Aside from the fact that stereotyping usually betrays a very simplistic way of looking at the world, it is most dangerous in the effects that it can have on other people. Basically, when you stereotype someone, you deny them their own personhood and assume that they are the same as everyone else in whatever group you assign them to. While there are positive stereotypes (that can be pernicious in their own right) many stereotypes are negative in nature, and tend to degrade, even dehumanize their subjects. This in turn can be used to justify discrimination. It is also the case that people tend to internalize negative stereotypes, which can have devastating consequences. The "tyranny of low expectations" for minorities in education remains one of the major causes of the achievement gap in this country, to cite one example.