Signaling molecules called chemosignals have been implicated in everything from the conveyance of fear to the lure to mate, and even to the synchronizing of women's menstrual cycles. Now researchers out of Israel have written in the journal eLife that one of the reasons we shake hands is to pass these chemosignals between one another. "We emit odors that influence the behavior and perception of others, but, unlike other mammals, we don't sample those odors from each other overtly," one researcher says. "Instead, our experiments reveal handshakes as a discreet way to actively search for social chemosignals."
To test this, neuroscientists had 280 people greeted either with or without a handshake, as per a press release; hidden cameras then helped tally how many times people touched their faces after handshakes. Those who received a handshake from someone of the same gender were far more likely to sniff their own hand, while cameras also revealed that we are constantly sniffing our own hands, with one at our nose roughly 22% of the time. The scientists then used sterile gloves to shake hands and found that squalene and hexadecanoic acid, which are involved in social signaling in dogs and rats, were present on the previously sterile gloves. Handshaking "may have evolved to serve as one of a number of ways to sample social chemicals from each other," says the researcher, and "it still serves this purpose in a meaningful albeit subliminal way." (Chemosignals aren't the only things transferred in a firm handshake.)