“No, I haven’t washed it yet. It probably smells disgusting. Here, take a sniff.” He hands me his jacket, which I hold to my nose. Disgusting? No, definitely not; more like delicious. You’ve probably experienced it before, when your significant other or someone you like smells so good even when they shouldn’t. But why does this happen?
It’s in your genes
There is a collection of genes in your immune system, called the Major Histocompatibility Complex (MHC), that plays a large role in the connection between scent and attraction. The two important functions of the MHC are one, recognizing pathogens (bad things like viruses) that your body should fight, and two, controlling each person’s unique scent. As humans, we are naturally and subconsciously attracted to the smell of people with different immune systems than our own. Each person’s genes that control their immune system are slightly different from everyone else’s. If two people’s genes have significant differences from each other, then their immune systems are good at targeting different types of pathogens.[1]The behavior of being attracted to people whose immune systems can fight different pathogens from your own immune systems is evolutionarily advantageous because it ensures that your offspring will be able to fight both types of pathogens.[2]If your body can recognize and fight one set of pathogens, and your partner’s body can fight another, then your children will be able to fight both (thanks biology!). Your offspring will have stronger immune systems, and hence would be more likely to survive–and so evolution is happy. Now the next time you think the girl in your math class is cute, you’ll have to save yourself from thinking, “Is she actually cute, or does she just have an immune system that my nose likes?!”
The Wedekind t-shirt study
licensed under CC BY-NC 2.0
A study led by Swiss researcher, Claus Wedekind, in 1995 demonstrated the connection between human scent and attraction. The study used a group of people with a diverse representation of MHC genes. For two days, the men in the group wore a single t-shirt, which was then given to the women in the group to smell. Each of the women smelled t-shirts from three groups: shirts worn men with similar MHC genes, shirts worn by men with different MHC genes, and shirts that had not been worn at all (the control group). The results showed that women rated t-shirts as more pleasant smelling if the men who wore the shirt had different MHC genes.[3] This study has been replicated many times and provides the most current knowledge on the correlation between MHC genes and attraction.
This pill does what!?
The results of the Wedekind study also revealed a different phenomenon: if the women in the study were on hormonal birth control, they preferred shirts from men with similarMHC genes. Birth control pills simulate the state of pregnancy using hormones. Hormones are chemical transmitters that send information about how the body should function to different systems of the body. Males and females have specific hormones that control their sexual processes. It is speculated that women are attracted to people with similar immune systems when they are on birth control because during pregnancy, women naturally want to be around family (and thus people with similar immune systems), where one feels safe and supported.[3]This poses a difficult question: if you are a woman on birth control, how do you know that you’re actually as attracted to a person as you think you are?
Ripe with attraction
Not only can human noses detect the viability of a potential mate’s immune system, but noses can also detect other non-physical traits like fertility. A study performed at the University of New Mexico recorded the amount of money in tips female lap-dancers made over the course of two months and compared their tips to their menstrual cycles. When the women were ovulating (the best time in a woman’s cycle for conceiving a child), they made significantly more money than when they were not ovulating or on their periods.[4]The evidence indicates that human noses can sniff out some incredible details that relate to attraction!
Do humans have pheromones?
So, we know that humans can detect information like fertility and immune system genes (the MHC), but the question remains: how do humans sniff this knowledge? Through what processes does this information travel from one human to another? A possible answer to this question is human pheromones. Pheromones, which we know other animals have and use, are chemical transmitters (much like hormones) that animals release outside of their body to communicate with other animals of the same species (whereas hormones only work within a single organism). Pheromones can communicate many types of information, but they’re well known for communicating sexual attraction. Right now, we do not know if humans release pheromones or not. Other mammals use an organ in the nose called the vomeronasal organ to detect pheromones, but in humans this organ is not functional. Researchers speculate that if human pheromones do exist and humans can detect them, we are using our olfactory receptors (receptors in our nose) to detect pheromones, rather than the vomeronasal organ.[5]
A study led by Martha McClintock and Kathleen Stern at the University of Chicago in 1998 presents evidence for human pheromones. The study demonstrated the now dubbed “McClintock Effect,” or menstrual synchrony, where when females spend a lot of time around each other, their menstrual cycles oftentimes sync up. If you’ve ever gone to summer camp or had a roommate, you may have experienced this yourself! The study made women smell each other’s sweat to achieve this effect.[6]Though these famous results point to the presence of human pheromones, the results of this study and the McClintock effect itself is a controversial topic.
Though the McClintock study claims that its results prove that pheromones exist, other studies claim that they were not able to demonstrate menstrual synchrony. Many researchers and doctors also agree that menstrual synchrony is a myth, and if you feel like you are experiencing synchrony with your roomie, it’s probably just a coincidence.[7]However, the fact that other mammals use pheromones is a hint that humans might as well. In any case, researchers have not given up on this mystery yet.
There are still many unanswered questions about how humans can detect genetic and sexual information about each other. If pheromones do exist, we still don’t know through which organs and physical processes humans process them, or which chemicals and compounds they are made of. The current direction that research in human pheromones is going is isolating organic compounds that could potentially be pheromones, exposing humans to these compounds, and then analyzing their effects on the body’s responses.[8]Hopefully in the coming years we will start to get some answers to these persisting questions.
Despite the remaining mysteries, it is evident that human attraction is not just based on how people look. In fact, a lot of attraction comes down to how people smell. Beauty, it seems, is not in the eye, but rather in the nose of the beholder.
References:
[1] (2016, Nov 4) Why every immune system is different.Retrieved from: https://www.umcg.nl/EN/corporate/News/Paginas/why-every-immune-system-is-different.aspx
[2] Smith, K. (2015, February 11). Why do some people smell better to you than others?Retrieved from: https://www.memd.me/people-smell-better-others/
[3] Wedekind, C., Seebeck, T., Bettens, F. & Paepke, A.J. (1995) MHC-dependent mate preferences in humans. Proceedings: Biological Sciences, 260(1359), 245-249. doi:10.1098/rspb.1995.0087
[4] Miller, G., Tybur, J.M. & Jordan, B.D. (2007) Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus? Evolution and Human Behavior 28, 375–381. https://doi.org/10.1016/j.evolhumbehav.2007.06.002
[5] Brennan, P.A. & Zufall, F. (2006) Pheromonal Communication in Vertebrates. Nature, 444,303-315. doi:10.1038/nature05404
[6] Stern, K. & McClintock, M. K. (1998) Regulation of ovulation by human pheromones. Nature 392,177–179. doi:10.1038/32408
[7] (2016, May 13) Myth or Truth — Does Your Period Really Sync With Close Friends? Retrieved from: https://health.clevelandclinic.org/myth-truth-period-really-sync-close-friends/
[8] Katsnelson, A. (2016) What will it take to find a human pheromone? Chemical & Engineering News 94(46) 23-25. Retrieved from https://cen.acs.org/articles/94/i46/take-find-human-pheromone.html
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