In a series of posts involving hunting and various personal attributes, the question of how precise a proxy for IQ the simple ten question Wordsum vocabulary test is was raised. When evaluating Wordsum conversions against other good-faith estimation attempts by US geographical region, the answer seems to be "quite precise".
Comparing a paper by Helmuth Nyborg (via Bruce G. Charlton) with a post earlier this year by Inductivist reaffirms that impressive level of precision. Nyborg finds the same surprising thing Inductivist found--Episcopalians have slightly higher average IQs than Jews do*. Of the eight denominations the two comprised estimates for, the correlation between aggregate totals is an impressive .87 (p=.005). Both estimates are for whites only.
Relatedly, Razib of GNXP has done an enormous amount of work on the interplay of religion and intelligence.
* Both survey sources have atheist and agnostic categories, so these Episcopalians and Jews are at least theistic. The Pew US Religious Landscape Survey project found that 1 in 5 Jews are not very certain of the existence of God or do not believe in God at all, while only 1 in 25 Mainline Protestants (including Episcopalians) felt this way.