Most activities that are important for survival of the species, such as eating and sex, are also pleasurable; our brains evolved mechanisms to reward and encourage these behaviors. But we can learn to short-circuit the original activities and tap directly into these reward systems. We can eat foods that have no nutritive value and we can have sex without procreating; we can take heroin, which exploits the normal pleasure receptors in the brain; none of these is adaptive, but the pleasure centers in our limbic system don’t know the difference. Humans, then, discovered that cheesecake just
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