What a Handshake Smells Like

A new paper argues that the gesture may have evolved to allow people to judge one another's scent signals.

In 2010, to prepare its dealers for the launch of a new sales promotion, the U.K. arm of Chevrolet commissioned Geoffrey Beattie, a professor of psychology at the University of Manchester, to devise a handshake that would “offer peace of mind and reassurance to its customers.”

Here’s what Beattie came up with:

PH = √ (e2 + ve2)(d2) + (cg + dr)2 + π{(42)(4

2)}2 + (vi + t + te)2 + {(42 )(42)}2

(e) is eye contact (1=none; 5=direct) 5; (ve) is verbal greeting (1=totally inappropriate; 5=totally appropriate) 5; (d) is Duchenne smile - smiling in eyes and mouth, plus symmetry on both sides of face, and slower offset (1=totally non-Duchenne smile (false smile); 5=totally Duchenne) 5; (cg) completeness of grip (1=very incomplete; 5=full) 5; (dr) is dryness of hand (1=damp; 5=dry) 4; (s) is strength (1= weak; 5=strong) 3; (p) is position of hand (1=back towards own body; 5=other person's bodily zone) 3; (vi) is vigour (1=too low/too high; 5=mid) 3; (t) is temperature of hands (1=too cold/too hot; 5=mid) 3; (te) is texture of hands (5=mid; 1=too rough/too smooth) 3; (c) is control (1=low; 5=high) 3; (du) is duration (1= brief; 5=long) 3.

Chevrolet, the company announced that July, now had the mathematical formula for a perfect handshake, with 12 variables ranging from the sincerity of the shaker’s smile down to the ideal degree of sweatiness, though the simplified version that made its way into the staff training materials—dry palm, firm grip, eye contact—could also be described as “common sense.” But beneath the weirdness of the whole campaign, Chevrolet actually had a fair point. Hands offered forth for a shake—timid or strong, lingering or prematurely withdrawn—can reveal a lot, especially between people who might otherwise reveal very little to one another. Many historians believe that the handshake originated not as a greeting, but as, to borrow Chevrolet’s phrasing, something that would “offer peace of mind and reassurance” by allowing two people to prove that neither was carrying a weapon.

The custom of shaking hands may have evolved primarily as an excuse for people to judge each other by way of scent.

But new research proposes another theory: that the most important information relayed by a handshake isn’t conveyed through touch at all. Rather, according to a team of neurobiologists from Israel’s Weizmann Institute, the original handshake was all about smell. In a paper published last week in the journal eLife, the researchers argue that the custom of shaking hands may have evolved primarily as an excuse for people to judge each other by way of scent, gleaning information from the chemicals passed from palm to palm.

For the study, neurobiology researchers at Israel’s Weizmann Institute of Science secretly filmed 271 volunteers over the span of a few minutes as they interacted with a researcher, who either did or did not shake the subject’s hand upon entering the room. When the subjects were left alone, those who had shaken sniffed their hands more than twice as much as those who hadn’t—demonstrating, the authors said, that the shake itself had transmitted something worth sniffing.

Strangely, the researchers also noticed that the effect was strongly gendered. After shaking hands, people tended to sniff their right hands more if the researcher had been of the same gender, and the left hand if the researcher was the opposite gender. The study’s authors didn’t have an explanation for why this might be the case—but the important finding, they said, is that the sniffing happened at all.

“We can argue that this serves an evolutionary purpose,” said Idan Frumin, a Ph.D. candidate at the Weizmann Institute and the lead author of the study. “We don’t necessarily know what’s transferred, but we’re pretty sure that it’s relevant information. Otherwise we wouldn’t have found this behavior.”

To test whether a handshake alone was enough to transfer scent, the researchers first shook hands with 10 volunteers while wearing surgical gloves. When they analyzed the gloves to see what chemicals they had picked up from touching another person’s bare palm, they found three “chemosignals,” or chemicals thought to convey some kind of information about the person whose body created them. One study found that chemosignals in women’s tears, for example, may dampen sexual arousal in heterosexual men; another found that chemosignals in the sweat of people experiencing fear may inspire fear in those who smell it as well. (Frumin was also quick to clarify that the “chemosignals” transmitted by the handshake were different than “pheromones,” a contentious term among scientists studying smell. “Pheromones are species-wide—it’s the same chemical across the whole species,” he explained, while chemosignals are “specific signals that are unique to each individual, like an olfactory fingerprint.”)

After tallying up the amount of hand-sniffing that took place, Frumin and his colleagues measured nasal airflow on an additional set of volunteers to make sure the pattern wasn’t just nervous face-touching. (One study found that people touch their faces an average of 3.6 times per hour, while another put it as high as 15.7. Either way, it happens a lot.) When people put their hands near their noses, the researchers found, they tended to take in twice as much air as when they were breathing normally. “In other words,” the authors wrote, “when subjects brought their hands to their nose, they concurrently sniffed.”

But the handshake itself isn’t the evolutionary end game—after all, the gesture is the norm only in certain segments of the world. Rather, Frumin said, it’s one culturally specific means towards a more universal end: “In Western culture, it’s not very common to have close physical contact,” he explained. “The handshake is basically the only acceptable way of touching and getting chemicals passed between people,” he explained. A handshake does in one part of the world what a cheek kiss does in another. For example, the so-called “Eskimo kiss”—commonly portrayed as two people rubbing the tips of their noses together— is actually a misinterpretation of the kunik, an Inuit greeting in which one person presses the nose against another person’s skin and inhales.

In fact, the handshake and the kiss—used to convey very different things in the U.S.—may stem from the same place. “Many anthropologists believe that first ‘kisses’ may have been delivered via our noses rather than our lips, as we closely inhaled the scent of our loved one’s cheeks,” Sheril Kirshenbaum wrote in her book The Science of Kissing. “As humans developed better language skills, smell probably became less necessary for recognizing one’s relatives, but remained an important means to strengthen the bonds between people … Despite all the variability, affectionate nibbling and muzzling may be rooted in our common evolutionary lineage.”

If the eLife study’s findings are true, then the handshake is its own strange little study in contrasts: the go-to gesture for strangers and co-workers, also incredibly intimate; a practice governed by social norms, built on a basic biological instinct. A boardroom formality, boiled down to humans’ animal need to get to know each other. For all of Chevrolet’s complicated calculations, a perfect handshake may only be as good as the hand-sniffing that follows.