I would think the trend would also need to be evenly distributed. If some groups have higher-than-replacement birth rates, they will simply come to dominate over time.
The authors discuss this a bit. They note that even “higher fertility” subcultures are trending down over time, so it’s not sufficiently clear that anyone is going to remain “above replacement” in the long run. That said, this does seem the weakest point for thinking it an outright extinction risk. (Though especially if the only sufficiently high-fertility subcultures are relatively illiberal and anti-scientific ones—Amish, etc. - the loss of all other cultures could still count as a significant loss of humanity’s long-term potential! I hope it’s OK to note this; I know the mods are wary that discussion in this vicinity can often get messy.)
I would think the trend would also need to be evenly distributed. If some groups have higher-than-replacement birth rates, they will simply come to dominate over time.
The authors discuss this a bit. They note that even “higher fertility” subcultures are trending down over time, so it’s not sufficiently clear that anyone is going to remain “above replacement” in the long run. That said, this does seem the weakest point for thinking it an outright extinction risk. (Though especially if the only sufficiently high-fertility subcultures are relatively illiberal and anti-scientific ones—Amish, etc. - the loss of all other cultures could still count as a significant loss of humanity’s long-term potential! I hope it’s OK to note this; I know the mods are wary that discussion in this vicinity can often get messy.)