Science Advances sniffs out something peculiar in a “body volatile” (an actively smelly chemical) called hexadecanal, or HEX, which – based on a test using an unfair video game – appears to make men who smell it less aggressive, but women who smell it more aggressive:
Here, we tested whether hexadecanal (HEX), a human body volatile implicated as a mammalian-wide social chemosignal, affects human aggression. Using validated behavioral paradigms, we observed a marked dissociation: Sniffing HEX blocked aggression in men but triggered aggression in women. Next, using functional brain imaging, we uncovered a pattern of brain activity mirroring behavior: In both men and women, HEX increased activity in the left angular gyrus, an area implicated in perception of social cues. HEX then modulated functional connectivity between the angular gyrus and a brain network implicated in social appraisal (temporal pole) and aggressive execution (amygdala and orbitofrontal cortex) in a sex-dependent manner consistent with behavior: increasing connectivity in men but decreasing connectivity in women. These findings implicate sex-specific social chemosignaling at the mechanistic heart of human aggressive behavior.
In each round of the ultimatum game, both “players” are allotted a sum of money (~$9) that they can keep if they agree on how to distribute it between them. The game is programmed such that the purported partner agrees only to distributions that substantially discriminate against the participant. Five such rounds served as an effective provocation. At this point, there were no differences between the groups …. Following this is an aggression discharge phase where the participant is again misled to believe that they are playing against the same partner (who previously provoked them) but now in a reaction-time task where they compete to identify a change in shape of a target. The first to react is then allowed to blast his/her opponent with a loud noise blast. The volume applied by the participant is taken as a measure of aggression…. The game was rigged such that the participant was faster than the fictitious opponent on 16 of 27 trials, and in trials where the fictitious opponent was faster, the participant endured noise blasts randomly ranging in volume.
This further points to what may be considered a physiological counterpart of this brain mechanism: As stress increases, men become more aggressive and women become less aggressive (46). As stress decreases, men become less aggressive and women become more aggressive (47). In this manner, a non–sex-specific effect of HEX on the stress response (always reducing stress) may evolve into a sex-specific effect of HEX on aggression (increased aggression in women yet decreased aggression in men).
The above detailed neuroanatomy and mechanism may underlie a direct circuit from reception to action without the mediation of conscious perception.