Given the focus of this web site, it is only natural to link to this blog posting:
Racist and ethnic stereotypes often arise from common fears of the unknown, with variations on a theme playing out in different ways and contexts throughout history. They are often supported by confirmation bias, a logical fallacy by which people tend to look for information that confirms their current beliefs and discount or ignore evidence to the contrary.
Source: 10 Logical Explanations For Ethnic Stereotypes – Listverse