Sniffing a chemical in human body odour – commonly known as BO – triggers aggression in women but blocks aggression in men, a new study shows.  

Researchers in Israel looked at the effects the chemical compound, called hexadecanal (HEX), has on the human brain. 

HEX is not like other compounds that produce body odours. However, it has a distinct smell for humans and can be detected by most mammals.   

The researchers found HEX decreases connectivity in parts of the brain that regulate social decision-making in women – including the decision to become aggressive – while in men it increases this connectivity.

HEX also appears abundantly on heads of infants. 

The researchers believe that HEX was an evolutionary tool, used to control aggression thousands of year ago in order to lessen the risk of harming the infant.

A chemical compound in body odour called HEX is also found in abundance on the heads of newborn babies, scientists in Rehovot, Israel report (stock image)

Researchers in Rehovot in Israel reported that HEX, a chemical compound found in body odours, was also abundant on the heads and necks of infant babies (stock image).

What do we know about body odor?

Every animal, including us humans has a unique body odour.

Although our sense of smell is heavily influenced by genes, it can also be affected by physiological and other conditions.

You can alter your sense of smell by exercising, hot weather and medication. 

The role of human smells might have been greater for early ancestors.

New research suggests that people who smell different to us are attracted more to each other than previously thought.  

Contrary to common belief, women are also more affected by smell than men.  

The new study was led by Dr Eva Mishor, a researcher at the Weizmann Institute of Science in Rehovot, Israel. 

“Hexadecanal or HEX is a volatile molecule that emits no odour from the human body. 

‘We found that HEX has no perceptible odour, but that when you sniff it, it affects the way you behave toward others – specifically, your aggressive responses to others.’

Key to the process is a brain region on both the left and right sides called the angular gyrus, which she describes as ‘a hub of the social brain’. 

Already, it is known that the angular georus has a significant role in memory, number processing and reasoning. 

It’s activated under exposure to HEX in both men and women when provoked. HHowever, it blocks aggression in men and triggers aggression in women. 

Dr. Mishor proposes an evolutionary explanation for why the same molecule can affect both sexes in a different way.

‘Male aggression translates many times into aggression toward newborns – infanticide is a very real phenomenon in the animal kingdom,’ she said. 

“Meanwhile. Female aggression typically translates to defending offspring. 

The angular gyrus (highlighted in blue in this diagram of the brain) is activated under exposure to HEX in both men and women when provoked

If provoked, both women and men are able to activate the angular gyrus in their brains by exposing them to HEX.

Researchers aren’t sure if emitting HEX body odour has evolutionary benefits for humans. 

It is possible that HEX-emitted concentrations change in different situations, depending on whether aggression is needed as an appropriate response, Dr Mishor told MailOnline. 

For the study, the academics recruited 127 participants for a ‘double-blind’ test – where neither the participants nor the experimenters know who is receiving a particular treatment. 

This study was based on two scientifically proven methods to gauge aggressive behaviour in humans: the so-called “aggression paradigms” (TAP and PSAP). 

Researchers used TAP to test 130 participants. Half of them were exposed to HEX and the other half to a control drug. 

An additional 50 people were tested using the PSAP technique. They were each given both HEX or the control drug.  

Participants in the experiment were offered an outlet for their aggression in the form of blasting their purported game partner with loud, unpleasant noises. The louder the noise blast, the higher the measure of aggression. Women exposed to HEX consistently selected louder noise blasts than women in the control group. Yet the opposite effect was observed for men - those who were exposed to HEX consistently selected milder noise blasts than those who were not exposed to the molecule

Participants were given an opportunity to express their anger through loud and unpleasant sounds blasting at their game-partner. A higher level of aggression can be achieved by blasting louder noises. Women who were exposed to HEX tended to choose louder noise blasts over women who were not. However, the reverse effect was seen for men. Those who had been exposed to HEX tended to select milder noise blasts than those not.


Aggression is not a single brain area. 

Aggressive behavior is actually linked to the networks of communication among different parts of our brain. These network connections regulate how we interpret social cues, and whether or not we abide by them. 

Although previous research has suggested that people emit aggressive body odours, we don’t know how social chemical signals might affect human aggression. 

Both methods have two stages – a provocation stage intended to frustrate participants and a response stage intended to gauge their aggression. 

To measure participants’ aggression, researchers created a computer-based game. 

The participants were given either the molecule or the control substance and asked to play against two different types of computer. 

