> Another theory suggests that there was selective breeding for Talmudic scholarship. This seems unlikely to have been an important selective factor, since there weren’t very many professional rabbis, certainly less than one percent of the population.

Although there are very few rabbis, majority of ultra-orthodox men spend most part of their time in rabbinic studies until they're very deep into adult life, and their level of success in that field is directly converted into their social status. (Which also means that they don't work and depend on their wives (who are also expected to care about the house and have a lot of children) as well as welfare from state and charities to survive).