Scent a deal? Not always Saul Loeb/AFP/Getty Images

You won’t believe you do it, but you do. After shaking hands with someone, you’ll lift your hands to your face and take a deep sniff. This newly discovered behaviour – revealed by covert filming – suggests that much like other mammals, humans use bodily smells to convey information.

We know that women’s tears transmit chemosensory signals – their scent lowers testosterone levels and dampens arousal in men – and that human sweat can transmit fear. But unlike other mammals, humans don’t tend to go around sniffing each other.


Wondering how these kinds of signals might be exchanged, Noam Sobel and his colleagues at the Weizmann Institute of Science in Rehovot, Israel turned to one of the most common ways in which people touch each other – shaking hands. “We started looking at people and noticed that afterwards, the hand somehow inadvertently reached the face,” says Sobel.

To find out if people really were smelling their hands, as opposed to scratching their nose, for example, his team surreptitiously filmed 153 volunteers. Some were wired up to a variety of physiological instruments so that airflow to the nose could be measured without them realising this was the intention.

Take a good sniff

The volunteers were filmed as they greeted a member of the team, either with or without a handshake. The researchers recorded how often the volunteers lifted their hands close to their nose, and how long they kept them there, the minute before and after the greeting.

Before the greeting, both men and women had their hand near their nose 22 per cent of the time, on average. Airflow in the nose more than doubled at the same time, suggesting they were smelling their hands.

After shaking hands with someone of the same sex, on average volunteers sniffed their shaking hand more than twice as much as they did before the handshake. If the person was of the opposite sex, they smelled their non-shaking hand twice as much as before the handshake. This usually happened once the experimenter had left the room.

The team also carried out the experiment with people wearing sterile gloves. The chemicals the gloves picked up from the experimenter’s hand included squalene and hexadecanoic acid, both of which are involved in social signalling among dogs and rats.

“People constantly have a hand at their face, they are sniffing it, and they modify that behaviour after shaking hands. That demonstrates that the handshaking is a chemosignalling behaviour,” says Sobel.

Just like rats

It may seem counter-intuitive that the volunteers smelled their shaking hand more when they encountered someone of the same 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.” There are plenty of instances where signalling happens within the same sex, he says, such as women synchronising their menstrual cycles or rodents sniffing out dominance. The behaviour could also be context-specific, he suggests. In a bar, for example, the pattern might be reversed.

“I am convinced that this is just the tip of the iceberg,” says Sobel. “This is just one more instance where chemosignalling is a driving force in human behaviour.” One surprise was just how much the volunteers were smelling their hands. “When we were coding the videos we would see people sniffing themselves just like rats. It’s like blindsight – you see it all the time but you just don’t think of it.”

Charles Wysocki at the Monell Chemical Senses Center in Philadelphia agrees. “It fits with the general idea that there is a lot more chemical communication going on that we are unaware of”.

As well as trying to work out exactly what sort of information might be transmitted, Sobel’s team is now looking at how chemosensory signalling through handshaking might be affected in behavioural conditions such as autism spectrum disorders.

Journal reference: eLife, DOI: 10.7554/eLife.05154