It happens with Abraham and Sarah. It happens with Oedipus and Jocasta. And certainly, it happens in a big way with Jaime and Cersei Lannister. From the Bible, to the plays of Sophocles, to the hit TV series Game of Thrones, we humans enjoy juicy stories about incestuous relations. Most of us want to keep it on the pages of fiction. But not everybody. Some of those who do it for real defend their actions, saying they're afflicted with a condition called genetic sexual attraction. Sounds scientific, but don't bother looking for GSA in the Diagnostic and Statistical Manual of Mental Disorders 5th Edition (DSM-V). You won't find it because it doesn't exist. There is neither a disorder, or a sexual orientation, that gives people a sexual preference for their siblings, or their children. Writing last year for Salon Magazine, Amanda Marcotte did a thorough job debunking GSA.
The premise of GSA is that there must have been some evolutionary benefit to incest way back in history. Getting close to people outside of your own group could have been dangerous in the Stone Age, after all. He or she might attack you. If there was a benefit, logic dictates there would be genes for this trait floating around the human gene pool. That would lead some siblings to be uncontrollably attracted to one another. While this concept does dovetail with some insights and evidence from modern genetics suggesting that mixing between distant cousins --third cousins possibly being optimal-- could produce more benefits than harm, if falls apart if one considers matches between first degree relatives and first cousins. This raises the question what the interaction between genetics and sexual attraction and mate selection really is. It turns out that biology does not generally drive people to seek their opposites, but it does make them attracted to people who aren't a genetic match.
Genetic basis of sexual desire
Genes do exert a powerful, direct effect on sexual desire. One key example that was uncovered more than a decade ago involves the DRD4 gene, which affects receptors for dopamine, which plays a major role in pleasure. Research by a team at Hebrew University, in Jerusalem, demonstrated in a watershed paper in the journal Nature in 2006 that simple variations in DRD4 affect the libido level. Without any pathology coming into play, some people are simply more driven than others by a desire for sex, simply because of their genetic makeup. If you have one version of the gene, you are more horny than someone who has a more common version of the gene, and that person is more horny still than someone else who has yet another version.
Along with dopamine receptors and other matters of brain chemistry, genes control body shape and proportions, and today we know that both men and woman are attracted to each other's shapes. Men like curves, and in very specific proportions.
The HLA system: Insight from the immune system
Perhaps most telling is the outcome of a research study published last year in Scientific Reports involving a complex of genes known as the human leukocyte antigen (HLA) system. The term antigen refers to proteins that are expressed on the surfaces of cells in many different tissues, proteins that are encoded by one's HLA genes. The immune system uses the HLA antigens to distinguish an individual's own tissues from what is foreign, for instance a virus or a bacterium. This keeps the immune system from attacking "self" tissues and cells, meaning those that belong to the individual who owns the immune system. Not all humans have the same combination of HLA antigens on their tissues, but some do. There are various HLA groups, just as there are various blood groups. Consequently, HLA genetics is at center stage in transplant medicine. It also comes into play in autoimmune diseases. Rheumatoid arthritis and systemic lupus erythematosus (SLE) are common examples of autoimmune diseases. In such diseases, the immune system misidentifies tissues a foreign, when actually they are self.
Since research on non-human animals has shown that HLA antigens also are related to odors released by individuals and recognition of odors by others, scientists thought to explore whether such an effect also could occur in humans. It turns out that a similar thing happens in humans. Volunteers consisted of couples who were tested for their HLA patterns. They were compared in different ways with their reported degrees of sexual satisfaction with their partners, and with their partners' HLA patterns. Each member of a couple also was asked about his or her preference, or lack thereof, for the partner's body odor. The findings suggested that humans tend to be more attracted to people who do not match their own HLA patterns. In other words, someone who is a good recipient of your bone marrow is less likely to want you as a mate than someone who is not a good tissue match with you.
But you may know that whenever somebody requires bone marrow, the first place that doctors check is within the family, because that's where HLA matching is most likely. Considering all of this, a tendency for incest does not appear to be worked into our genetic programming. How nice for us that Sophocles, the biblical writers, and the characters in Game of Thrones were not up to speed on HLA genetics.