I suspect personal experience is behind most " prejudice "

Letter: David Andrews: Historical facts shows that George Washington was a bigot

    He is certainly NOT a " moron ". General Intelligence seems to be rather equally distributed on the RIGHT and on the LEFT. I learn a lot from " conservative " thinkers like H.L. Mencken. Even Hitler was no dope. Hitler tries to explain his racial psychosis in " My Vienna Days " ( Mein Kampf ). I suspect personal experience not the accumulation of scientific " evidence " is what makes the bigot, what is behind so called PREJUDICE. It is just a rational process - often misleading- to generalize from personal experience. 

A syllogism encountered in your high school Plane Geometry textbook : 

All men are mortal 
Socrates is a man 
Socrates is mortal 

But how do we know ALL men are mortal ?. No record of any man living suspiciously too long Unless we believe in vampires and Count Dracula ? Even Oscar Wilde's " Dorian Gray " was certainly fiction. 

H.L. Mencken titled his book " Prejudices ". As far as I can tell he was prejudiced only against " morons " . He was also prejudiced against " socialists " but forgave them because they could not get rich on their politics. He never met Bernie Sanders . The Russian revolutionary Leon Trotsky would say that Sanders was the ideal socialist leader for successful dentists. But Trotsky was prejudiced against " socialist " Democrats .