Why do we shake hands? It may be to collect information about other people—by smelling them. A recent study has found that after a handshake, we often bring our own hands to our face in order to get a whiff of the other person. And we do it without even noticing.
Scientists believe the technique helps us exchange "social chemical signals" the way other animals do and might just be the reason why we started shaking hands in the first place.
"Handshaking is already known to convey a range of information depending on the duration of the gesture, its strength and the posture used," says neurobiologist Noam Sobel, one of the study’s authors. "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."
A press release on the study breaks down its methods:
During the experiment, around 280 people were greeted either with or without a handshake. They were filmed using hidden cameras and observed to see how many times they touched their face. One finding of the study was that people constantly sniff their own hands -- keeping a hand at their nose about 22% of the time. Subjects greeted with a handshake significantly increased touching of their faces with their right hand.
Subjects were found to touch their faces about twice as often after shaking hands than they usually do, and even more when greeting those of the same gender. That could be because we’re gauging dominance, rather than a mate.
To prove that people were actually sniffing rather than just inadvertently touching, researchers outfitted some subjects with nasal catheters that monitor airflow. Sure enough, those touching their faces right after a good shake were sniffing deeply.
The team also had subjects conduct grip-greetings wearing sterilized gloves to test for chemicals exerted during the practice. “The chemicals the gloves picked up from the experimenter's hand included squalene and hexadecanoic acid,” reports New Scientist, “both of which are involved in social signaling among dogs and rats.”
"I am convinced that this is just the tip of the iceberg," said Sobel, and previous studies on human chemical signaling suggest he’s right. Research conducted in 2009 found that the smell of sweat can trigger fear responses in humans. And a 2011 study found that the scent of women’s tears may lower a man’s testosterone levels, squashing arousal. It isn’t a huge leap, then, to believe that we pass on other signals through touching.
Now, just try not to think about sniffing your hand the next time you meet someone new. We bet it’ll be hard to do.