Handshakes may have evolved to help us sniff each other out, scientists believe.
A study has found that after the traditional greeting, people tend unconsciously to sniff their own hands.
The behaviour could be a subtle way to pick up and sample chemical scent signals, according to the researchers.
Professor Noam Sobel, chairman of neurobiology at the Weizmann Institute of Science in Israel, said: "It is well-known that we emit odours that influence the behaviour and perception of others but, unlike other mammals, we don't sample those odours from each other overtly.
" Instead, our experiments reveal handshakes as a discreet way to actively search for social chemo-signals."
During the study 280 people were greeted either with or without a handshake while being filmed with hidden cameras to see how many times they touched their face.
The team discovered that people constantly sniff their own hands after shaking, keeping a hand at their nose about 22% of the time.
Individuals greeted with a handshake significantly increased the amount of time they touched their faces with the right hand.
However, this only seemed to occur when someone was greeted by a person of the same gender.
Measurements of air flow through the nasal passages showed that it doubled when a hand was close to the nose - proving that the participants were actively sniffing.
Previous studies have suggested that human scent signals, or pheromones, could play a role in mate selection, conveying fear, altering brain activity and synchronising women's menstrual cycles.
In a further test, the scientists analysed sterile gloves used to shake the hands of volunteers. They found that two chemicals thought to be involved in social signalling in dogs and rats - squalene and hexadecanoic acid - were transferred on to the gloves.
Prof Sobel said: "Handshaking is already known to convey a range of information depending on the duration of the gesture, its strength and the posture used.
"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."
The research is published in the journal eLife.