Here is a great article from Razib Khan which makes many of the points I made in my “Demographic Math” post, but with more detail and fancy charts.
He is reviewing a book by Eric Kaufmann,
The theme is a point I made, that fertility estimates based on population averages that ignore whether there is a structure involving a mixture of high- and low-fertility groups will misjudge the size and composition of the future population. In particular, religious subgroups will tend to predominate and the effect of secularization in slowing this down is the big unknown.
But there is way too much there for me to summarize quickly. I’m very busy working today so I’ll have more to say in the comments tomorrow, but don’t let that stop you from reading there and commenting here.