I see what you're saying, but that only works if the process is continuous - if the religious culture that values high birth rates continues for several generat

I see what you're saying, but that only works if the process is continuous - if the religious culture that values high birth rates continues for several generations. For a counterexample, look at Saudi Arabia, another religion-driven state. In 1960, the birth rate was 7.22. Today, it is 2.58. So religious parents have 8 children, and all 8 survive, because it's the 21st century and they're getting vaccinated and not drinking their neighbors' poo anymore. Will all 8 of those children go on to have big families of their own, in keeping with their religious sect's values? Or will they move to town, get better jobs, and start using family planning to have a family they can afford, knowing their children aren't going to die of cholera or smallpox or simple starvation? Interestingly, I did some googling on the Haredim. As of 2010, the poverty rate for the Haredim in Israel was 60%, far higher than the rest of Israel's Jewish population (probably more in line with the occupied Palestinians, who also have a very high birth rate). Population size or not, this is extremely limiting for their political power. Money counts for a great deal. If they can't afford to expand their communities, and refuse to participate in the broader economy, they'll necessarily be limited in their growth. So no, I'm not concerned about this problem. Religious groups that refuse to participate in mainstream society may have high birth rates, but will also have high poverty rates. And I see no evidence at all of this behavior in American society, outside of tiny cults. Even the Mormons, probably the best and largest analogy, have seen their birth rate drop drastically, falling 18% from 2007 to 2014 alone. I'm not expecting the LDS church to overrun the nation anytime soon.