(photo credit: INIMAGE)
Shaking hands – mandatory in election season – may be a friendly way to spread one’s germs around, but the ancient custom seems to derive from the intention to check out another person’s odors. Even if we are not consciously aware of this, handshaking may provide people with a socially acceptable way of communicating via the sense of smell.
Neurobiologists at Rehovot’s Weizmann Institute of Science just published a study in the journal eLife showing that not only do people often sniff their own hands, but they do so for a much longer time after shaking someone else’s hand. The number of seconds the subjects spent sniffing their own right hand more than doubled after an experimenter greeted them with a handshake, according to research student Idan Frumin, whose research was guided by Prof. Noam Sobel.
“Our findings suggest that people are not just passively exposed to socially-significant chemical signals, but actively seek them out,” said Frumin. “Rodents, dogs and other mammals commonly sniff themselves, and they sniff one another in social interactions, and it seems that in the course of evolution, humans have retained this practice – only on a subliminal level.”
To find out whether handshakes indeed transfer body odors, the researchers first had experimenters wearing gloves shake the subjects’ bare hands and then tested the glove for smell residues. They found that a handshake alone was sufficient for the transfer of several odors known to serve as meaningful chemical signals in mammals.
“It’s well known that germs can be passed on through skin contact in handshakes, but we’ve shown that potential chemical messages, known as chemosignals, can be passed on in the same manner,” Frumin explained.
Next, to explore the potential role of handshakes in communicating odors, the scientists used hidden cameras to film some 280 volunteers before and after they were greeted by an experimenter, who either shook their hand or didn’t. The researchers found that after shaking hands with an experimenter of the same gender, subjects more than doubled the time they later spent sniffing their own right hand (the shaking one). In contrast, after shaking hands with an experimenter of the opposite gender, subjects increased the sniffing of their own left hand (the non-shaking one). “The sense of smell plays a particularly important role in interactions within gender, not only across gender as commonly assumed,” Frumin says.
The scientists then performed a series of tests to make sure the hand-sniffing indeed served the purpose of checking out odors and was not merely a stress-related response to a strange situation. They measured nasal airflow during the task and found that subjects were truly sniffing their hands and not just lifting them to their nose. It turned out that the amount of air inhaled by the volunteers through the nose doubled when they brought their hands to their face.
The scientists also found they could manipulate the hand-sniffing by artificially introducing different smells into the experimental setting. For example, when experimenters were sprayed with a commercial unisex perfume, the hand-sniffing increased. In contrast, when the experimenters were sprayed with odors derived from sex hormones, the sniffing decreased. These final tests confirmed the olfactory nature of the hand-sniffing behavior.
“Handshakes vary in strength, duration and posture, so they convey social information of various sorts,” concluded Sobel. “But our findings suggest that at its evolutionary origins, handshaking might have also served to convey odor signals, and such signaling may still be a meaningful, albeit subliminal, component of this custom.”