The altruistic gene doesn't help just any randomly chosen individual. In a sense, it helps copies of itself in a different individual. Generally speaking, full siblings share 50 percent of their genes, so if I can help more than two of my sisters, even at the expense of sacrificing myself, then, on average, such behavior will be favored by natural selection. Hence the famous quip by the evolutionary biologist J. B. S. Haldane. When asked whether he would give his life to save a drowning brother, he replied: "No, but I would to save two brothers or eight cousins."
( Peter Turchin )
[ Ultrasociety: How 10,000 Years ]
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