show similarities in number of lifetime partners between identical twins, indicating a genetic influence on sexual behavior. A recent paper in PLoS One takes on this question of the genetics and sexuality by asking how variations in the human dopamine D4 receptor (DRD4 - not my abbreviation this time, don't ask me why the letters are reversed) gene affect people's sexual behavior. Specifically, the researchers wondered if the gene affected people levels of promiscuity and infidelity. In order to test this, the researchers combined anonymous surveys on sexual behavior with a genetic test for variants of DRD4.

Dopamine is involved in reward pathways in the brain. Its presence in the brain is associated with pleasure and reinforcement of certain activities. The D4 receptor is one of the cellular receptors which bind dopamine when it's released. These receptors signal to the cell that dopamine is present and trigger a cellular response to the molecule (that's how all hormones and neural transmitters work, by binding a receptor which then induces some specific response in your cells). Variations in the gene for this particular receptor have been previously linked to novelty-seeking behavior. The gene has a region that involves tandem repeats of a certain sequence, and individuals with more than 7 such repeats (heretofore known as the 7R+ genotype) on at least one of their two copies of the gene are more likely to engage in novelty and sensation-seeking behaviors. This particular variation of the gene results in receptors with decreased ability to bind dopamine and decreased levels of the receptor in the reward centers of the brain. In other words, people with this variation have a lowered capacity to respond to dopamine and therefore may seek out activities that lead to heightened levels of dopamine more than people without the sequence repeats. Since sex is a pretty sensation-heavy activity, it follows that these differences may affect sexual behavior.

And indeed, that's what the researchers found. They surveyed 181 individuals (median age 20.11) for levels of promiscuity and infidelity and looked for correlations between these behaviors and the 7R+ variant of the DRD4 gene. In this context, promiscuity was defined as uncommitted sex with non-monogamous partners and infidelity was characterized as uncommitted sex with someone other than one's relationship partner (survey questions aren't provided, but as far as I can tell this measure lumps cheating in with open relationships). The study participants included 118 women and 63 men. 61% were of European descent, 19% were of Asian descent, 9% Hispanic, 1% African America, 4% mixed race, and 6% 'other.' The subjects filled out an anonymous survey on their sexual history, although they were given the option to not answer questions that made them uncomfortable in order to increase the levels of honesty in the replies (something that's pretty hard to control for). The demographics skew white and female, and the sample size is relatively small, as the researchers acknowledge, but the results are certainly striking. 24% of the participants had the 7R+ variant of the DRD4 gene. Frequency of the variant was independent of gender. 77% of the participants reported being sexually active, and the following statistics are based on that subset, which means the actual sample size is actually more like 140. The researchers found that the levels of promiscuity in individuals with the 7R+ variant of the gene were almost twice those of 7R- individuals (45% vs. 24%). The numbers for infidelity were similar, 50% for 7R+ individuals and 22% for 7R- individuals. Of those who reported a history of infidelity, the 7R+ individuals also reported higher numbers of extra-relationship partners. The 7R+ variant of DRD4 does appear, then, to have a substantial affect on the rates of both promiscuity and infidelity.

There are several issues with the study, which the authors themselves point out, so these results should of course be taken with a grain of salt. For one thing, 7R+ individuals may just be more open about their sex lives, or may have more opportunities for uncommitted sex. It's very hard to pick a single gene out and claim it has a causal relationship on human behavior. The authors caution that these results are merely associative - not all individuals with the 7R+ variant of the DRD4 gene will be promiscuous/non-monogamous, and clearly some 7R- individuals will be (about a quarter, based on the numbers above). This gene is just one of many possible factors that contribute to how people actually behave, some of which are likely to be genetic and some social. That said, these preliminary results (more testing is obviously needed to confirm the current findings) do indicate a strong correlation between a particular variant of the gene and certain sexual behaviors, and it makes sense from an evolutionary perspective that there would be genetic variations in sexual behavior. In societies where promiscuity is advantageous, you would expect genes that promote it to more be widespread, while societies where monogamy is an advantage would have lower rates of such genes. Indeed, the authors point out that the rates of the 7R+ variant differ across populations. The Yanomamo Indians of South America, for instance, who practice polygyny, have high frequencies of the 7R+ variant.

I've argued before that humans likely have multiple mating strategies, and this particular study seems to back that up. Some people may be genetically predisposed to find monogamy difficult, while others may be just the opposite. Which is not to say that people don't ultimately bear responsibility for their actions. Non-monogamy is fine, as long as all partners agree to it, but trying to force yourself into a particular kind of relationship and then blaming forces outside your control when you find you can't maintain it isn't cool. Genetics is not an excuse for cheating; genetics gives you a set of tools to work with, and how you use them is at least to some extent your own responsibility. It's best to figure out what works for you and find a relationship context (or lack thereof) that works with your personal disposition. The results of this study reinforce the idea is that neither monogamy nor non-monogamy is the "natural," state of human beings. They're both normal variations of human behavior, representing alternative evolutionary strategies. That doesn't mean that your behavior is dependent on your genes, only that it may be influenced by them.

Dr. Pisaster has a doctorate in biophysics, not actually anything sexy. She does however enjoy having sex, reading about sex, and talking about sex. Especially when she's had a little whiskey.




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