You need *four* kids per couple to double the population.
A couple is made up of two people. So for the population to stay the same size, each couple has to produce two kids. For the population to *increase* they need to have more than 2 kids. And if they have *less* than two kids, the population will be shrinking.
Right now the average couple does not have 2-3 kids. They have 1-2 kids.
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