..."we all use categories--of people, places, things--to make sense of the world around us. "Our ability to categorize and evaluate is an important part of human intelligence," says Banaji. "Without it, we couldn't survive." But stereotypes are too much of a good thing. In the course of stereotyping, a useful category--say, woman--becomes freighted with additional associations, usually negative. "Stereotypes are categories that have gone too far," says John Bargh, Ph.D., of New York University. "When we use stereotypes, we take in the gender, the age, the color of the skin of the person before us, and our minds respond with messages that say hostile, stupid, slow, weak. Those qualities aren't out there in the environment. They don't reflect reality."
Bargh thinks that stereotypes may emerge from what social psychologists call in-group/out-group dynamics. Humans, like other species, need to feel that they are part of a group, and as villages, clans, and other traditional groupings have broken down, our identities have attached themselves to more ambiguous classifications, such as race and class. We want to feel good about the group we belong to--and one way of doing so is to denigrate all those who who aren't in it. And while we tend to see members of our own group as individuals, we view those in out-groups as an undifferentiated--stereotyped--mass. The categories we use have changed, but it seems that stereotyping itself is bred in the bone.
source: http://www.findarticles.com/p/articl..._20526120/pg_2