The smell of death isn’t just gross—it might be downright scary. Scientists have identified a chemical associated with death and decay that appears to function as a warning signal, activating the body’s threat management response and making people more vigilant. 

A University of Kent psychologist and an Arkansas Tech University behavioral scientist argue that putrescine—a chemical compound unleashed by the fatty acids that break down when dead tissue decays—is an olfactory threat signal for humans. In four different tests published in a study in the journal Frontiers in Psychology, putrescine exposure put people on edge, eliciting cognitive reactions related to escaping threats. 

In one trial, 60 people were asked to open a jar and give its contents a sniff, inhaling either the pungent smell of putrescine or the similarly gross smell of ammonia. Afterward, those who smelled the death smell were quicker to react to a red dot randomly presented on a screen in the lab, indicating that the smell made people more vigilant. 

In two more tests, a total of more than 100 people were stopped on a university campus and asked if they had time to participate in a smell test. After sniffing putrescine, people walked away faster than those who had smelled ammonia or water (as timed by a hidden experimenter with a stopwatch).

In a fourth experiment, 65 people filled out questionnaires that were subtly scented with putrescine, water, and ammonia, such that none of the participants were aware of the scent. They read an essay designed to elicit a response against an "out-group member," someone who didn’t share their values. In this case, it was supposedly written by a Middle Eastern exchange student in the UK who criticized Western values and predicted their decline. Afterward, people filled out a questionnaire about how likable the essay’s author was and whether his ideas should be publicized. Those who smelled putrescine were significantly more hostile towards the foreign student than those who filled out ammonia-soaked questionnaries, suggesting that subconsciously, the scent of putrescine elicited a defensive response. 

The chemical compound “could serve as a warning signal that mobilizes protective responses to deal with threats,” the authors write, evoking people’s fight-or-flight response. This is one of the first studies to point to a chemical threat signal of this type that isn’t transmitted through sweat. 

[h/t: Brain Decoder]