Pheromones are chemical compounds released by animals that trigger specific reactions in the recipient, such as mating behavior or aggression. These chemicals are in essence a form of nonverbal communication that is widespread from insects to mammals. However, while the existence of pheromones is well established in animals, it remains controversial for humans. Humans with our rich language skills have likely lost most, if any, dependence on pheromones for modulating our behaviors. Still, some evidence has been reported for potential human pheromones, and a new study in Science Advances reports on an interesting candidate called hexadecanal.
Hexadecanal (HEX) is an odorless and volatile chemical produced by mice that is known to reduce aggression and promote calmness in other mice who encounter it. Interestingly, humans also produce and excrete HEX on the skin, in feces, and in our breath. Also like mice, humans possess a receptor for binding HEX which suggests that we could respond to this chemical. To examine the effect of HEX on humans, the study authors developed a computer game designed to create frustration and hostility in the game players. The test players thought they were playing against another human opponent but were actually just playing against the computer. A player’s aggressiveness was determined using a variable volume noise blast that the player could use against his/her opponent, the higher the noise blast the greater the perceived aggression. In a second version of the experiment, the test player could deduct money from the opponent in response to a provocation, and the greater the deduction the greater the aggression. Half the 127 test subjects (67 males and 60 females) were exposed to HEX while playing the game while the remainder served as the control group. Among men, HEX reduced aggression by roughly 20% while in women HEX increased aggression by a similar amount. The researchers also used functional MRI (fMRI) to monitor brain activity characteristic of aggression in response to HEX. Women again showed increased aggression (13%) as opposed to men who showed a 20% reduction in aggression. In aggregate their results support the conclusion that HEX can modulate human behavior but does so in a sex-specific fashion with a calming effect on men and an aggression-stimulating effect on women.
In an intriguing speculation, the authors hypothesize that the HEX could play a role in parenting. It turns out that infants’ heads produce abundant HEX (unpublished communication to the study authors) as do their feces, meaning that parents likely are exposed to HEX during normal child care. Exposure to HEX could reduce the father’s possible aggression to the infant, a danger seen in many species. In contrast, HEX might increase a mother’s aggression making her a fiercer protector of the infant. Both of these effects would likely have been evolutionarily advantageous, especially among our early progenitors. While this hypothesis is completely untested at this point, the study does raise interesting questions for further investigation about a biological role for HEX that might still be active in modern humans.