Research by Dr. Cikara and others shows that when one group feels threatened, it makes it much easier to think about people in another group as less than human and to have little empathy for them — two psychological conditions that are conducive to violence.

A 2011 study by Dr. Fiske and a colleague looked at “social cognition” — the ability to put oneself in someone else’s place and recognize “the other as a human being subject to moral treatment.” Subjects in the study were found to be so unempathetic toward drug addicts and homeless people that they found it difficult to imagine how those people thought or felt. Using brain M.R.I., researchers showed that images of members of dehumanized groups failed to activate brain regions implicated in normal social cognition and instead activated the subjects’ insula, a region implicated in feelings of disgust.

As Dr. Fiske has written, “Both science and history suggest that people will nurture and act on their prejudices in the worst ways when these people are put under stress, pressured by peers, or receive approval from authority figures to do so.”