第二种形式是互惠的利他主义，或者说“你帮我搓背我也帮你搓背”。
The second form is reciprocal altruism, or "you scratch my back and I'll scratch yours".
他认为，自然选择对于那些曾经是利他主义者的人是有利的，因为我们的祖先生活在小而密集的群体中，人们的利他主义可以指望得到相互间的利益互惠。
He argues that natural selection favored humans who were altruistic because in the small, close-knit groups in which our ancestors lived, altruists could expect reciprocity.
与此同时，其他一些表面上看来的利他主义行为往往也仅仅是互惠。
Meanwhile, other ACTS of seeming altruism are often just reciprocity.
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