Sniffing a chemical in human body odor blocks aggression in men but triggers it in women, an analysis published Friday by the journal Science Advances found.

The chemical in question, called hexadecanal, or HEX, is also emitted by infants when under stress, the researchers said.

This may be why the odor it produces leads to more aggressive behavior in women, as it taps into the maternal instinct to protect their offspring, according to the researchers.

At the same time, the HEX scent may also suppress male aggression by design, as it could put the child at risk.

The findings suggest that sex-specific differences in the human olfactory system result in divergent reactions to these "social odors," the researchers said.

"Impulsive aggression is a major factor in the human condition, yet how exactly aggression is triggered or blocked in the human brain remains unclear," wrote the researchers, from the Weizmann Institute of Science in Rehovot, Israel.

However, "we observed that sniffing a body volatile, namely, HEX, significantly decreased aggression in men yet significantly increased aggression in women," they said.

For this study, the researchers recruited 127 participants for a double-blind test in which half were exposed to HEX in unmarked specimen jars.

Study participants took part in a computer game used to measure aggressive behavior, in which each player competes against another "player" -- in reality, a game algorithm -- designed to provoke them.

In a later phase of the game, the participants get to unleash their aggravation by blasting their opponents with a loud noise, and the volume of the blast is recorded as a measure of aggression.

The noise blast data indicated that HEX significantly lowered aggression in men but significantly increased it in women, researchers said.

In addition, whole-brain analyses using magnetic resonance imaging and other scanning technologies revealed that HEX increased activity in the left angular gyrus of the brain, the region involved in perceiving social cues, in both men and women.

In men, however, smelling HEX increased connectivity between the angular gyrus and a brain network involved in social appraisal and aggression, but decreased this connectivity in women.

Although a study published in 2020 indicated that humans emit body odors related to aggression, it has not been known how human aggressive behavior may be affected by social chemical signals, the researchers said.

"HEX may exert its effects by modulating functional connectivity between the brain substrates of social appraisal and the brain substrates of aggressive execution," the researchers wrote.

"This places chemosignaling at the mechanistic heart of human aggression and poses but one added example to the rapidly growing body of evidence implicating social chemosignaling as a major, albeit mostly subconscious, power in human behavior," they said.

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