Intelligence.

I heard/read (can't remember) an interesting theory on why this could be. Back in the day... in many religions, the smartest people of the community became leaders of their religion (rabbi, priest, etc). Rabbi's tend to have many children and therefore passed on their genes for intelligence. Priest on the other hand do not (or shall I say, weren't allowed - maybe some did) marry and reproduce. Therefore their intelligence was not passed on.