The Centers for Disease Control and Prevention (CDC) says one of the many ways you can reduce your risk of catching Covid-19 is to "avoid touching your eyes, nose, and mouth." It perhaps sounds like the easiest of all their guidelines to follow, but as a couple of weeks of lockdown reveals, this seemingly simple advice is surprisingly hard to stick to.
Have you ever wondered why? Well, it might be that we’re subconsciously trying to sniff ourselves, according to a hypothesis proposed by a team of neurobiologists from the Weizmann Institute of Science in Israel. Writing in the journal Philosophical Transactions of the Royal Society B, they speculate that touching our own face is linked to the human tendency to smell ourself, a tool we still use today to understand our surroundings, other people, and ourselves.
To kick off their investigation, the researchers carried out an online survey involving more than 400 people from 19 countries (many in Europe and North America) and asked them about their sniffing habits. Around 91 and 94 percent reported smelling their own hands and armpits, respectively. A further 55 percent said they had smelled their hand after placing it in their armpit, while 73 percent of men and 55 percent of women said they smelled their hand after placing it near their crotch. Up to 94 percent of people also reported sniffing their romantic partner and 60 percent of respondents said they sniff strangers.
The researchers also studied video footage of people watching a lecture and noticed a “large proportion of the audience” dabbling in hand-to-nose contact, with many displaying the “unmistakable action of sniffing.” By their workings, humans touched their face dozens of times an hour, and touched near their nostrils around 5 to 7 times per hour.
"We conclude from all this that although in our view most of olfactory social sniffing is subconscious, upon being asked, humans are nevertheless quite aware of engaging in this behaviour," the researchers write.
So the theory goes, there are evolutionary advantages to keeping tabs on how our hands smell. For one, it can provide reminders of where our hands have been, such as a dirty bacteria-ridden place, and other information about our surroundings. They go as far as to argue that self-sniffing could be a form of “social chemosignalling” used to suss out other people we came across.
To test out this idea, the researchers used gas-chromatography mass-spectrometry to show that shaking hands was enough to transfer smell-giving particles, known as volatile organic compounds, from one hand to another. They next got human participants to shake hands with a researcher, then covertly recorded and quantified hand-to-nose contact before and after handshaking, observing that the rate of self-snigging significantly increased after handshaking.
“After observing the extent of human hand-sniffing, it occurred to us that this behavior may provide information not only regarding one's own body, but also regarding the bodies of others,” they continued.
Granted, this research has a few limitations, as well a leaps in logic that need further research to back up. As just one of many examples, the researchers were not able to identify whether the face touching was an itch or truly a subconscious attempt to sniff ourselves. After all, it could be argued that the face is far more sensitive than, say, your shoulder, so you would expect more scratching and incidental touching.
There’s surprisingly little scientific literature on the subject of self-sniffing, which the team suspect is due to social taboos about self-sniffing appearing to be animalistic.