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Below-replacement fertility is a fake idea and I say that as a demographer https://twitter.com/JDVance1/status/1323061111185440768
"Replacement level" is the fertility rate that would lead to zero population growth over the long term in a mathematical model population called a stable population (closed to migration and with constant rates of fertility and mortality).
Whether a real population's fertility is above or below that level, especially in the short- or medium-term, means basically nothiiiiing
Like idk what to tell you, fertility has been "below replacement" for over 30 years (except 2006-2007) but we've never had negative population growth. We've never even had negative natural increase (births minus deaths, no consideration of migration).
