Love at first sniff: Humans form friendships over similar body odours

A team of researchers in Israel have discovered that people may have a tendency to form friendships with individuals who emit a similar body odour. The researchers were even able to foresee the quality of social interactions between complete strangers by initially ‘smelling’ them with an electronic nose device (eNose).

The study was published recently in Science Advances. It suggests that the sense of smell may be of bigger significance to human social interactions than previously believed.

Dogs often sniff each other when they meet, quickly deciding if their encounter is one of friendship or a challenge for domination. The sense of smell often plays an important role in social interactions in terrestrial mammals, but there have not been studies that focused on humans and their sense of smell.

“Olfaction is a dominant sensory input underlying social interaction. This statement is largely acknowledged with respect to other terrestrial mammals, but it is often rejected with respect to humans,” the researchers write. “Recent evidence, however, implies a significant role for olfaction in human social interaction.”

Do humans not use their noses in social settings, unlike dogs and other terrestrial mammals? Or does the sense of smell play a more covert – and less conscious– role in humans?

Inbal Ravreby, a graduate student working in Professor Noam Sobel’s laboratory in Weizmann’s Brain Sciences Department, posited that humans, too, rely on their sense of smell when determining whether someone is a friend or foe, albeit discreetly.

“Nonhuman terrestrial mammals sniff themselves and each other to decide who is friend or foe. Humans also sniff themselves and each other, but the function of this is unknown,” the authors write.

Moreover, people, when socialising, lean towards becoming friends with others who share similar backgrounds, values, appearance, and even such measures as brain activity. Ravreby hypothesised that people, when subconsciously sniffing themselves and others, may be making subliminal evaluations, after which they selected those with similar body odours.

In order to test her hypothesis that people trust their sense of smell when forming friendships, Ravreby drafted same-sex nonromantic friends, click friends whose friendships had initially formed very quickly. She speculated that because such friendships emerge before the pair really knew each other in detail, they may be affected by physiological characteristics such as body odour.

Ravreby collected body odour samples from the click friends and carried out two sets of experiments to contrast the samples with those collected from random pairs of individuals.

In one set of the experiments, she made use of eNose to compare the chemical signatures of the odours. In the second set, she asked the pairs that she drafted to smell the two groups of body odour samples to determine similarities measured by human perception.

In both types of experiments, click friends were determined to smell significantly more like each other than did the individuals in the random pairs.

“[C]lick friends have greater similarity in body odour chemistry and in body odour perceived smell in comparison to random dyads,” the researchers write. 

“There are at least three alternative explanations for this: First, consistent with our hypothesis, this similarity may be related to the root causes of friendship. Second, and alternatively, this similarity may somehow be a consequence of long-term friendship, following common body odour–shaping experiences, eg, living in the same area, eating together, etc. Last, we acknowledge that this similarity may be related to some independent unknown factor and that this same unknown factor may, in turn, be driving friendship.”

To rule out the first two alternatives, they carried out another study in which they “tested whether similarity in body odour as determined by eNose can predict the quality of social interaction between complete strangers.” They had the strangers carry out a mirror game for two minutes without talking, then asked them how much they liked the other person.

The researchers wrote they “observed that dyads who reported clicking were significantly more chemically similar than dyads who did not report clicking” as measured by the eNose device. 

They also were able to predict which pairs would like each other with 71 percent accuracy when they fed the eNose data into a computational model.

The discussion section of the paper notes: “General similarity within a dyad is indeed a strong positive predictor of friendship, which acts at the very early phase of interaction. Here, we add the observation that body odour similarity may also support rapid friendship formation.”

“These results imply that, as the saying goes, there is chemistry in social chemistry,” Ravreby concludes.

Sobel clarifies: “This is not to say that we act like goats or shrews – humans likely rely on other, far more dominant cues in their social decision-making. Nevertheless, our study’s results do suggest that our nose plays a bigger role than previously thought in our choice of friends.”