Historically (e.g. medieval times) the priesthood drew many of most intelligent/academic segment the population, as a compelling alternative to manual labor.
Beginning in the 4th century, Christian priesthood adopted celibacy, which later was made mandatory by the Roman Catholic Church, which sought to prevent inheritance from draining its coffers. http://en.wikipedia.org/wiki/Celibate#Christianity
Meanwhile, Jewish Rabbis have been encouraged to marry and bring up children throughout this history.
Over centuries, you get a selective drift which results in a different distribution of intelligence among the populations. This explains in part the statistical incidence of academic achievement in the jewish population: http://en.wikipedia.org/wiki/Ashkenazi_Jews#Achievements
Note that I'm speaking here of statistical measures. There are plenty of dumb jews, just as non-jews. But thousands of years of policy from the Catholic church has led to a population shift which has excluded many of the historically most academic in nature from procreating over 25 - 50 generations.
An explanation I first came across in Nozick's http://en.wikipedia.org/wiki/Anarchy,_State,_and_Utopia