Autism and the smell of fear

Is there an odor associated with fear? A recent study at the Weizmann Institute not only confirms that there is, but also reveals that individuals with autism perceive fear-associated odors—like sweat from an anxious skydiver—in a way that is different from how smells are experienced by individuals who are not on the autistic spectrum.

It has long been recognized that certain emotions—including happiness, aggression, and fear—are associated with specific smells produced by the body, and that people respond to such odors, even if they are not consciously aware of their existence. Prof. Noam Sobel and his team in the Department of Neurobiology set out to test whether, like other forms of social communication, this “smell sense” is disrupted in the case of autism.

Working with a group of participants on the high-functioning end of the autism spectrum who volunteered for the study, as well as a control group of individuals without ASD, the scientists began by verifying that all participants had a similar ability to sense and identify odors that can be consciously detected. Then, the two groups were exposed to an odor that went under the olfactory radar: the smell of fear.

Prof. Sobel and his team exposed the study participants to two different samples of human sweat. In the first sample, the sweat was collected from people while they were exercising, in a fear-free environment. The second sample was gathered from the same people while they participated in an activity that is highly correlated with fear: skydiving classes.

Presented with the “fear-neutral” and “fear-enhanced” sweat, the two groups of participants responded in two very different ways. In the control group, smelling the fear-induced sweat produced measurable increases in skin conductivity—a physiological response associated with heightened anxiety in which the skin momentarily becomes a better conductor of electricity. (This is the same difference in conductivity that provides the “true or false” indication on lie detector tests). Exposure to the sweat gathered under calm conditions did not trigger this response in this same control group.

Participants with autism, on the other hand, responded in the opposite manner: fear-induced sweat lowered their fear responses, while the odor of “calm sweat” measurable raised anxiety levels.

In a separate set of experiments, Prof. Sobel and his team evaluated how the unconscious perception of fear-associated odors affected participants’ response to social cues. The scientists created talking robotic mannequins that emitted different odors through their nostrils. These mannequins gave the volunteers—who were unaware of the olfactory aspect of the experiment—different tasks designed to evaluate the level of trust that the volunteers placed in the mannequins.

The mannequins emitted the same sweat-based odors as in the previous experiment, and again, they found that the response to the smells was reversed in autistic participants as opposed to the control group. The ASD participants placed more trust in the “fear smell” mannequin, as opposed to the mannequin that exuded a “calm smell.” These results suggest that people with ASD can misread olfactory cues, something that may contribute to their confusion about social cues.