For humans, sex for propagation is crucial until a population stabilizes. When human populations reach a state of balance and security, sex for propagation decreases. In the United States, where most parents expect their children to survive and also expect that they themselves won’t be out on the streets with a cup when they’re old, the average number of offspring per family is fewer than two. However, any population that is threatened will initiate reproduction earlier and reproduce more—they’re unconsciously doing the calculation that some of their children are not going to survive and that
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