Why the smell of babies may increase aggression in women but decrease it in men
All terrestrial mammals use airborne molecules to communicate, through secretions such as body odour. These scents contain meaningful information about behaviour, health, reproduction status, and more. The EU-funded SocioSmell project sought to examine this phenomenon in humans. “For various reasons, this behaviour is largely overlooked in humans,” says project coordinator Noam Sobel. “We humans don’t walk up and sniff strangers. That is not part of accepted behaviour.” The goal of the SocioSmell project was to uncover the role sociochemical signalling plays in human behaviour, health and disease, and to understand the molecules and brain mechanisms involved.
Sniffing skydivers
In an early result, Sobel and his team at the Weizmann Institute of Science found that humans surreptitiously sniff their hand after shaking hands with a stranger. “Humans rub their faces, touch their faces all the time. It’s almost impossible to stop doing it,” adds Sobel. “This showed we are constantly exchanging this type of information.” He hypothesised that sociochemical signalling may be altered in cases where social behaviour is dysfunctional. In a subsequent experiment, typically developed adults and adults with autism spectrum disorder (ASD) were exposed to ‘the smell of fear’, in this case body odour collected from skydivers. “The typical reaction is an increase in autosomal arousal – fear is contagious,” explains Sobel. “But we saw a profoundly different physiological and psychological response in adults with ASD.” The research suggests that some of the social dysfunction experienced by adults with ASD may be caused by changes in how sociochemical signals are received or processed in the brain.
Chemical messenger
Sobel and his team were able to isolate one of the molecules central to sociochemical signalling, hexadecanal, from more than 5 000 volatiles found on the skin. Young babies produce significant amounts of hexadecanal in their scalp. The group was able to show that exposure to this chemical reduces aggression in men, but increases it in women. The difference was highly conserved between the sexes – Sobel and his colleagues could differentiate female fMRI brain scans from male ones with 80 % certainty simply by the pattern of activity elicited by the molecule. But why should such a difference exist? “Maternal aggression increases the life expectancy of offspring, as maternal aggression is typically protective,” notes Sobel. “In contrast, paternal aggression reduces the life expectancy of offspring, as it is often directed at the infants themselves.” The Weizmann Institute of Science has applied for a patent on the molecule.
Missing bulbs
The project also uncovered a group of women who lack olfactory bulbs, yet retain a normal sense of smell. The discovery was made serendipitously during a routine brain scan of a woman involved in one of Sobel’s experiments. A search through the Human Brain Connectome Project database revealed several more women with an identical condition. Curiously, all these women are left-handed. “It made absolutely no sense. Three per cent of left-handed women have perfect olfactory ability without olfactory bulbs,” says Sobel. “If humans can have a normal sense of smell with no olfactory bulb, it’s a particular challenge to our understanding of how odour is encoded in the brain.” The work was supported by the European Research Council. “Without this funding we would be lost,” remarks Sobel. “It gave us the opportunity to think freely and test freely.” Sobel says he and his team will continue to publish from the collected data for several years.
Keywords
SocioSmell, odour, behaviour, human, sniff, shaking hands, fear, ASD, autism, babies, volatiles, hexadecanal
