What causes people to be mean?

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Let's face it. Mean people are real. They're everywhere and according to some members there are a number of them on this site. There are members who provoke other members into anger. There are others who easily get slighted at innocent comments or remarks made by other members. There are others who feast on the misery of others by simply rebutting everything that he or she says. There are others who are always on the lookout for others' faults and make reports to admin. Why? What causes people to be mean? Is it genetics? Does meanness have anything to do with race? status? education? what?