A defining characteristic of human society is its tribalism: its tendency to gather in groups which define themselves by certain common characteristics, and differentiate themselves—set themselves apart from and at odds—with other groups and individuals who do not share these characteristics... The fact is that people identify with the group of which they are a part, and are suspicious or fearful of those who belong to other groups. This tribalism is true in countless dimensions, from income to accent to national origin to sexual activity to race to musical taste...