Your Nose Knows: Even An Undetectable Smell Of Death Triggers Fight-Or-Flight Response

When animals die they release an unpleasant smell created in part by putrescine, a chemical compound resulting from the breakdown of fatty acids in the putrefying tissue of dead bodies. New research suggests that we humans, just like animals, perceive and respond to this scent as threatening.

“These are the first results to show that a specific chemical compound (putrescine) can be processed as a threat signal,” wrote Dr. Arnaud Wisman, co-author and a psychologist at the University of Kent, and Dr. Ilan Shrira, co-author and a visiting assistant professor of psychology at Arkansas Tech University. “Thus far, nearly all the evidence for threat chemosignals has come from those that are transmitted by body sweat.”

Power of Scent

“We do not know why we like (or dislike) someone’s smell, and we’re usually not aware of how scent influences our emotions, preferences, and attitudes,” Wisman and Shrira told Medical Daily in an email, adding that “it is hard to think of a scent as frightening.”

However, in the animal kingdom, scent is fundamental to survival and various studies have shown that putrescine acts as a powerful chemosensory signal that triggers animals to leave or avoid an area. Humans similarly respond to obvious threats, which generally increase our vigilance and sharpen our reactions, preparing us for “fight-or-flight,” the researchers explained. Despite the popular belief that “fight” is the more common reaction to a threat, the co-authors said “flight” is actually the preferred human response in most cases. To underline their point, the researchers cited a study where people confronted by a threatening stranger chose to distance themselves and only became aggressive (and this included verbal as well as physical aggression), if the get-away option did not exist.

Considering a wide variety of past research on scents and behavior, Wisman and Shrira wondered, Would putrescine prompt threat response behaviors in humans?

And so they conducted a series of four experiments in which participants were exposed, consciously or unconsciously, to putrescine. Observing participants' behavior, the researchers compared them to participants who had been exposed to other scents. For example, in the fourth experiment, participants were primed with one of three scents — putrescine, ammonia, or water (neutral). Next, participants read about a foreign student who criticized their own value system and then they were asked to evaluate this student.

Putrescine led to greater hostility compared to the other two scents, the researchers discovered after analyzing the results. Interestingly, the ammonia group and the neutral group participants responded more or less identically. Similarly, the other three experiments revealed how even brief exposure to putrescine increased vigilance, in turn leading to fight-or-flight readiness.

“Participants in our studies were most likely not aware of the effect of putrescine on their behavior because people are not familiar with putrsescine and do not consciously associate it with death or fear,” said Wisman and Shrira, and their research supports this claim: None of the participants in the fourth experiment reported being aware of any scent.

When asked if putrescine might be considered a kind of “opposite” scent of pheromones, the researchers said, “Putrescine signals a different type of message than pheromones, but people’s responses to putrescine (avoidance and hostility) do seem indeed to be the opposite of responses to many sexual pheromones.” In either case, both putrescine and pheromones elicit a very human reaction. Apparently, sometimes our noses know more than we do.

Source: Wisman A, Shrira I. The smell of death: evidence that putrescine elicits threat management mechanisms. Frontiers in Psychology. 2015.

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