An odorless molecule can make us more aggressive

The experiment: hexadecanal exposure affects human behavior: it makes women aggressive and men more docile.

According to a study published in Science Advance, there is a small, odorless molecule that, when "sniffed," is able to make us more aggressive, or more docile.

It is a molecule emitted by humans and other mammals, and it seems capable of changing women's behavior in a more aggressive direction, while it would make men more docile.

The molecule that makes women aggressive

That many instinctive reactions are guided by chemical processes that still have no explanation is well known, as well as the important role of smell in the processes that in mammals guide the instinctual responses to stress or danger situations.

Not too long ago, a study showed that the presence of hexadecanal could reduce stress in mice; and it is still on hexadecanal that scientists are focusing their efforts to explain how the chemistry can influence mood and behavioral patterns in humans.

It is a chemical compound found in human skin, saliva and other typical mammalian "expressions." It has no odor, but when "sniffed" it appears to be able to decisively influence the aggressiveness of human behavior.

Eva Mishor, of the Weizmann Institute of Science in Rehovot, Israel, led her research team in an experimental study investigating the influence of hexadecanal on human behavior.

During the first experiment, 67 men and 60 women were purposely cheated during a game show with a small amount of money up for grabs; participants were then given the option of disturbing their "uncooperative" gaming partner by emitting a powerful sound whose volume they could control.

Half of those involved had a small patch containing hexadecanal under their noses, while the patch of the other half of the participants bore no traces of the molecule.

The differences were immediately apparent, as Mishor states: women exposed to hexadecanal indulged much more than other women in the disruptive maneuver, while men exposed to the same molecule proved more docile than others.

A survival strategy?

In a second experiment, 25 men and 24 women took part in a more complex game, during which money was occasionally taken from them by the opposing player.

The players - under functional MRI - were then given the opportunity to take resources from the other players in turn. Once again, the researchers observed that women were more likely to engage in aggressive behavior when exposed to hexadecanal, while men subjected to the molecule became more docile and less competitive. Hexadecanal may therefore influence human behavior.

Instrumental tests have shown that exposure to hexadecanal activates areas of the brain involved in perceiving social cues, and it does so differently in men and women.

There may be a purely biological mechanism behind the effects of hexadecanal: the molecule is among the most abundantly produced substances by infants, who emit the odorless molecule from their heads.

It could therefore be a survival mechanism that induces mothers to be more aggressive in defense of their offspring, and tames fathers so that they don't attack them: in other mammals it has in fact been noted that the mother's aggression is directed toward external aggressors, while that of the father can also turn against offspring.

"Our results imply that sniffing a baby can increase aggression in mothers" concludes Mishor "but decrease aggression in fathers": the emission of hexadecanal would thus be a tool that can ensure a higher survival rate of mammalian pups.