Human beings by nature identify with others similar to themselves. The more similar two people are, the more closely they will identify with one another. This instinct has evolved from a protection of self. On a simplified level, if I am predisposed to help people who are predisposed to help in the same way, we will all perpetuate the same behaviors, and that behavior will survive. Then simply use this and apply its understanding to understanding why humans help those who are similar to themselves.
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