Wikipedia:No Ethnic Epithets
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Some people pushing a political agenda find it convenient to dismiss sources disagreeing with their position as being affiliated with "That Evil Tribe Down the River". In the worst case, identifying unpleasant sources and attaching their (supposed) ethnic affiliation to the article in effort to depict such a source as partisan and unobjective can develop into a sort of cottage industry.
Wikipedia's policies regarding ethnic classification of sources are simple: don't. Most importantly, it runs counter to No Original Research, one of the basic rules of Wikipedia.