An academic ethics web site reporting a review of the evidence;
They cite existing evidence 'pointing to a negative relationship between intelligence and religious belief (coming) from four sources. These are (1) negative correlations between intelligence and religious belief; 2) lower percentages holding religious beliefs among intelligence elites compared with the general population; 3) a decline of religious belief with age among children and adolescents as their cognitive abilities increase; 4) a decline in religious belief during the course of the 20th century as the intelligence of the populations has increased.' There has been plenty of empirical research to establish that case from within nations, eg a representative study in the Netherlands found that non-believers scored in the average a statistically significantly higher IQ than the God squad.
I had always thought that there was a different 'locus of control' with people with religion (ie they see external influences running their life) but wasn't sure if that was as a consequence of having God on their side or because they were the kind of person who had this need and having God to blame/congratulate for everything made the thinking and rationalising easier, as well (maybe) as not having to admit to themselves that they had golly gosh up and having the internal anguish of that.
Now it seems that they are just not very bright (with apologies to those of you with faith).