Handshaking may have originally been a sign of peace—in ancient times, for example, to show a fellow soldier that you held no weapon.
But scientists believe that the handshake may have evolved a social chemosignaling function. In other words, we sniff our hands after a handshake to acquire information about the person we’ve just met. It’s similar in more ways than one to animals’ social sniffing; in fact, the chemicals picked up during a handshake include squalene and hexadecanoic acid, both used in social signaling amongst dogs and rats. The difference is just that—at least by human standards—we’re more polite about it.
The study was conducted by neurobiologist Noam Sobel and his colleagues at the Weizmann Institute of Science in Rehovot, Israel. Sobel had previously discovered that the smell of a chemical contained in female tears makes men less sexually aroused. And other studies have shown that we can “smell fear” in people around us through the chemicals transmitted in sweat. Now, Sobel sought to illustrate what olfactory processes occur during one of the most common human greetings.
Sobel secretly filmed 271 participants so that they could observe behavior and measure airflow to the nose without subjects knowing this was the intent. Before a planned introduction, both men and women participants had one hand near their nose 22% of the time, on average. During that 22%, airflow to the nose doubled, indicating that the participants had actually smelled their hands.
After shaking hands with someone of the same sex, participants sniffed their shaking hands more than twice as much as before the handshake. After a handshake with a person of the opposite sex, participants typically smelled their shaking hand twice as much as before and their non-shaking hand more than twice as much. The study, published on the online journal eLife and free to read (check out the videos they’ve embedded!), suggests that after a handshake we actively sniff our shaking hand—even if we don’t realize it.
Here’s Catherine de Lange, writing for New Scientist:
It may seem counter-intuitive that the volunteers smelled their shaking hand more when they encountered someone of the same sex than the opposite sex, but that’s the wrong way to think about it, says Sobel. “We tend to think of social chemosignalling as a cross-gender story but it’s not.”
So depending on the situation, handshaking may give us subliminal information about social status, dominance, or health. And an individual’s hand-smelling behavior may be not only gender-specific, but sexual-orientation-specific, too. While chemosignalling is certainly not the only important function of the handshake, the authors of the study write that “given these effects, which we speculate are only the tip of the iceberg, humans likely evolved social chemosignal-sampling strategies and we propose that handshaking is one of them.”