Have you ever met a perfect stranger and felt like you 'clicked' almost immediately? Well, that feeling of chemistry might actually be chemistry.
An Israeli study recruited twenty pairs of same-sex, platonic friends who said they clicked when they met.
Using an electronic nose (and volunteers who heroically agreed to sniff T-shirts worn overnight), the study concluded that the pairs of friends smelled more similar than randomly paired strangers.
Plenty of other mammals sniff each other to decide whether to play with, attack, or run away from a strange animal.
"In humans, the role of olfaction has been denigrated in part because of various social taboos, culminating in the view that olfaction is unimportant for human sociality," the researchers wrote in the paper.
But what if perfect strangers "begin to interest us at first sniffs rather than at first sight alone?", the neuroscientists asked. After running several experiments to rule out alternative explanations, they concluded that "there is indeed chemistry in social chemistry".
In the main experiment, 20 pairs of friends were asked to wash with unscented soap, avoid foods like curry and garlic, and sleep in a separate bed from their partner.
They were instructed to wear a fresh cotton T-shirt each night for at least six hours and then put the shirt in a resealable plastic bag. The shirts were then frozen and thawed out an hour before the sniffing experiments.
The body odor samples were run through a compact gas sensing device called PEN3 eNose, which contains a gas sampling unit and a sensor array. A simple graphing technique comparing five different sensor responses was then used to determine similarities between odors.
In another test, 25 'smellers' were recruited to compare odors using a Shirt Sniffing Device (a glass jar with the T-shirt inside connected to a mask that makes sure no other smells interfere with the test).
The smellers examined two odor samples at a time, some of which belonged to random pairs of people, while others were from friends who clicked.
In this experiment, the volunteers and the eNose both detected more similarities between the friends than the strangers.
Not all of the experiments gave such clear results, though. When 24 volunteers compared three odor samples – two from friends and one from a stranger – they failed to identify the friendships as easily as those comparing just two samples.
"We think that the task was maybe just too hard," says neuroscience PhD student Inbal Ravreby, who was the lead author on the paper.
The volunteers were also asked to rate the 40 body odors from friend pairs by pleasantness, intensity, sexual attraction, competence, and warm personality. When the researchers combined all five scores, the pairs who clicked had been rated as more similar than random pairs.
And that's not where the research ended, either. The team also wanted to test if they could use smell similarities between complete strangers to predict whether they would click when they first met.
To do this, they recruited volunteers who had never met before and asked them to play the silent 'mirror game', in which participants try to copy each other's hand movements at close range for two minutes without speaking.
The researchers did this experiment as a round robin, producing 66 different pairings; one third of the pairs said they clicked with their partner. After running their odor samples through an eNose, the pairs that clicked were significantly more chemically similar than the pairs that didn't click.
"I think this may be the strongest result as it shows that we can predict clicking with 71 percent accuracy," Ravreby told ScienceAlert.
Previous studies have suggested that smelling a stranger activates the amygdala, the fear center of the brain, whereas smelling a friend doesn't.
We also know that people born with no sense of smell are often disadvantaged in many social situations because of their condition; meanwhile, people with autism spectrum disorder ( ASD) have altered social chemosignaling. (In one study, people without ASD responded to skydiver sweat by becoming more alert, whereas those with ASD had the opposite reaction.)
COVID-19 causes a very small proportion of people to lose their sense of smell long-term. It is hard to say what impact this might have on social interactions, says Ravreby.
"In many cases, it is not a total anosmia but partial, where people can report on some smells but not on others," she said. "I would guess that body odors will keep playing a role."
The study was published in Science Advances.