Most of us know that incest is wrong, either through cultural conditioning or what appear to be innate evolutionary cues, yet some humans can’t seem to resist the urge to have sex with their relatives. In fact, since the DNA of every living human is 99.9 percent the same, it may be much more common than we think. Still, that doesn’t mean incest can be acceptable in any case. As we will see, breeding with a close relative can result in some dire consequences.
Fortunately, Mother Nature has developed her own strategies to deter us from mating with close relatives. According to evolutionary psychologists, just observing your mother care for another baby is a cue that that other child is a sibling and discourages sexual attraction. If you happen to be the youngest, being raised with other children by the same set of parents has the same effect. This may explain why relationships with adopted family members are still considered taboo even though they don’t share the same genes.
Females are more heavily invested in the genetic fitness of their offspring, and experts believe that’s why women tend to be particularly averse to incest. A 1994 study showed that women might actually be able to smell the best genetic mate to father their future offspring. In the experiment, which would eventually be known as the “smelly t-shirt” test, women were asked to sniff shirts that had been worn by random men for three days and rate their scents for intensity, pleasantness, and sexiness. After analyzing the participants’ DNA, researchers found that the women preferred the scents of the men who were most genetically different from them.
A more recent study also found that couples who had the most genetic similarities also had the lowest sexual satisfaction. Women who were partnered with genetically similar mates were even more likely to cheat on their husbands. These studies suggest that genes can not only influence mate choice , but determine your overall satisfaction with your partner.