The computer was purposefully annoying, goading its human playmates as a form of provocation – for example, in the first game, which required divvying up money, the computer would offer to keep most of the funds for itself. 

A second game followed, where the human players could ‘punish their friend’ by blasting loudly. 

This was used as a metric for gauging aggressiveness – the louder the blast, the more aggressive the participant was judged to be. 

Dr Mishor discovered that people who were exposed to HEX had different behavior than those not. 

The researchers discovered that HEX affects men and women in different ways when they take sex into consideration. 

A 3D reconstruction of the brain, displaying the brain regions where the difference between women and men was most pronounced (yellow-orange). For both female and male participants, the researchers showed that HEX modulated the way the angular gyrus, a brain region that integrates social cues, talks with areas of social-emotional decision making. This may imply that HEX exerts its effect by altering social control over emotional reactivity

The 3D brain reconstruction shows the areas of brain where differences between men and women were most apparent (yellow/orange). The researchers found that HEX had a significant effect on the communication between the angular gyrus and areas that are involved in decision-making about social emotions. This could indicate that HEX may alter emotional reactivity by altering the social control. 

Female participants exposed to the molecule showed increased aggression compared to those in the control group. Male participants, however, displayed a different behavior and were less aggressive.  

They reached out to Kobe University Japan researchers who were studying molecules excreted by babies. 

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’. 

Noam Sobel of the Weizmann Institute of Science, said that babies cannot communicate using language so chemical communication is vital for them.

It is your right as a baby to be more aggressive than your mother and less aggressive towards your dad.  

Functional magnetic resonance imaging scans (fMRI), which detect changes in blood flow and brain activity, showed that HEX is perceived by both men and women as having no odour.

Illustration of an fMRI experiment. The experimental setup made innovative use of pressure-measuring balls that allow participants to express their aggression in a more natural manner

Illustration of an experiment using fMRI. This experimental set made use of innovative pressure-measuring ball technology that allows participants to show aggression in more natural ways. 

But their neurologic responses were quite different. Both sexes HEX activated the left angular gyrus in both sexes, which is involved with the integration social cues. 

It was, however, sex dependent in the manner it talked to brain regions.

According to Dr Mishor, HEX affects men because there is more regulation of social behavior. Their aggression was controlled and that served as a “cooldown” signal. Women, however, have less regulation and can consider it a signal of freedom. 

This means that the ability to communicate between parts of the brain responsible for social regulation and aggression control differs among men and women.

Researchers claim that they have found a direct link between the behaviour of humans and one molecule, based on the use of smell. 

Science Advances has published the paper today. 


Scientists reported that the chemical responsible for body odour was identified in 2020. 

A bacteria enzyme has been discovered to create the pungent scent known as BO. 

It is also known as the “BO enzyme” and it’s made from bacteria Staphylococcus Hominis, which our ancestors now extinct. 

Unilever collaborated with researchers from York University to discover that Homo sapiens has suffered body odour since the beginning of our evolution. 

This bacteria was passed on from more primitive peoples.   

Unilever Dr. Gordon James said: “This research has been a real eye-opener.

“It was exciting to discover that an important odour-forming enzyme is only found in select armpit bacteria. It evolved there millions of year ago.

Academics have discovered a method to create deodorants which neutralize BO. 

Michelle Rudden of the University of York’s Department of Biology said that “Solving” the structure of “BO enzyme”, has enabled us to identify the molecular mechanism inside bacteria responsible for making the odour molecules.

“This breakthrough in body odour science will allow for the development of specific inhibitors to stop BO production at the source, without disrupting microbiome. 

The bacteria produces enzymes that latch onto the odourless chemicals made by our apocrine glands. 

These are in the skin and produce sweat and open into hair follicles. These are found only under the arm, near the nipple, and in the external genitalia. 

Human’s also have eccrine glands which are all over the body and do not open into hair follicles. 

Thermoregulation is a function of eccrine and somatosensory glands, but little is known about their hairy hairy counterparts. 

Scientists have discovered that there are bacteria living in the soil and that this microbiota is crucial to their function. 

The study, published in the journal Scientific Reports, found odourless precursor chemicals secreted from the glands are sliced up by the enzyme.

This transforms the harmless, odour-free chemicals into a thioalcohols, which the researchers describe as ‘most pungent volatiles’ in sweat despite being found only in trace levels.