In A Nutshell
Unlike animals that directly sniff each other, humans are more discreet. After shaking hands with another person, we raise our hands to our faces and smell the scent without realizing it. Researchers believe it’s an unconscious form of chemical communication. Earlier studies found that the scent of a woman’s tears turns off men and the smell of sweat can signal fear.
The Whole Bushel
Unlike animals that directly sniff each other, humans are more discreet. After shaking hands with another person, we raise our hands to our faces and smell the scent without necessarily realizing it. Scientists believe it’s an unconscious form of chemical communication.
To test their theory, researchers from the Weizmann Institute of Science in Israel secretly filmed about 280 people greeting a member of the research team with or without shaking hands. Before shaking hands, participants had at least one of their hands around their nose about 20 percent of the time. However, after a handshake with someone of the same sex, participants had the hand used in the handshake close to their nose more than 40 percent of the time.
Surprising, though, the time spent sniffing the shaken hand remained at 20 percent when participants greeted someone from the opposite sex. That seemed counterintuitive because earlier studies have shown that men and women often assess their potential mates by smell. But what was really strange was that people greeting someone of the opposite sex doubled the amount of time their unshaken hand was near their nose. The researchers really couldn’t explain that other than to guess that it might be a reassurance mechanism of some kind.
In a follow-up study, 153 participants were secretly filmed shaking hands while wearing nasal catheters that could measure how much air they were sniffing without their knowledge. The increased airflow showed that people were actually smelling their hand after a handshake, not just brushing their face or scratching their nose or doing something else.
“Handshaking is already known to convey a range of information depending on the duration of the gesture, its strength and the posture used,” said lead researchers Noam Sobel. “We argue that it may have evolved to serve as one of a number of ways to sample social chemicals from each other, and that it still serves this purpose in a meaningful albeit subliminal way.” Basically, he’s saying we sniff each other like dogs. We’re just more sneaky about it. Scientists disagree about why we do it, however.
The researchers also performed an experiment where participants wore sterile gloves to see what types of chemicals were being transferred during a handshake. They discovered two chemicals, palmitic acid and squalene, which are used as social signals by dogs and rats. This shows that we shake hands as a form of chemosignalling behavior, even if we’re doing it unconsciously. Whether it’s to establish dominance, synchronize menstrual cycles or signal something completely different, the researchers just don’t know yet.
An earlier study by this group showed that the smell of a woman’s tears is a sexual turnoff for men. It didn’t matter whether the men were in a happy or sad mood or even if they were viewing movies that were sexually arousing. By testing the men sniffing saline or actual tears on cotton pads, researchers found that the scent of a woman’s tears in each case reduced the level of sexual arousal in the men who participated in the study. The level of arousal was measured by the amount of testosterone in saliva and through patterns of brain activity recorded by an fMRI scanner.
An experiment by another group in 2009 suggested that the smell of our sweat changes when we’re terrified, causing other people who smell it to change their behavior as well. “Stress sweat” made people more alert to threats, causing them to be correct 43 percent more often when deciding if someone’s face was threatening or nonthreatening.
The US military funded some of this research, which raises concerns about how it will be used. However, one psychiatrist said that the active chemical in stress sweat would probably not cause mass panic if released in a crowd of people.
Show Me The Proof
The Guardian: Palm scent: the science of smelling after a handshake
New Scientist: After handshakes, we sniff people’s scent on our hand, The scent of a woman’s tears wards off men, Scent of fear puts brain in emergency mode