Serendip is an independent site partnering with faculty at multiple colleges and universities around the world. Happy exploring!

The Chemicals Underlying Physical Attraction

kdillard's picture

“It was love at first sight.”  Society would have us believe that true love can, indeed, be found upon first laying eyes upon someone.  It has been seen countless times in literature, most notably with Romeo and Juliet. Maybe thislove at first sight is possible but is beyond our conscious control. What would Romeo and Juliet say if we were to be able to tell them that they did not consciously choose one another but rather were merely responding to the “body odors” of the other person?  Like many people, I have often wondered why my friends are attracted to certainpeople.  I have never been able to understand what about that other person was appealing to my friend.  Such is the mystery of physical attraction and how much it varies from person to person.  With some people finding certain traits, such as a hairy body and pale skin, attractive while others find those same traits completely unappealing, it is clear that there is no simple way of classifying particular traits or characteristics as universally desirable.  Indeed, when we consider ideas ofbeauty in several different cultures, it becomes even harder to come up withuniversally attractive features. For example, traditional Mexican men like their women to be fat while intraditional China women crippled their feet in an attempt to become more beautiful.  While it is relatively easy to appreciate that the culture a person grows up in and the type of people they are surrounded by greatly impacts what they find physically attractive, itis much more difficult to accept that there are certain chemicals that underlieour attraction to certain people that is largely beyond our control.  I have always believed that I am incomplete control of what and whom I find physically attractive and have, onoccasion, been very surprised to discover what others are attracted to. If Iwas not in complete control of whom I was attracted to and there was some underlyingchemical that was responsible for my attraction, then wouldn’t all human females be attracted to similar qualities?  In addition, if pheromones do exist, to what degree do theyimpact behavior in ways that people do not suspect?  To what degree do they cause people to make choices in lover relationships that could determine the entire direction of their lives?

Pheromones are defined as “substances whichare secreted to the outside by an individual and received by a secondindividual of the same species, in which they release a specific reaction, forexample, a definite behavior or a developmental process” (Karlson and Luscher,1959).  In addition, pheromones aredirectly related and play a key role in mating behavior.  Many animals use these chemical signalsto identify possible mating partners and, in the case of female moths, are able to attract a mate from many miles away. As such, it is clear that pheromones are not to be underestimated.  They are capable of significantlyaltering behavior of both males and females and are able to act over very long distances. 

The chemical signals from pheromones arereceived and processed by the VNO, or vomeronasal organ.  The VNO is located bilaterally on theanterior nasal floor of the nasal septum, which opens into the nasalcavity.  All lower mammals posses aVNO, however, there is much debate over its presence and function inhumans.  The VNO is characterized as a vestigal organ, since there is no evidence for any anatomical connectionbetween the VNO and the brain.  In addition, the VNO is not present in all humans (Pines, Maya).  However, this absence of a VNO does not mean that pheromones do not exist and that humans are not susceptible to theirpowerful effects.  There are currently two strong candidates for human pheromones.  The first is Androstadienone (AND), a compound found in male sweat.  When AND comes in contactwith the olifactory mucosa, several psychological and physiological effectshave been observed, including its activation of the hypothalamus, theprefrontal cortex, and the superior temporal cortex in females.  These physiological changes have theninduced phycological changes, such as triggering sexual arousal and ovulation (Stern, Kathleen).  In contrast,the other possible candidate for a human pheromone is EST, an estrogen derivative found in female urine, and is also sex-specific.  EST triggers increased blood flow to the hypothalamus of heterosexual men. Although there is evidence that humans respond to sex-specific chemicalsignals, these chemical signals cannot be classified as pheromones since the majority of humans lack a VNO.  In addition, in studies both heterosexual women and homosexual men experienced increased blood flow to the hypothalamus when they were exposed to AND whilehooked up to a PET, which monitors blood flow to certain parts of thebrain.  Heterosexual men did notrespond to AND.  In addition,neither group responded to EST.  

In addition to pheromones, humans possesshuman leukocyte antigens (HLA’s), genes that assist in odoridentification.  Studies have beendone that show that females are attracted to males that have a different HLAprofile than their own.  Thisattraction to those with different HLA’s makes sense from an evolutionary standpoint.  If females and males are attracted to those who are genetically different, which they are able to subconsciously tell through odor, then there would be much greater genetic diversity within that population.   

This issue becomes one more example of the ancient debate between determinism and freedom.  It seems oxymoronic thathumans are still susceptible to pheromones even though we no longer possess the VNO, the region that processes these chemical signals and sends them to thebrain, resulting in behavioral changes. However, if we no longer possess this region, then how can we be susceptible to certain chemical signals,unless the VNO, served merely to amplify a signal, or modify it in someway.  Also, if we are stillsusceptible to these chemical signals, which, experimentally, seems to be true,then why did the region critical for their processing become vestigial?  Maybe, with the regression of the VNO,humans gained more conscious control over their own behavior or maybe the VNOwas just an evolutionary backup, in case the primary chemical processor failed.  It is interesting to thinkabout the possible difference in our species if we still possessed the VNO.  Would evolution have selected differently, had the selection process been much more tightly regulated?  Would humans, on the whole, be taller?Larger? More susceptible to diseases? Finally, if pheromones do exist, how much control do we actually haveover whom we are attracted to? 



Broad, Kevin."More to pheromones than meets the nose." Nature 11 (2008):128-29.


Pines,Maya.  “A Secret Sense in the HumanNose: Pheromones and Mammals.”  HowardHughes Medical Institute. 


Stern, Kathleen and McClintock, Martha.  “Regulation of ovulation by human pheromones.”  Nature  392 (1998): 177-179. 




Paul Grobstein's picture

pheromones and life

"one more example of the ancient debate between determinism and freedom"
Or, maybe, a new set of observations that help us to better appreciate the relation between the two? Maybe we are attracted to things without knowing why we are attracted to them, and that's a starting point for new explorations into ourselves from which, in turn, we acquire greater ability to influence our own choices? Whether or not the VNO is vesitigial?