In fact, the human penchant for drugs may be the accidental by-product of two completely different adaptive behaviors. This at least is the theory Steven Pinker proposes in How the Mind Works. He points out that evolution has endowed the human brain with two (formerly) unrelated faculties: its superior problem-solving abilities and an internal system of chemical rewards, such that when a person does something especially useful or heroic the brain is washed in chemicals that make it feel good. Bring the first of these faculties to bear on the second, and you wind up with a creature who has
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