What's in a handshake?

Remember when “firm” or “limp” were the only metrics by which a handshake could be evaluated?  Well, those simple days might be over.   Have you ever considered the molecules that are being transferred from you to your new acquaintance during this handshake?  Or, more disturbingly, from your new acquaintance to you?  And what if these molecules served some purpose in controlling how we respond or relate to this person?

A recent paper published in eLife poses a few of these questions through the investigation of handshaking as a behavior that facilitates chemosignaling between humans.  While shaking hands is considered a cultural norm, it could also be a way for us to convey information about ourselves to others without the hassle of using words.

As described in our earlier post “Butterflies or Butterlies”, the production and sensing of pheromones (or chemosignals) is one of the most fascinating forms of chemical communication.  Almost all vertebrates and invertebrates are believed to have evolved to perceive volatile molecules – whether to locate an apt mate, track down food or sense danger.   While it is assumed that pheromones are likely an aspect of human chemo-communication (possibly a made-up term), studying human systems is complicated because of our acute awareness of our own biology.  In other words, it’s pretty confusing to analyze “natural” human behavior or to even define “natural” when it comes to humans.   

The Sobel lab, a research group at the Weitzmann Institute, is interested in human chemosignals and their effects on behavior, and they have previously published research investigating the presence of and response to chemosignals packaged in human emotive tears.   This study, which is absolutely worth a glance, explored the effects of female emotional tears on the sexual arousal of male.  In short, to induce “negative emotion” tears the donor women “watch sad films in isolation” and the tears are collected in a vial.  These tears are then soaked onto a pad, which is placed under the nose of the male subject.  Ultimately, the conclusion of this study was that compared to a saline control, these emotive tears caused a decrease in “self-rated” arousal, various physiological measures of arousal, as well as testosterone levels in the male subjects.   While it’s somewhat comforting to know that men are not aroused by crying, it’s generally important to consider the biological relevance of this study.  Does it make biological sense for a male to be so proximal to female tears?  What is the physical range of this chemosensation?  Do we think this is truly an evolutionary adaptation?  Or do these tear-secreted compounds have another function unrelated to human-human signaling?

The recent study on chemosignaling and the handshake is, in my opinion, even more thought provoking than the previous study.  The experimental design is the following:  The volunteers are aware that they are participating in a scientific experiment, though they do not know the nature of the research.  Upon arrival each subject is greeted by the scientists using a standard greeting – the handshake.   The volunteers, thinking that they are still waiting for the experiment to begin, are then observed for several minutes and the frequency and duration with which they draw their hands to their face is recorded.  Researchers observed that after the handshake the subjects were significantly more likely to bring the hand-shaking hand close to their face, specifically in the vicinity of their nose.   Interestingly, this behavior was only true for interactions between people of the same gender.  Additionally, researchers attempted to confirm the olfactory (smell-related) nature of this interaction by “tainting” the experiment using perfume or putative male or female chemosignals.   These additives altered the sniffing behavior of the subjects suggesting that this behavior is indeed a result of olfactory sampling.

So basically, in shaking hands with a new acquaintance, we are receiving and sensing molecules from their skin surface, a behavior these researchers imply may be related to how we perceive or sense or respond to other human beings.  

This article really raises more questions than it answers.   What’s the deal with the gender specific behavior?  Are these molecules produced by us?  Or microbes on our skin?  What sort of physiological or chemical response do we have to these molecules? While clearly much (or all) remains mysterious, raising interesting questions and generating hypotheses is a great way to generate more research in the field.  

When I describe this study to my friends, acquaintances, colleagues, almost everyone recoils with disgust.  “Wtf! I would never do that!” is the general response.  And, honestly, I kind of feel that way too.   But maybe that’s what makes this behavior so interesting.  While it is always important to note that such human studies have significant error bars, and generally inadequate controls (despite an honest effort by the researcher), it is truly interesting to ponder how chemo-signaling can drive subconscious behavior.   Ultimately we are all slaves to chemistry in some form or another.