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Studies by other psychologists suggest an unconscious mental link between immorality and actual dirt and infection. In a much-noted 2006 study, Chen-Bo Zhong and Katie Liljenquist found that thinking of a past immoral deed made people want to clean their hands with a disinfectant wipe, and that doing so actually made them feel better afterward about their transgression. Zhong and Liljenquist called it the “Macbeth effect,” after the guilt-stricken hand washing of Lady Macbeth.

Issues like abortion and gay marriage, of course, intimately implicate the body, so it’s less surprising that disgust would play a role. But other researchers have found the emotion at work in more abstract moral judgments.

In a study published early last year in the journal Science, a team led by Hanah Chapman, a psychology PhD student at the University of Toronto, looked at disgust and unfairness. Test subjects who played a game and considered the results unfair, the researchers found, reacted with the exact same instinctive facial expression as those exposed to more straightforwardly disgusting stimuli. Unfairness, it seems, can disgust us.