In 2004, 57 police officers of different races were divided into two groups for a simple experiment. Half of them were shown two photo lineups, one with an array of white faces and one with black faces. This group was more visually attuned to the white faces. A second group looked at the same lineups after words like “violent,” “crime” and “shoot” flashed on their screens, at the edge of their field of vision. This group of officers’ eyes were mostly drawn to the black faces. In a similar test, using pictures of guns and knives instead of words, a group of white college students exhibited a similar shift in attention.

The psychologists who did the research described their findings in terms of a “bias” in perception, rather than of intentional prejudice. The distinction is important: The test measured the split-second reactions of the police officers and the students, not their considered judgments. This kind of bias lies beneath the surface, implicit and often automatic. “Just as black faces and black bodies can trigger thoughts of crime, thinking of crime can trigger thoughts of black people,” the psychologists wrote in an article called “Seeing Black.”

Read more here