" The problem with keeping our own prejudices to ourselves and just trying not to act on them is that stereotypes influence us even when we do not realize it. Several psychological experiments have shown this quite clearly. In one (Bargh and Chen, 1996), subjects were given a long, boring task on a computer screen. Before each trial, a White or Black male face was shown on the screen so quickly that the subject did not have conscious awareness of the image. When an error message flashed and the subject was told he or she would have to begin the task again, those subjects (White) who had been subliminally shown a Black face showed more hostility than those who had been shown a White face. Our emotional reactions are so automatic that we do not even have to be aware of the stimuli that provoke our prejudices."

http://www.apa.org/pi/oema/racism/q10.html)