Parents have little or no effect on childbearing. I often half jokingly tell my three sons that they’re required to have three kids each, but twin studies say I’m wasting my breath. While fertility runs in families, the reason nowadays is almost entirely genetic. A major study of Danish twins born in 1870–1910 found moderate nurture effects on family size. Half a century later, though, these nurture effects had disappeared. Upbringing had a tiny influence on when Danes tried to start a family, but none on the total number of children produced by those thirty-five to forty-one years old. A
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This is incredibly counter intuitive. You'd expect in a pre birth control era to have a larger nature impact and in a birth control era to have a larger nurture impact....
