What could explain why school officials saw a bomb, not a science project? Why did police see a terrorist-in-training, not a mild mannered honors student in a NASA t-shirt?

Our research suggests a simple but troubling answer — they might not have seen him as fully human.

Dehumanization is a phenomenon by which people see others as somehow “less than human.” Psychologists have linked this perception with everything from perceiving others as primitive and backward to seeing the dehumanized as savage, and aggressive.

In a recent set of studies, we assessed this perception in the most explicit terms possible. We showed 1,065 American participants photos of the “ascent of man” image and asked them to rate (on a 0 to 100 scale) how “evolved” they perceived different groups to be. A score of 100 corresponds to an image of a modern “full human,” and a score of 0 corresponding to an image of an ape-like human ancestor.

Muslims, we found, are the most readily dehumanized groups. Our American participants rated Muslims about 12-15 points lower than Americans on our scale (they were rated over 20 points lower in a smaller sample of British participants). Consistent with the idea that dehumanization is driven by a sense of threat, we found that dehumanization of Arabs was greater in the days after the Boston Marathon attacks.

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Most importantly, dehumanization is associated with less sympathy and more aggression. For example, in one study, we gave participants a story about two children (one Arab, one white) caught shoplifting in a store. The police detained the Arab child but sent the white child home. Those who dehumanized Arabs and Muslims were less likely to feel sympathy towards the Arab child. Even more troublingly, we observed that dehumanization of Arabs and Muslims was associated with supporting highly aggressive policies such as drone strikes in the Middle East and torture of Arabs and Muslims. Across our work, dehumanization tends to be associated with aggressive responses even when we statistically account for individuals’ dislike of Arabs and Muslims, suggesting that dehumanization has a unique influence.