The Daily Cow

A friend of mine shared with me a research paper (also available here) which discussed the correlation between religious belief and measured intelligence (IQ). I wonder which way the causation goes, or whether there is a third variable that directly influences intelligence and religious belief. Is it that smart people are more skeptical of unproven (and in all scientific senses, unjustifiable) superstitions (intelligence => skepticism), or is it that the resignation of the inquiring scientific mode of thinking due to satisfaction derived from non-scientific sources causes believers to think less deeply about abstract things (unscientific belief => less inquisitive mind)? Or is it that by "religious belief" the real meaning is "Catholicism" (popularity of Catholicism => religious belief and lack of scientific curiosity)? (Since it's the most popular religion in the world) Or maybe, those with lower IQ tend to be more impoverished and are in need of "hope" (Need => religious belief)?