Chemical emitted by babies could make men more docile, women more aggressive

Scientists have argued for decades over whether humans have pheromones, chemical compounds that trigger aggression and mating in insects and other animals. Although the notion has great popular appeal—search Amazon for “pheromone” and you’ll get the idea—there’s scant evidence for this kind of signal in our species.

A new study could change that. Researchers have identified an odorless compound emitted by people—and in particular babies—called hexadecanal, or HEX, that appears to foster aggressive behavior in women and blunt it in men. “We cannot say that this is a pheromone,” says study author Noam Sobel, a neuroscientist at the Weizmann Institute of Science. “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.”

Humans emit HEX from their skin, saliva, and feces, and it’s among the most abundant molecules babies emit from their heads. When researchers isolated the odorless compound and piped it into mouse cages, it had a relaxing effect on the animals, says Sobel, who studies the role of scent in human interactions.

To test how HEX affects people, Eva Mishor, who earned her Ph.D. in Sobel’s lab, created a series of computer games designed to evoke intense frustration—and a measurable response to it—in 126 human participants. Half of the volunteers wore a HEX-infused adhesive strip on their upper lips while they played, whereas the other half wore strips that smelled identical but were HEX-free.

In one task, participants negotiated with an unseen partner to divvy up a sum of virtual money. The participants thought they were playing with another person, but they were actually playing against computers. If a player offered their “partner” anything less than 90% of the whole amount, the computer rejected their proposals with a bright red “NO!” preventing them from earning any money.

Next, participants played a game in which they earned opportunities to blast that same “partner” with noise. Players could choose how loud the blasts were by selecting buttons that bore emojis expressing varying levels of pain—and in doing so, displayed their varying levels of aggression.

Sniffing HEX did not calm all the participants down, but had different impacts on men and women, the team reports today in Science Advances. Women exposed to the chemical behaved 19% more aggressively in the noise-blast task, whereas men were 18.5% less aggressive.

In a second experiment, the scientists compared how individuals behaved when exposed to HEX or to the control odor while monitoring their brain activity in a functional magnetic resonance imaging scanner. HEX again increased aggression in women (by an average of 13%) and dampened male aggression (by 20%). The chemical also had different effects on brain activity, reducing neural communication between brain areas that control aggression in women, and boosting communication between those regions in men.

The study provides “pretty convincing evidence that HEX can modulate aggression in humans in a sex-specific way,” says Dayu Lin, a neuroscientist at the New York University Grossman School of Medicine who was not involved with the work.

The authors speculate their results may have to do with infant survival. HEX is one of the most abundant molecules that babies emit from their heads. Deploying the compound as adults sniff infants’ downy crowns could be advantageous for newborns, Mishor hypothesizes: In mammals, mothers are more likely to use aggression to defend their babies, whereas fathers are more likely to attack their offspring. But the study didn’t address human parents.

In fact, any explanations for how HEX may affect people are speculative, because the team hasn’t shown that either infants or adults emit enough HEX to alter people’s behavior, says biologist Tristram Wyatt of the University of Oxford, also not involved with the work. A more rigorous approach to figuring out which chemical signals influence human aggression would be to capture the compounds people emit when they’re actually feeling aggressive or threatened, he says. Psychological experiments of the kind used in the study are also notoriously difficult to replicate, he cautions. “It’s fascinating research, but I’m not sure how much weight to put on it.”

source: sciencemag.org