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Why You’re Not Completely Messed Up If You Have a Crush on Your Cousin

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 The following is an excerpt from Kayt Sukel’s book, This Is Your Brain on Sex: The Science Behind the Search for Love.

Nearly 150 years ago Charles Darwin pointed out that being smelly—you know, in the right way—could help male ducks, elephants, and goats procure a mate. In The Descent of Man, and Selection in Relation to Sex, Darwin argued that chemical signals are just as important as visual or auditory ones when it comes to animal attraction. Specifically, animals release a variety of chemical signals through their skin and waste that cue members of the opposite sex to come calling. Those chemical signals would be the pheromones.

Originally defined by Peter Karlson and Martin Luscher in 1959, a pheromone (from the Greek pherein, “to transfer,” and hormōn, “to excite”) is a small chemical molecule that helps an animal communicate with other animals within its species.

Though the word pheromone is often used as a synonym for a sexual attractant, that’s a bit of misnomer. There are several types of pheromones that have been identified in the animal kingdom: primers, releasers, modulators, and signalers.

In the fifty-odd years since Karlson and Luscher first came up with the term, there has been some debate about whether there is a human pheromone. But as of yet, a unique human pheromone has not been identified. The only candidate, discovered by Martha McClintock at the University of Chicago, is a chemical that helps to sync the menstrual cycles of women who live together.

One chemosensory compound that has been the subject of intense study is the major histocompatibility (MHC) complex. As the name says, it’s a complex: a mixture of hundreds of different compounds, perhaps more. Although the MHC complex is often talked about as if it were a pheromone, it doesn’t fully meet the criteria. It is a group of genes that leads to a specific odor-print. As it turns out, every human has a signature odor—like a fingerprint.

The MHC complex aids animals in identifying their own offspring and family members. It also helps them assess a potential mate. The variability seen in the MHC complex is not just responsible for that odorprint; it’s also linked to a healthy immune system.

It’s no different in humans: the more variability in your HLA, the better your immune system. Because of this, it was long hypothesized that folks were most attracted to those who had an MHC complex as different from their own as possible—in other words, we seek an MHC complex that’s different from our own.

Over ten years ago Swiss researchers at the University of Bern undertook what I call the “stinky tee” experiment. The group genotyped 49 female students and 44 male students, looking specifically at their HLA gene sequences. The men were then asked to live “odor-neutral” for a few days, avoiding sexual activity, odor-producing foods, and cigarettes; on two consecutive nights they slept in a T-shirt provided by the researchers.

When the T-shirts were returned to the experimenters, the women were asked to sniff six different shirts and rate them for intensity, pleasantness, and sexiness. The researchers found that HLA mattered: women rated the odor of the men whose HLA systems were less like theirs as sexier, compared to the men whose HLA systems were similar to theirs. This trend was reversed if the woman was on the pill, interestingly enough.

A more recent study, led by McClintock, challenged the notion that the more different, the more attractive. The group did their own “smelly tee” test with forty-nine women. In this case the researchers selected a diverse group of male sweaty T-shirt donors, but made sure there was some overlap with common HLA alleles found in the female smellers’ families.

McClintock and her colleagues discovered that women were able to discriminate the differences in HLA genotypes. However, they did so based on the HLA alleles they inherited from their father. What does this mean, exactly? These women did not prefer odors that were completely different from their own HLA system–influenced odorprint. Rather, they preferred odors that had a couple of alleles in common with dear old Dad.

So if at last year’s family reunion you found your fourth cousin (twice removed) on your dad’s side more attractive than you thought you should, you’re not quite as messed up as you thought. Your olfactory system was just on the lookout for optimal genetic diversity.

Several online dating companies now offer HLA matching services as part of their programs. For a cool $1,000, you can have potential matches genotyped to see how closely related your HLA systems are. Though of course, there’s no guarantee that similar HLA genes make for a happy relationship.

We’re giving away a copy of This Is Your Brain on Sex: The Science Behind the Search for Love! Leave a comment below by April 10 at 5:59 p.m. eastern and we’ll choose one winner at random. Winner will be contacted via email. Good luck! 

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