Humans seek to understand the world around them..they do this through simplifying the world into more easily understandable categories. Culture race religion age sanity intelligence etc all have no definate boundaries but for the most part this is ignored. one main problem is that people fail to acknowledge when these categories are false or even harmfull as this threatens thier view of the world which creates fear. Equilibrium can only be reached when a human feels they know enough about the people around them to assuade that fear...fear being an evolutionary precursor to danger and death.