at first whiff Women rely partly on smell when choosing friends College women smelled each other's T-shirts in new study to evaluate "friendship potential." Jennifer Ouellette – Apr 18, 2025 1:16 pm | 42 Credit: Universal Pictures Credit: Universal Pictures Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only   Learn more There are so many factors that can influence how we perceive others, which in turn can determine the people we choose as platonic friends or romantic mates. We certainly make snap judgments based on physical appearance, but scent can have a powerful influence, too. According to a new paper published in the journal Scientific Reports, two heterosexual women meeting for the first time rely partly on scent to judge whether they want to be friends with each other, deciding within minutes—practically at first whiff—whether there is friendship potential. Social olfactory research largely stems from evolutionary psychology, specifically the work of Swiss biologist Claus Wedekind in 1995. Subtle chemical signals from pheromones are known to play a role in attraction in many species. Scientists had already found evidence in fish and mice that the major histocompatibility complex (MHC) genes, which are critical for immune system function (and useful in determining tissue compatibility for transplants, for instance)—showed a marked preference for sexual partners with different MHC genes, perhaps as a way of keeping the gene pool well-mixed and protecting against inbreeding. Wedekind introduced the so-called "sweaty T-shirt" method to study the possible role of MHC in mate preferences in humans. Male participants wore the same T-shirt for two days, which were then placed in identical boxes. Women participants then smelled each shirt and indicated which ones they found most sexually attractive. Wedekind found that the women overwhelmingly preferred the T-shirt smells of men who had the most dissimilar MHCs to their own. The only caveat: The preference was reversed in women who were taking oral birth control. There have been many other smelly T-shirt studies since Wedekind published his findings, even inspiring the trend of "pheromone parties" for amorously inclined singles. The psychologists who authored this latest paper were more interested in studying the role of olfactory cues in platonic friendships. Smelly T-shirt studies often try to isolate a person's "natural" odor, devoid of scents from toiletries, pets, and other environmental factors. The Cornell team focused on so-called "diplomatic" odor, which incorporates all those other factors. Why can’t we be friends? “It’s not just perfume,” said co-author Jessica Gaby of Middle Tennessee State University. “It’s your dietary choices. Are you a cat person or dog person? What laundry detergent do you use? All these judgments come together into what we call ‘diplomatic odor.’ You live in this odor space–does it match with the odor spaces of the people you interact with?” For their study, Gaby et al. organized an on-campus "Speed-Friending" event for 40 female volunteers, consisting of four distinct phases. First, participants had their headshots taken. Next, they looked at pictures of all the other women participating and rated friendship potential based solely on visual cues. Then the women wore a T-shirt for 12 hours as they went about their daily activities, which were then collected and placed in plastic bags. Finally, participants rated the friendship potential of anonymized participants based solely on smelling each T-shirt, followed by a live session during which they interacted with each woman for four minutes and rated their friendship potential. This was followed by a second round of smelling the T-shirts and once again rating friendship potential. The results: There was a strong correlation between the in-person evaluations of friendship potential and those based solely on smelling the T-shirts, with remarkable consistency. And the ratings made after live interactions accurately predicted changes in the assessments made in the final round of odor-based testing, suggesting a learned response element. "Everybody showed they had a consistent signature of what they liked,” said co-author Vivian Zayas of Cornell University. “And the consistency was not that, in the group, one person smelled really bad and one person smelled really good. No, it was idiosyncratic. I might like person A over B over C based on scent, and this pattern predicts who I end up liking in the chat. People take a lot in when they’re meeting face to face. But scent—which people are registering at some level, though probably not consciously—forecasts whether you end up liking this person.” The authors acknowledged that their study was limited to college-aged heterosexual women and that there could be differences in how olfactory and other cues function in other groups: older or younger women, non-American women, men, and so forth. "Future studies might consider a wider age range, investigate individuals at different stages of development, focus on how these cues function in male-male platonic interactions, or examine how scent in daily interactions shapes friendship judgments in other cultures," they wrote. Scientific Reports, 2025. DOI: 10.1038/s41598-025-94350-1  (About DOIs). Jennifer Ouellette Senior Writer Jennifer Ouellette Senior Writer Jennifer is a senior writer at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. Jennifer lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban. 42 Comments