Stereotypes

Somewhat in connection with the previous post:

IMO it's natural for people to form stereotypes of various groups of people: one extrapolates from the evidence available, and uses those rules in the absence of the further evidence.

Moreover, if the issue in question is something really important to the person forming the stereotypes, they tend to be stronger: it is really important to all of us not to be attacked in the street, and we avoid the kinds of people that we deem to be the most likely attackers, even if we are fully aware that most likely they aren't. We just don't want to take the risk.

Going from the specific to the general seems quite natural to me. What I don't understand about the stereotypes is some people's tendency to go from the general to the specific, especially in the presense of the evidence to the contrary. Surely people must realize that not everything in the world conforms to their stereotypes, and that stereotypes are just guidelines, and not always good ones. Many of them are rooted in reality, but quite a few aren't.

What is this tendency to ignore the evidence based upon? Surely can't be evolutionary.