The publication of the Cochran-Harpending Theory of Ashkenazi Intelligence has stimulated a revival of the popularity of alternative theories tracing back to Sir Francis Galton. Darwin's half-cousin, who coined the term "eugenics," pointed out that since the Catholic Church was the main "career open to talents" in the medieval world, the celibacy rule for priests, monks, and nuns might have lowered the intelligence and morality level of Christian Europe. I have also heard it asserted that Galton pointed out that the Jews practiced the flip side of this by encouraging rabbis to have a large family, but I haven't found proof of that. That particular theory was advanced at least as far back as cyberneticist Norbert Wiener's autobiography.
But is Galton's theory of Christian celibacy, which was only mandatory for priests from 1000 onward, quantitatively plausible? I've never seen an in-depth study. A reader raises doubts bout how how important the effect might have been:
As far as the 'smart boys went to the monastery' theory, birth order is the determining factor in vocations, even into the 20th century. Generally the first son was to inherit the feudal obligations of the father (to work land, fight, etc.). First daughters were to marry others of the same class. It usually was only the third child of either sex that COULD go to the church, so much work was needed and so many children died before adulthood. The Benedictines had no IQ tests to filter their recruits, and even local priests were often illiterate. (Also, they were often fathering children surreptitiously...)