People sniff their hands twice as much after a handshake to help them pick up chemical signals about others, according to a new study.
The number of seconds the subjects spent sniffing their own right hand more than doubled after an experimenter greeted them with a handshake, researchers wrote in the journal eLife.
“Our findings suggest that people are not just passively exposed to socially-significant chemical signals, but actively seek them out,” said Idan Frumin, the research student who conducted the study under the guidance of Professor Noam Sobel of Weizmann Institute’s Neurobiology Department.
“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,” said Frumin.
To examine whether handshakes indeed transfer body odours, the researchers first had experimenters wearing gloves shake the subjects’ bare hands, then tested the glove for smell residues.
They found that a handshake alone was sufficient for the transfer of several odours 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 said.
Next, to explore the potential role of handshakes in communicating odours, the scientists used covert 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 said.
The scientists then performed a series of tests to make sure the hand-sniffing indeed served the purpose of checking out odours and was not merely a stress-related response to a strange situation.
First, 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.
Next, the scientists found they could manipulate the hand-sniffing by artificially introducing different smells into the experimental setting.
These final tests confirmed the olfactory nature of the hand-sniffing behaviour.