I disagree - very high fertility combined with very low rates of extramarital pregnancy are totally normal for traditionalist religious groups-not just ultra-Orthodox Jews, but also the Amish, Salafists, Deobandis, vilayet-e faqih-e faqih Twelvers, the SSPX, inter alia - what Eric Kaufmann (see his book Shall the Religious Inherit the Earth?) calls “endogenous growth sects”. There is nothing unique to Kiryas Joel about it; Kiryas Joel is simply the most easy-to-point-to example of a wider phenomenon