Limp or firm, your handshake conveys subliminal social cues. Now, research reveals it also transmits chemical signals that could explain why the greeting evolved in the first place.
In the study, published in the journal eLife, scientists from Israel's Weizmann Institute of Science found that people use the touch of a handshake to sample and sniff signalling molecules.
During the experiment, around 280 people were greeted either with or without a handshake. They were filmed using hidden cameras and observed to see how many times they touched their face. One finding of the study was that people constantly sniff their own hands -- keeping a hand at their nose about 22% of the time. Subjects greeted with a handshake significantly increased touching of their faces with their right hand. However, this only seemed to be the case when the subject had been greeted by a person of the same gender.
To check that the observed face-touching was being used as a way to subtly sniff the hand used in handshaking, subjects were fitted with nasal catheters to measure airflow. They found that when a hand was in close proximity to the nose airflow through the nasal passages doubled. In other words, the subject was sniffing.
“It is well-known that we emit odours that influence the behaviour and perception of others but, unlike other mammals, we don't sample those odours from each other overtly,” says Professor Noam Sobel, Chair of Neurobiology at the Weizmann Institute of Science.
"Instead, our experiments reveal handshakes as a discreet way to actively search for social chemosignals," he said.
Previous studies have suggested that human chemosignals play a role in mate selection, conveying fear, altering brain activity, and synchronising women's menstrual cycles. To confirm that handshaking is effective at passing on this type of chemical, the scientists analysed the content of sterile gloves used to shake the hands of the subjects. They found that squalene and hexadecanoic acid, both chemicals thought to play a part in social signalling in dogs and rats, were transferred onto the gloves.
"Handshaking is already known to convey a range of information depending on the duration of the gesture, its strength and the posture used," says Professor Sobel. "We argue that it may have evolved to serve as one of a number of ways to sample social chemicals from each other, and that it still serves this purpose in a meaningful albeit subliminal way."
We spoke to Idan Frumin, PhD student at the Department of Neurobiology, Weizmann Institute of Science, and lead author about the group's work.
1: What kind of signals could be passed between humans via a handshake?
IF: We can only speculate, drawing from previous research on human chemosignaling. For instance, we know from research by Martha McClintock in the 70s that women tend to synchronize their menstrual cycles. A signal may be transferred between women to drive that effect. We also know that the choice of a mate is influenced by a person having a compatible immune system. Such data could potentially be transferred skin-to-skin contact via a handshake. But these are speculations and a more rigorous analysis is needed to explore these possibilities.
2: Before the handshake became a staple of boardroom etiquette, could it have evolved to serve a more primitive function?
IF: We suggest exactly that. It is quite possible that “boardroom etiquette” is actually a later explanation for an early type of behaviour that appeared in order to compensate for scarcity of physical contact between human individuals. As we don’t overtly sniff each other like other mammals, we needed a way to pass olfactory data between our peers. The handshake might be the best way to convey chemosignals.
3: Do the findings contribute to the debate about the presence of a vomeronasal organ in humans?
IF: Not at all. We don’t delve into the argument of whether people have or don’t have a functioning vomeronasal organ (VNO). Even without adequate evidence for the presence of such organ, a host of human chemosignaling effects suggest that we do communicate chemically. Other labs and ours’ have demonstrated that human secretions and synthetic molecules can influence human behaviour and physiology. Whether fear sweat or arousal-reducing tears, the effects exist whether there is or there isn’t a VNO. Our novelty here is in the way of conveying these signals, and we don’t discuss the specific chemosignaling subsystem of perceiving them.
4. What was it about the experiment’s setting that could have made within-gender sniffing more frequent, while not having a significant impact between genders?
IF: First, I should stress that it was not a case on "no impact" between genders. In fact, between genders, there was a significant reduction from baseline – in contrast to the increase within gender. This too is a meaningful result.
Beyond this, we cannot be sure that these interactions are a staple of the effect we found – it is well possible that in various settings a different kind of interaction will arise. For example, in a different setting, the direction of the effect might change. The main idea here is that there is an effect and that it is controllable by the type of greet (handshake or its absence) and also by olfactory cues. Another main idea here is that people constantly touch their own faces while sniffing, regardless of the manipulation we employed.
5: What might have to change to increase the frequency of sniffing between genders?
IF: One can speculate on various possibilities. For example, a gap in social status between the participants might play a role. In our experiment, subjects were mostly of equal age and potential status. Also, sexual orientation rather than gender alone may be a factor.
Although our experimental design protected against it, we cannot exclude the possibility that a specific individual experimenter could somehow drive excessive sampling. Finally, the particular setting might significantly impact behaviour. Our setting was quite formal and a more social setting might produce a different result.
6: What most surprised you about the findings?
IF: Without hesitation: The level of baseline olfactory hand exploration that occurs with people being totally unaware of it. Subjects "doing nothing" had a hand at their nose more than 20% of the time. If we look at rats and mice sniffing themselves all the time, it looks natural to us. It’s strange to think about but it’s possible that we are also sniffing ourselves a lot of the time.
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