There is a discussion about religion and IQ, specifically that of atheists. Are atheists smarter than others?
As luck would have it there are great datasets that can answer questions like this. The NLSY79 and NLYS97 have one of the most accurate measures of general intelligence for large datasets (the surveys are designed so that we can get nationally representative results from them).
From the NLSY97 I have the IQ, and religion as measured in 2005 (where most of the sample are in their 20s).
Atheists and Agnostics have far higher IQ than the average. On average atheists/agnostics have 8 I.Q points higher than the national average, which is half a standard deviation.
However, and on closer inspection not surprisingly, people who don’t believe in god and who identify as instead having “a personal philosophy”, are around average in intelligence. This group is much larger than atheists and agnostics.