Book excerpt: Here biologist Lee Dugatkin outlines four paths to cooperation shared by humans and other animals: family dynamics, reciprocal transactions (or "tit for tat"), so-called selfish teamwork, and group altruism. He draws on a wealth of examples—from babysitting among mongooses and food sharing among vampire bats to cooperation in Hutterite communities and on kibbutzim—o show not only that cooperation exists throughout the animal kingdom, but how an understanding of the natural history of altruism might foster our own best instincts toward our fellow humans.