A chemical that babies give off from their heads calms men but makes women more aggressive, according to new research in the journal Science Advances.
It could be a chemical defense system we inherited from our animal ancestors, the authors speculate, making women more likely to defend their babies and men less likely to kill them.
Odors affect behavior in the animal world in plenty of ways. A rabbit mom will attack her pups if they smell like another female rabbit. Mice whose sense of smell is damaged don't attack other mice intruding on their territory.
We humans like to think we are above all that. But scientists are increasingly finding that odors affect us more than we think.
In the latest study, scientists tested how people responded to a chemical called hexadecanal, or HEX.
HEX is found in body odor and breath. It's also found in feces, and raising babies is "the one social setting where humans have extensive exposure" to poo, the authors note. They also discovered that HEX is the most abundant of the many chemicals babies' heads give off.
The study tested people's responses to HEX using rigged games designed to aggravate the player. In one game, when the aggravated player is allowed to win, he or she gets to blast the opponent with a loud noise. The louder the noise, the higher the scientists rated the player's aggression level.
When players sniffed HEX before playing, women's blasts were louder and men's were quieter. The effect was somewhat subtle. On a six-point scale, the differences were, on average, roughly between half a point and less than a point in either direction.
The first time he saw the results, they "made absolutely no sense to me," co-author Noam Sobel, head of the neurobiology department at the Weizmann Institute of Science, said in an interview. "I personally did not see any possible ecological reason for a molecule to increase aggression in women and decrease it in men."
But lead author Eva Mishor, who was studying signals of aggression for her doctorate at the Weizmann Institute, noted that in animals, female aggression is usually aimed at defending their young, while male aggression is often directed at the offspring themselves.
"This was totally 100% Eva's eureka moment," Sobel said. "If you're an offspring, you have a vested interest in emitting a molecule that will make women more aggressive and men less aggressive."
"I said, 'OK, it's plausible,' " he added. " 'But I want to see it again.' "
So they did another experiment, this time testing subjects' reactions while in a brain scanner.
The results were the same. And they saw that HEX activated a part of the brain involved in judging social interactions. This region seemed to turn connections to brain regions that control aggression up or down, depending on the subject's gender.
There are still plenty of questions to answer. The study did not test babies directly. And the authors noted that they didn't know if the amount of HEX their subjects smelled was the same as what they would get from sniffing babies' heads.
"In the beginning, I found it a little bit far-fetched," said neuroscientist Jessica Freiherr at Friedrich-Alexander University, who was not involved with the research. But "it makes sense," she said in an interview.
Smelling sweat from angry people made others anxious, according to research by Freiherr and her colleagues. Other studies have found that subjects identified fear in faces faster when they smelled sweat collected from people who were afraid. And women's tears lowered testosterone and sexual arousal in men, another study from Sobel's lab found.
"We still are those animals," Freiherr said. "Maybe not having our nose on the floor all the time, but we can still sniff out those signals."