By Nathan Jeffay

The smell of a newborn boosts their chances of survival by making moms more aggressive and dad less aggressive, Israeli scientists have suggested in peer-reviewed research.

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,” said Dr. Eva Mishor, lead author of the study. “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 main experiment for the study involved getting participants to play a game, believing they were competing against another person while the opponent was really a computer.

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.

Women who sniffed hexadecanal blasted the noise louder than other women, while men who had smelled the chemical generated gentler sounds.

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.

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 sniff themselves and each other all the time,” Sobel said, commenting on the new research, which he co-authored with Mishor. “Now, perhaps we know the outcome of sniffing newborns and have a better understanding of the mechanisms involved, and of its possible evolutionary role.”