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One thing that most "ethnic cleansing" seems to have in common is a feeling by the aggressors that they are the injured party. The hated minority is blamed for society's failings.

So in the US the blacks are blamed for crime and high taxes (caused by giving too much to people as welfare). In Germany the Jews were blamed for the economic conditions after WWI.

Even when the differences aren't racial the minority group is stigmatized as with intellectuals during China's Cultural Revolution.

There is always a leadership group who benefits by redirecting the anger of the majority group against the hapless minority and thus distracts people from examining the real causes of their discontent.

The fact that so many people are so easily turned into willing followers is another area that has received much study. After WWII it started with Adorno and his colleagues. Most recently has been the work of Robert Altemeyer who has called this the Authoritarian Personality type. Here's a nice literature review of the correlation between those who need to be directed and right wing political beliefs:
Political Conservatism...

I'm sure there are some innate behaviors that makes us treat the "other" differently, but for it to turn into mass action seems to require an economic/political goal.  

Policies not Politics
---- Daily Landscape

by rdf (robert.feinman@gmail.com) on Sat Feb 10th, 2007 at 02:02:20 PM EST

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