Baby smell triggers aggression in women but calms men down

Baby smell triggers aggression...
The researchers speculate an evolutionary origin to the odor signal helping increase a baby's survival
The researchers speculate an evolutionary origin to the odor signal helping increase a baby's survival
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The researchers speculate an evolutionary origin to the odor signal helping increase a baby's survival
The researchers speculate an evolutionary origin to the odor signal helping increase a baby's survival

An intriguing new study has found a chemical excreted by babies can influence aggression in humans. The findings reveal curious sex-specific responses to the chemical, triggering aggression in women but blocking aggression in men.

Hexadecanal (HEX) is a chemical recently discovered playing a role in mouse behavior. A 2015 study reported HEX reduces stress responses in mice through a process known as social buffering. More recent research found exposure to HEX in humans may reduce a person's startle response, suggesting the chemical plays a role in modulating human behavior.

This new research builds on those prior findings by investigating the influence of HEX on aggression. More specifically, the research focused on the differences in aggression induced by HEX between men and women.

Recruiting nearly 200 subjects, the researchers first explored the effect of HEX on aggression across two different behavioral experiments. These experiments were designed to measure a person’s aggressive responses and the findings revealed exposure to HEX did indeed influence aggression.

“We found that HEX has no perceptible odor, but that when you sniff it, it affects the way you behave toward others – specifically, your aggressive responses to others,” explains Eva Mishor, lead researcher on the project.

While HEX did indeed affect aggression responses in the cohort, the results were discordantly sex-specific. Women exposed to the chemical displayed increased aggression while men displayed decreased aggression.

A small number of the participants completed one of the behavioral tests while brain activity was measured using functional magnetic resonance imaging (fMRI). In both men and women exposure to HEX increased activity in the left angular gyrus, but connectivity from that brain region to others significantly differed between sexes.

“HEX, it would seem, affects men in that there was more social regulation, their aggression was kept in check and it served as a ‘cool down’ signal for them, while in women the regulation decreased and it can be thought of as a 'set free’ signal,” explains Mishor.

Why HEX exposure results in such dramatically different responses between men and women is a mystery. But the researchers hypothesize these sex-specific responses could be a tool babies evolved to enhance their chance of survival.

Part of the new study explored the presence of HEX on newborn babies. The researchers discovered HEX is most abundant on a newborn baby’s scalp. Noam Sobel, another researcher working on the project, speculates HEX could be an important chemical signaling tool from a baby to its parents.

“Babies cannot communicate through language, so chemical communication is very important for them,” says Sobel. “As a baby, it is in your interest to make your mom more aggressive and reduce aggressiveness in your dad.”

Sobel refers to the commonly observed phenomenon of infanticide in the animal kingdom. Heightened paternal aggression has been linked to infanticide in mammals, while increases in maternal aggression generally translate into a mother defending her offspring.

“If babies had a mechanism at their disposal that increased aggression in women but decreased it in men, this would likely increase their survival,” the researchers speculate in the newly published study.

There are plenty of caveats in the study, limiting how definitive its findings can be. The associations between HEX exposure and brain activity alterations are still observational, with plenty of further work needed to understand how simply sniffing HEX can induce these changes. Plus, although significant levels of HEX can be detected on newborn baby scalps, it isn’t yet known if the volume of chemical secreted by babies is enough to induce any measurable behavioral change.

Nevertheless, the compelling findings offer some insight into the role odors could play influencing the behaviors of new parents, beyond simply releasing feel-good chemicals such as dopamine.

The new study was published in the journal Science Advances.

Source: Weizmann Institute of Science

It's not at all clear to me that increasing aggression in women would necessarily increase infant survival. I can think of a lot of scenarios where the effect would be quite the opposite. (Oddly, this is a situation where animal studies might actually be useful, because it's likely that whatever is going on, it's something that evolved long before modern humans did.)
Christian Lassen
Momma Bear ftw