Parents of newborns will, in detail, for all who are willing to listen, describe the wonder that is their baby’s smell – and they may not be mistaken. A new study by researchers at the Weizmann Institute of Science suggests that a molecule secreted by humans, and perhaps especially by babies, plays an important social role: regulating aggression in adults.
The study, led by Dr. Eva Mishor from Prof. Noam Sobel’s research group at the Weizmann Brain Sciences Department and the Azrieli Institute for Human Brain Imaging and Research, has found that a molecule that is likely to be sensible by all mammals and that has been found. in abundance in the scalp of newborns, sparks the brain and behavioral changes in adults who are exposed to it.
In addition, the study is published today in The progress of science, found that the molecule affects women in one way and has the opposite effect on men. The molecule, the researchers found, reduces the connection in parts of the brain that regulate social decision-making in women, while in men it increases this connection. These brain differences may underlie the different behavioral outcomes.
The finding is among the first to provide a direct link between human behavior and a single molecule that is captured through the sense of smell. Furthermore, the diametrically opposite change that it effected in women and men sheds new and surprising light on the mediating role that sex plays in the perception of smell and its resulting neurological processes.
All animals secrete molecules that emit certain signals that are perceived through our olfactory abilities. Studies have shown that they can also have clear social and behavioral effects on mammals. However, the science behind how humans are affected by these social signals of chemical communication is still limited.
In their study, Mishor and Sobel focused on a molecule called hexadecanal or HEX, which humans emit and which most mammals can sense. “We found that HEX has no noticeable odor, but that when you sniff at it, it affects the way you behave towards others – specifically your aggressive reactions to others,” explains Mishor.
HEX was previously proposed to be a “social buffer” chemo signaling agent. For example, it reduces stress in mice when excreted by their peers. However, the question of whether it plays a similar role in humans was unknown.
The study used two validated scientific methods to measure aggressive behavior in humans, the so-called “aggression paradigms” known as TAP and PSAP. The researchers used the TAP method on about 130 human participants, half of whom were exposed to HEX and half to a control substance. The PSAP method was used on about 50 additional participants, each exposed to both HEX and control. The goal was to see if HEX acted as a chemo signaling signal that affected aggressive behavior and associated brain mechanisms.
Both methods have two phases: a provocation phase that aims to frustrate participants, and a reaction phase that aims to measure their aggression. Therefore, the researchers created a computer game to measure the participants’ aggressive behavior. After the participants had been exposed to the molecule or to the control substance, they were asked to play two sets of games against what they thought was a person but was actually a computer.
The computer was purposefully annoying and teased its human playmates as a form of provocation. For example, in the first game, which required splitting money, the computer would offer to keep most of the money to itself. This game was followed by another game that allowed people to “punish” their interlocutor with a loud outburst. This was used as a measure to measure aggression – the higher the explosion, the more aggressive the participant was judged to be.
Mishor found that those who were exposed to HEX exhibited a different behavior than those who were not exposed to it. However, the researchers were surprised to see that the results seemed inconsistent.
“I’m very sensitive to gender issues, and it was very important to me to include women in our study,” Mishor notes. We tend to assume that men and women generally have equal brains. But when the researchers took into account sex, they discovered that the different behaviors followed a clear logic: HEX affects men and women differently. While women exposed to the molecule exhibited increased aggression compared to female participants in the control group, male participants behaved the opposite and their aggression decreased.
Why should this molecule affect the sexes differently? Mishor suggests an evolutionary explanation: “Male aggression is often translated into aggression against newborns; infanticide is a very real phenomenon in the animal kingdom. Meanwhile, female aggression usually translates into defending offspring,” she says, explaining how the gender-differentiated outcome can help babies with survival.
To investigate this hypothesis, they reached out to researchers in Japan who had studied babies, specifically the molecules secreted from their scalp. This led them to discover that HEX “is among the most abundant, if not the most abundant molecule in the aromatic bouquet found on a baby’s head,” Sobel mentions.
“Babies cannot communicate through language, so chemical communication is very important to them,” Sobel explains. “As a baby, it’s in your interest to make your mother more aggressive and reduce your father’s aggression,” he adds.
To further validate their findings, the researchers used the second aggression paradigm method, which allows imaging of brain activity during the provocation and aggression stages. Also in this case, a marked difference was found between men and women.
There is not a single area in the brain associated with aggression. Rather, aggressive behavior is associated with communication networks between different parts of our brain that regulate the way we process social signals and either obey them or ignore them.
fMRI scan revealed that although men and women similarly perceive HEX as no odor, their neurological response to it was radically different. In both sexes, HEX activated the left-angle gyrus, an area involved in the integration of social signals. But the way it “spoke” to other regions of the brain was gender dependent.
“HEX, it seems, affects men by the fact that there was more social regulation, their aggression was kept in check, and it acted as a ‘cool down’ signal for them, while regulation in women fell, and it can be perceived as a ‘set free’ signal, “explains Mishor. In other words, the communication between the parts of the brain that are responsible for social regulation, and thus help to keep aggression at bay, is different in men and women.
“Like all mammals, humans sniff at themselves and each other all the time,” Sobel explains. Now we may know the result of sniffing for newborns and have a better understanding of the mechanisms involved and its possible evolutionary role.
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Eva Mishor et al, Sniffing the volatile hexadecal channel of the human body blocks aggression in men, but triggers aggression in women, The progress of science (2021). DOI: 10.1126 / sciadv.abg1530
Provided by the Weizmann Institute of Science
Citation: Scent of newborn infants blocks aggression in men, stimulates aggression in women (2021, November 22) Retrieved November 24, 2021 from https://medicalxpress.com/news/2021-11-scent-newborn-infants-blocks-aggression .html
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