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Helping hand

　　According to the theory, people or animals can pass on their genes by helping their relatives. That means they should be more willing to help brothers or sisters, who share half their genes on average, than more distant relatives who share less.

　　But relatives also have a nasty habit of competing with you for resources, be they chicks squabbling over food, or heirs fighting for the throne. And this is where the flip side of Hamilton's rule comes in.

　　If you kill two brothers, say, you've essentially wiped out your genetic identity rather than preserving it for future generations, says McCullough. "Then you have really damaged yourself in an evolutionary sense. So you can eliminate relatives but only up to a certain level."