Birth rate doesn’t seem to be very heritable, given the amount it can change over relatively short periods of time. For example, https://en.wikipedia.org/wiki/

Birth rate doesn’t seem to be very heritable, given the amount it can change over relatively short periods of time. For example, https://en.wikipedia.org/wiki/Birth_rate#/media/File%3A1910-_Fertility_rate_-_United_States.svg https://en.wikipedia.org/wiki/Birth_rate#/media/File%3A1910-... shows it going from 8 around 1940 to 12 around 1957 and back down to 7 around 1972 in the USA.