Suppose you were a member of a tribe, and you knew that, in the near future, your tribe would be subjected to a resource squeeze. You might propose, as a solution, that no couple have more than one child—after the first child, the couple goes on birth control. Saying, “Let’s all individually have as many children as we can, but then hunt down and cannibalize each other’s children, especially the girls,” would not even occur to you as a possibility. Think of a preference ordering over solutions, relative to your goals. You want a solution as high in this preference ordering as possible. How do
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