On April 1st, 2015 we learned about

Stealthy sniffing in social situations

You probably don’t realize it, but you’ve been sniffing your hand all day. No, it’s not that new hand-soap that’s got your nose excited, (even if that lavender scent is pretty refreshing.) You’ve actually been sniffing around for chemical information about your environment and the people you’ve come in contact with, even if neither you nor they were aware of it.

Lots of animals share information via smell. Think of cats rubbing their face against surfaces to leave pheromones, or less pleasantly, dogs peeing to mark territory. It’s even been discovered that humans can share things like a sense of fear through our sweat. So the concept doesn’t seem that far out there, but it’s just hard to imagine that we’re not more aware of sniffing our hands, especially after contact with new people.

Counting nasal contact

To really verify this, volunteers were recorded while meeting researchers. In addition to video, so that possible sniffs could later be tallied up, the participants’ breathing was being monitored. That way nose rubbing or scratching could be ruled out, and only hand-nose interactions that synced up with increased nasal airflow would be counted as a distinct sniff-event.

Looking at only the minute before and after a handshake, the totals were striking. Participants spent up to 22% of that time with their hand in range of their nostrils before contact with another person. In the instances where a handshake was made, that hand-nose time doubled after meeting someone of the opposite sex, and more than doubled when meeting the same sex.  There was some sense of decorum though, as people generally waited until the researcher had left the room before checking on the chemical calling-card they had just received, but even that seems to have been an unconscious adjustment (since nobody really remembers following this behavior pattern.)

It’s not yet known exactly what message is being delivered in these handshakes, but the researchers guess it’s something like when rats meet members of the same sex— a chance to assert some kind of dominance or pecking order is the most common interaction. This isn’t to say that this is the only way chemical communication is used by people, as it may be context-specific. But it does put a twist on the purpose of a handshake. Maybe a firm grip isn’t as good as a good sweaty palm if you really want to get a message across.

Source: After handshakes, we sniff people's scent on our hand by Catherine de Lange, New Scientist

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