Body odor can influence aggression in humans, but we are not all equal victims of its stench – in men, it blocks aggression, while in women it triggers it – a new study finds. 

The reason for this malodorous mismatch is ecological, suggest the authors of the study, published today in the journal Science Advances. To ensure the survival of babies, protective maternal aggression is provoked, while potentially threatening male aggression is curbed, they hypothesize.

“Maternal aggression is typically directed at intruders, yet paternal aggression, and more so nonpaternal male aggression, is often directed at the offspring themselves. If babies had a mechanism at their disposal that increased aggression in women but decreased it in men, this would likely increase their survival,” the authors write.

Hexadecanal (HEX) – which babies release from their heads – is a signaling molecule emitted from human skin, breath, and feces that is conserved across mammalian species. Participants in the study sniffed HEX before scientists measured their aggression and imaged their brains. Sex-specific differences in the olfactory system underlie the differing reactions of males and females to HEX, the study findings suggest. 

In both men and women, HEX increased activity in a part of the brain responsible for perceiving social clues – the left angular gyrus. However, in men, the connectivity between this region and a brain network linked to aggression was increased, whereas, in women, connectivity was reduced.

To discover this, the team recruited 127 people to take part in a double-blind test – neither the participants nor the researchers knew who was sniffing what. Half of the participants were exposed to HEX, while the other half huffed a placebo.

The study then used a computer game to measure aggressive behavior. Each player competed against an algorithm designed to aggravate them, before being given the opportunity to “blast” their “opponent” with an unpleasant noise. The players controlled the volume of the noise blast, which was used to quantify aggressive behavior. The louder the blast, the more aggressive the player.

Previous studies in non-human mammals have also found that social odors can both trigger and block aggression. “For example, a rabbit mother will attack and even kill her pups if they are tainted with the body odor of a stranger female,” the study authors write. Pigs, on the other hand, are known to be less aggressive in response to a certain sexy pheromone.

“In animals ranging from insects to rodents, aggression is sexually dimorphic at levels ranging from genes to cells, and this dimorphism in aggression has been linked to dimorphism in the olfactory system. Here, we find the same in humans.”

There are, however, notable limitations of the study, which should be addressed. Namely, the hypotheses drawn by the authors are exactly that – hypothetical. Their suggested connections between HEX, infant rearing, and aggression were not directly tested. Likewise, the links they suggest between brain patterns and HEX are purely correlative at this point, and unfortunately, the tests necessary to prove them are very difficult to conduct in humans. Also, the concentrations of HEX used in the study are not necessarily physiological – the concentrations that humans emit are not actually known.

Nevertheless, the study’s principal finding – that chemicals in body odor can trigger different aggressive responses in the sexes – demonstrates the power that chemical signaling holds over human behavior. And that’s not to be sniffed at.