They were astounded to find that inhaling hexadecanal, a chemical that is sometimes emitted from the human skin, especially on the heads of newborns, has an entirely different effect on men and women.
The scientists say their experiment, newly published in Science Advances, is among the first to provide a direct link between human behavior and a single molecule picked up through the sense of smell.
A team from the Weizmann Institute of Science got volunteers to sniff oil, some of it with hexadecanal and some without. Hexadecanal is odorless, but the men who sniffed it became less aggressive than others who didn’t — while the women who sniffed it became more aggressive.
“We were really surprised,” Dr. Eva Mishor, lead author of the study, told The Times of Israel. “Our hypothesis was that it was a social cue that reduced aggression, but we didn’t expect it to cause a different reaction in men and women.
“However, once we started to observe and understand it, there was logic,” Mishor said.
“The effect seems to make sense in childrearing, because in the animal kingdom male aggression is often directed against the infant, which endangers the offspring, while female aggression is directed against others as the mother tries to protect the infant.
“Male aggression translates many times into aggression toward newborns; infanticide is a very real phenomenon in the animal kingdom. Meanwhile, female aggression usually translates into defending offspring. So this smell mechanism we studied may actually be a means for an infant, who has limited communication, to increase survival by getting the effect they need from both parents.”
The opponent behaved unfairly and the participants were told to respond by generating sounds at the touch of a button, some of which were loud enough to hurt the ears of a human on the receiving end.
The scientists also ran brain scans on some volunteers, and found that while men and women perceive hexadecanal as odorless, their neurological reaction to it was very different. They actually identified specific brain regions that responded very differently.A 3D reconstruction of the brain, displaying the brain regions in yellow and orange where the difference between women and men was most pronounced after smelling hexadecanal. (courtesy Weizmann Institute of Science)
The research took place in the lab of Prof. Noam Sobel, a neuroscientist who researches the function of smell in human interactions, and famously concluded in 2015 that the handshake may have originated as a socially acceptable way of checking out each other’s odors.
Like all mammals, humans engage in reactive aggression from a very early age (9) and throughout life (10). Although aggression is a major factor in the human condition, the mechanistic neural substrates of human aggression are not well understood (11, 12).
Could human aggression mechanisms be tied to chemosignaling mechanisms as they are in all other terrestrial mammals? Two studies have suggested that humans emit aggression-specific body odors (13, 14), and an electroencephalography (EEG) study suggested that these may be processed differently in the brains of men and women (15), but whether and how human aggressive behavior is then affected by social chemosignals remains unknown
“Now, perhaps we know the outcome of sniffing newborns and have a better understanding of the mechanisms involved, and of
its possible evolutionary role.”
Results from the Weizmann experiment, which shows that women responded more aggressively when given the chance to ‘blast’ noise at opponents in a game after smelling hexadecanal, while men responded less aggressively (Weizmann Institute of Science)