Newborn Smell Triggers Aggression In Women But Hampers It In Men: Study

Many people, especially parents, are familiar with the newborn smell that their babies emit, but this distinct scent may actually affect adults more than we think. The molecules released by babies may affect adults' aggression, but does so differently in men and women, a new study has found.

Terrestrial mammals' aggressive behavior can be triggered or even blocked by certain odors, the researchers of a new study, published in Science Advances, noted. For instance, certain components in mouse urine may trigger fighting in males, while a rabbit mom may even attack or kill her own pups if she detects another female's scent on them.

Studies have suggested that humans also emit "aggression-specific" odors, the researchers noted. However, how exactly the "social cues of chemical communication" actually affect humans remains unknown, the Weizmann Institute of Science noted in a news release.

For their study, the researchers tested "whether and how" a molecule called hexadecanal (HEX), which is emitted by humans and sensed by most mammals, affects aggression in humans.

Provocation Stage and Response Stage

To do this, the researchers measured humans' aggressive behavior with the help of 127 participants. Half of them were exposed to HEX, while the other half to a control substance.

After being exposed to the molecules, the participants were subjected to a "provocation stage" that was meant to frustrate them. Here, they played five rounds of an on-screen computer game that was meant to be annoying as they played with what they thought was another person but was actually a computer, the news release noted.

A specific amount of money was allotted for the players in every round. They could keep it if they could agree on how to distribute it between them. However, the game was programmed in such a way that the supposed partner would only agree to distributions that were unfair to the participant.

At this stage, "there were no differences between the groups," the researchers noted.

Next, the researchers conducted the "response stage," where they measured the participants' aggression. Here, the human participants were given the opportunity to essentially "punish" the partner they thought they were playing against by competing against them in a game. The one who reacted first could use a loud noise blast on their opponent.

"(T)he louder the blast, the more aggressive the participant was judged to be," the news release noted.

Aggression in Men and Women

The researchers found that those who were exposed to HEX behaved differently than those who were not exposed to it. Moreover, it affected the participants' "aggressive responses," Dr. Eva Mishor, one of the study leads, said in the news release.

When they took sex into account, they found that HEX actually affected men and women differently, in that it increased aggression in female participants compared to the control group, but decreased the aggression in male participants.

Specifically, the women exposed to HEX "consistently" selected louder noise blasts compared to the women in the control group. As for the men, those exposed to HEX chose milder blasts than the men in the control, the news release noted.

Purpose of the Newborn Smell

As the researchers explained, this difference between the sexes could have an "evolutionary explanation."

"Male aggression translates many times into aggression toward newborns; infanticide is a very real phenomenon in the animal kingdom," Mishor noted. "Meanwhile, female aggression usually translates into defending offspring."

The researchers reached out to experts in Japan who were studying babies so as to look into this hypothesis. Sure enough, they found that HEX is actually "among the most abundant, if not the most abundant molecule" on babies' heads.

"Babies cannot communicate through language, so chemical communication is very important for them," Prof. Noam Sobel, the other study lead, further noted in the news release. "As a baby, it is in your interest to make your mom more aggressive and reduce aggressiveness in your dad."

Brain Responses

The researchers looked further into the matter and studied the brains of men and women via fMRI. They found that HEX activated the left angular gyrus in both men and women, but triggered different responses between them.

In men, HEX served as a "cool down" signal but it served as a "set free" signal in women, Mishor noted.

Overall, the researchers found that even though HEX does not have a specific odor, it actually affects how adults behave and does so differently in men and women.

"The finding is among the first to provide a direct link between human behavior and a single molecule picked up through the sense of smell," Weizmann Institute of Science noted.