Your understanding of the statistics is lacking. A 84% reproductive cultural "replacement rate" means that in order to maintain the population protestants nee

Your understanding of the statistics is lacking. A 84% reproductive cultural "replacement rate" means that in order to maintain the population protestants need to have an average of 2.38 (2/0.84) kids to have the same number of protestants in the next generation. The real number is actually much higher than that because what you really need is to measure how many children of two protestants also marry a protestant. This percentage is lower than (pew estimates this as 75%) so now you need to have have an average of 3.17 (2/(.83*.75) kids just to maintain the protestant population. These are, of course, woefully inaccurate numbers and exclude all kinds of factors. The reason I present them isn't to try to estimate what the numbers are but to make it clear that "Any group that maintains a positive fertility rate will multiply indefinitely" is not simply not anywhere near true. You just can't accurately estimate the growth rate of a cultural identity by looking at the birth rate. > For a religion like Islam that integrates even more significantly into one's life (and correlates very positively with good fertility rates), these figures are going to be even higher and getting within statistical noise of 100%. That's not what the science says. The rates of transmission are only slightly higher and definitely don't approach 100%. These transmission rates also aren't "natural laws" but contigent effects that are driven by any of a number of different cultural, religious, economic and legal factors. Even if a minority cultural identity does manage to grow through reproductive practices, it can still be fragmented by schisms (especially common with religious identity) or out competed by memetically superior identities that pulls converters from a wide range of cultural identities. Your view is here is incredibly simplistic and ignores all kinds of basic evidence from throughout history.