A body odour chemical influences people’s behaviour

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A chemical that is sometimes emitted from human skin, breath and faeces has no detectable smell, but it appears to influence people’s behaviour, with men becoming calmer and women becoming more aggressive.

Although scientists have yet to determine when or under what conditions people and other mammals release hexadecanal, it seems clear that humans are “communicating” with each other subconsciously through their body odours, says Eva Mishor at the Weizmann Institute of Science in Israel.

“Humans [smell] each other – their children, their romantic partners, strangers – all the time,” says Mishor. “Our study gives more power to the notion that humans communicate from the chemical volatiles they emit, and that we get lots of information from them.”

Previous studies had shown that hexadecanal acts as a “social buffer” that reduces stress in mice, she says. In humans, EEG-based studies have suggested the compound triggers brain activity differently in men and women, although it wasn’t known how.

To find out, Mishor and her colleagues asked 67 men and 60 women – ranging in age from 21 to 34, all identifying as their gender assigned at birth – to play an online economics game against other human or robot players. All the participants had been asked to sniff clove oil prior to playing; for half the people, the clove oil was mixed with hexadecanal, which didn’t affect the oil’s perceived smell.

During the game, which scientists programmed to seem unfair after a few rounds, participants could “blast” their opponents with varying levels of explosive sound. The researchers found that the women who had sniffed the hexadecanal reacted more aggressively to the game, increasing the volume and blasting their opponents with 17.6 per cent greater intensity, Mishor says.

The men, however, responded with 18.4 per cent less volume if they had inhaled the chemical, compared with those who hadn’t, she says.

Functional MRI scans of the participants’ brains showed that after sniffing hexadecanal, both men and women had increased activity in the parts of the brain associated with recognising social cues. Then, when they felt provoked by the game, women had increased neural activity linking those regions to brain areas responsible for aggressive behaviour. The same activity along neural connections in men, meanwhile, were reduced.

Humans manufacture hexadecanal themselves as a by-product of metabolic processes; whether they can also acquire it from certain foods remains to be determined, says Mishor.

The study didn’t include measures of physiological reactions to the odour, which would have strengthened the findings, says Jasper de Groot at Radboud University in the Netherlands. Nevertheless, it has the potential for “groundbreaking implications”, he says.

“It’s below the threshold of conscious awareness, yet hexadecanal apparently influences the behaviour of males and females differently,” says de Groot. “These are really striking effects [that underline] the fact that just because we are not aware of it doesn’t mean it doesn’t strongly affect us.”

Even so, people don’t always act out the aggression they feel, he says.

Journal reference: Science Advances, DOI: 10.1126/sciadv.abg1530

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