> Also the groups that have many kids (the Amish have 7 children on average, and you also have Mormons and Orthodox Jews) will expand exponentially
I agree with your overall point, and about the Amish and Orthodox Jews (especially the ultra-Orthodox, Hasidic/Haredi).
However, for Mormons, I'm not sure that is true. Mormons, in the US at least, have plummeting fertility rates, and also a lot of problems with retaining their younger members. They don't belong to the same category as Amish and ultra-Orthodox do, they are converging to the American secular mainstream while those groups remain on a clearly different trajectory from it.
I agree with your overall point, and about the Amish and Orthodox Jews (especially the ultra-Orthodox, Hasidic/Haredi).
However, for Mormons, I'm not sure that is true. Mormons, in the US at least, have plummeting fertility rates, and also a lot of problems with retaining their younger members. They don't belong to the same category as Amish and ultra-Orthodox do, they are converging to the American secular mainstream while those groups remain on a clearly different trajectory from it.