Charles Darwin was the first to define the psychology of disgust, writing in the 19th century that it was a feeling of revulsion related to the senses most notably the sense of taste.

Today, psychologist Paul Rozin is one of the world’s leading authorities on disgust describing it as “the fear of incorporating an offending substance into one’s body.” He has devised some ingenious experiments to tap into the feelings and expressions of disgust like, the dead-cockroach-in-drink test, and has developed a 32 item disgust scale.

While we tend to associate disgust with the smell of decay or the taste of spoiled food, disgust has cultural, political and social implications.

This week, Paul Rozin takes us into the inner workings of disgust: why we grimace when we feel disgust, the role of disgust in evolution, why we find some things nauseating, and the moral implications of disgust. Rozin has spent 50 years exploring this stomach-turning emotion and is a professor emeritus at the University of Pennsylvania.