Dogs greet each other using their noses, sniffing each other from bow to, above all, stern. Men are not that open to the idea of smelling each other, but the turnover of the perfume industry suggests that fragrance is also important in relationships between humans. Furthermore, there is evidence that men are able to identify a relationship, infer an emotional state and even detect a disease through the sense of smell.
Now Inbal Ravreby, Kobi Snitz and Noam Sobel of the Israeli Institute of Sciences have taken a further step: they believe they have shown, even within a rather small sample, that friends smell similar and that this phenomenon probably occurs. from the first contacts. This means that people choose friends, at least in part, on the basis of body odor, and not that people’s odors converge after they become friends.
As they write in Science Advances, Ravreby, Snitz and Sobel started their research by analyzing the smells of twenty pairs of same-sex friends who are tied by a solid, non-romantic relationship. To do this they used an electronic nose, e-nose, and two groups of human “sniffers”. The e-nose used metal oxide sensors to examine the T-shirts the participants were wearing, while the first group of human sniffers were tasked with determining whether the T-shirts smelled similar. The sniffers of the second group classified the odor of clothing on the basis of five subjective categories: pleasantness, intensity, sexual attractiveness, amplitude and warmth. The results of the e-nose and the opinion of the second group of sniffers were then subjected to a multidimensional mathematical process, to reduce them to simple and comparable numbers. All three processes produced the same result: friends’ shirts smelled more like that of strangers.
Friends therefore really smell similar. But is it friendship that causes a similarity of smells or is it the similarity of smells that causes friendship? Ravreby, Snitz and Sobel tested the e-nose to see if it is possible to predict positive interactions between strangers, that kind of “click” that immediately goes off at the start of a new friendship. To do this, they selected 17 other volunteers who were asked to wear some T-shirts to detect their body odor. Then the smells were analyzed by the e-nose, while the participants were asked to try their hand at a game: silently reproducing the hand movements of another individual.
Participants were divided into randomly selected pairs, and their reactions recorded. After each interaction they indicated how close they felt to their playmate by superimposing two circles (one representing themselves, the other their partner) on a screen. The more similar the two electronic olfactory signatures were, the more the overlapping of the circles coincided. Participants then rated the quality of their interaction based on twelve subjective emotional categories that define friendship. Similar smells corresponded to a positive rating in nine of these twelve categories. Interestingly, two participants with a similar smell weren’t necessarily more adept at the imitation game than the others, as evidenced by hidden camera recordings.
Why smell plays an important role in forming friendships is still unclear. Other qualities related to friendship, including age, appearance, education, religion, and race, are immediately obvious or become so quickly. But while some individuals have strong, recognizable body odor, for many it doesn’t, at least since the widespread use of soap. The smell is present, but it acts on a subliminal level. Dr. Ravreby speculates that there may be “an evolutionary advantage in having friends who are genetically similar to us.” Body odor is linked to the genetic heritage, in particular to the genes that are part of the important histocompatibility complex. Smelling others, as a result, could lead to subconscious inferences about genetic similarity.
But this is not enough to find an answer to the original question. Ravreby speculates that the similarity of smells may represent an extended form of parental selection. It is the mechanism that allows the spread of an individual’s genes not directly through their own reproduction, but by favoring that of relatives with similar genes. If people with similar smells are related enough to trigger this mechanism, so will their children. “By helping friends,” explains Ravreby, “we contribute to the spread of our genes”.
(Translation by Andrea Sparacino)