Lots and lots of email coming in making this point and attributing it to different folks. Here’s a representative email:

Jonah,
Ernest Van Den Haag made an interesting point in his book, “The Jewish Mystique.” During the Middle Ages both the Christian and Jewish communities encouraged their brightest to enter religious service. Priests were, at least nominally, discouraged from having children while Rabbis were encouraged to have large families. Over time this would have an impact on the intelligence of both groups. I have no idea if there is any validity to his claim, but Van Den Haag was adamant.