See: Of Badges, Bonds and Boundaries: In-group/outgroup differentiation and ethnocentrism revisited
Johan M.G. van der Dennen

We may imagine that so-called normal people could never believe in anything so ludicrous as the delusional systems of the insane. Yet, historical evidence suggests the opposite. Whole societies have been persuaded without much difficulty to accept the most absurd calumnies about minority groups (e.g., witches, heretics, Jews, 'enemies of the people') portrayed as enemies of the majority. Such accusations, Anthony Storr (1991) claims, originate from a particular type of fantasy which is comparable with, indeed equivalent to, paranoid delusions of the kind found in psychotic subjects.