In 2015, a team of Israeli researchers from the Weizmann Institute of Science published the results of a study on sociochemical signals in human behavior, in which they identified a molecule that characterizes the smell of newborns: hexadecanal (HEX). The subject was further explained in Italian news outlet Focus in March 2022, and it is their description I have followed here.
The characteristic odor of infants in the first 6 weeks of life
Newborns give off a characteristic odor that remains for the first six weeks of life, due to the presence of various molecules, some of which come from the amniotic fluid and from the vernix caseosa, the whitish substance that covers the fetus protecting its skin during gestation.
Even if the baby is cleaned and washed as soon as it comes into the world, Focus explains, these substances remain in traces on the skin and hair.
The study
The researchers recorded impressions and data from functional magnetic resonance imaging on volunteers of both sexes who had been made to smell an oil containing the HEX molecule, compared with a control group who had smelled the same oil without additions.
Testing the level of aggression
All participants were then asked to scare other subjects by making loud noises to test their level of aggression. Men who had inhaled the HEX showed less "noisiness" than those who had inhaled only the oil, while the exact opposite occurred for women.
Decrease of aggression in men
How to explain this result?
Scientists have centered their hypothesis on what happens in the animal kingdom, where the aggressiveness of males is often directed towards newborns, while mothers do everything to protect them from danger.
Increase of aggression in women
Therefore the different response to HEX in the two sexes could respond to the need to increase the chances of survival of the young: thanks to their odor, fathers would become less aggressive and mothers more combative to protect the life of her offspring.
The study also showed that this molecule is perceived by the brain even in the absence of the olfactory bulbs; this would underline the importance of this molecule for the purpose of survival.
In their article explaining the study and its results in more technical detail, the authors recognize that more studies are needed to confirm these findings, especially the hypothesized connection with child rearing. Although the evidence seems strong, “it remains to be experimentally verified,” they write, and other chemicals besides HEX may be involved, so “future testing of additional potential ligands remains an important question.”
The effect of the smell of newborns on mothers
Other studies have focused more specifically on the effect that the smell of newborns causes in mothers, with the hypothesis that it is able to stimulate the pleasure circuits in a similar way to drugs.
To study this hypothesis, the brain activity of 30 women was measured, of which 15 were new mothers, while they smelled the odor chemically extracted from newborns' clothes. Sure enough, the brain activity revealed stimulation of pleasure areas in all the participants. However, in line with the idea that this chemical is important for women with babies, the activation was higher in those who had recently given birth.
The importance of odors
Generally, body odors are seen in a negative light, and we do anything we can -- spending lots of money every day -- to camouflage or eliminate them.
However, the above mentioned studies show how odors, at least in the relationship between parents (especially mothers) and newborns, can be as important as visual and auditory stimuli, which are often given higher priority.
