Exposure to Chemical from Babies Linked to Aggression

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Pheromones famously trigger the olfactory system in animals, and have been linked to mating and aggressive behavior. For example, compounds in mouse urine can induce male mice to fight each other, and a rabbit mother will attack her own offspring if she smells a different female rabbit, according to Science. However, the presence of pheromones in humans has not been confirmed. In a study published in Science Advances on November 19, scientists identified a compound known as hexadecanal that seems to increase aggression in women who smell it but suppress aggression in men.

Eva Mishor, a study coauthor and neuroscientist at the Weizmann Institute of Science in Israel tells New Scientist, “Our study gives more power to the notion that humans communicate from the chemical volatiles they emit, and that we get lots of information from them.”

See “Do Human Pheromones Exist?”

Hexadecanal, abbreviated HEX, is a chemical that humans emit from their skin, saliva, and feces, and is particularly abundant on babies’ heads, reports Science. Previous research found that smelling HEX has a relaxing effect on mice. In the new study, Mishor and her colleagues investigated whether HEX might affect human behavior—and, by extension, what role scent might play in human social interactions.

The study exposed 127 participants to a computer game designed to frustrate them by resulting in an unfair division of money. (Participants were told they were playing against another person, but were in fact playing against the computer.) The experimental group had HEX scent strips applied to their upper lips while playing the game, and the control group had an identical strip without HEX applied to their upper lips. A follow up game allowed participants to blast their imaginary opponents with a noise loudness of their choosing, represented by increasingly angrier emojis. Women who smelled HEX responded 19 percent more aggressively in the follow up noise-blast test compared to women who didn’t, while men who smelled HEX responded 18.5 percent less aggressively than men who didn’t.

The scientists then used fMRI to measure the brain activity of participants while they were exposed to HEX delivered through an olfactometer. As they were scanned, participants were provoked by having money taken away via the computer game and expressed aggression by taking money from others. The scans showed decreased connectivity between brain regions associated with aggression and social cues—a marker of aggressive feelings—in women exposed to HEX compared to controls, and the opposite in men.

Study coauthor and neuroscientist Noam Sobel, also of the Weizmann Institute, tells Science that this study does not conclude that HEX is a pheromone because it did not explore whether people emit more of the chemical when feeling aggressive, an important criteria for it to be considered a human signaling pheromone, “But we can say that it’s a molecule expressed by the human body that influences human behavior, specifically aggressive behavior, in a predicted manner.” New York University neuroscientist Dayu Lin, who was not involved with the research, tells the magazine that the study exhibits “pretty convincing evidence that HEX can modulate aggression in humans in a sex-specific way.”

The paper’s authors hypothesize in the study that the differing effects of HEX may have to do with survival of babies. “Whereas maternal aggression has a direct positive impact on offspring survival in the animal world, paternal aggression has a negative impact on offspring survival. This is because maternal aggression (also termed maternal defense behavior) is typically directed at intruders, yet paternal aggression, and more so nonpaternal male aggression, is often directed at the offspring themselves,” the authors write.

Radbound University behavioral scientist Jasper de Groot, who was not involved in the work, tells New Scientist that a limitation of the study was that it did not measure physiological reactions to the HEX odor. University of Oxford biologist Tristram Wyatt, who specializes in the evolution of pheromones and was not involved with the study, calls the author’s explanations for HEX’s influence on human behavior speculative in comments to Science. He adds that psychological experiments are very difficult to replicate and that the researchers did not provide evidence that humans emit enough HEX to provoke an olfactory response. He calls for a more rigorous approach and cautions that “It’s fascinating research, but I’m not sure how much weight to put on it.”