Primary education has been mandatory for Jewish boys since the first century. Nearly all Jewish men were literate throughout the first millennium, even the farmers. This gave them a massive advantage in high-paying fields that required literacy, since at this time most of the world was still illiterate.
Ashkenazim were the only group of people where, due to persecution, virtually everyone worked white collar jobs. Finance, accounting, writing, money management, merchants, money lending; bookwork, basically. The more intelligent you were, the more money you made. The more money you made, the more kids you had, and the more likely you were to pass your high IQ genes on to the next generation, weeding out the less intelligent as time goes on. The persecution in itself created a survival of the fittest aspect, as intelligence was dire to prosper. There is also the theory of Rabbi's(whom were of high intelligence themselves) practicing eugenics.
Jews make up no more than 3% of the population, yet they make up 50% of the top 200 intellectuals, 40% of the Nobel Prize winners in science and economics, 20% of the profs at the top universities, 40% of the partners of the top law firms and financial institutions, 59% of the producers of the top grossing Hollywood films, 37% of the winners of the national medal of science, 80% of the chairmen and presidents of the Federal Reserve Bank and its Districts, 25% of the Executive branch czars to the President, and 50% of the world's chess champions. (Jews and the New American Scene, 1995)