Standard deviation (Photo credit: Wikipedia)

Studies relying on large sets of aggregate data are informative in understanding group trends but tell us next to nothing about individuals. That is, the results of such a study - no matter how big or how well done - cannot reasonably be interpreted as suggesting that a particular religious person is any less intelligent than a particular atheist.

Intelligence, as assessed by modern intelligence tests, appears to be normally distributed throughout the population. Using the mean and standard deviation from modern IQ tests (typically 100 and 15, respectively), we can calculate the portion of the population which will obtain IQ scores in various ranges. Most people (68% to be precise) will fall within 1 standard deviation of the mean (i.e., they will have IQ scores between 85 and 115). Another way to frame this would be to point out that most people are, by definition, of average intelligence.

Very small differences become statistically significant when the sample size gets large. This is important because these some differences, while statistically significant, are too small to have much practical importance.

Finding a negative correlation between intelligence and religiosity is certainly interesting, but is a far cry from indicating that religious belief somehow causes people to be less intelligent.

The abstract of a meta-analysis by Zuckerman, Silberman, and Hall (2013) recently appeared online pending publication in, and it is getting quite a bit of attention in the atheist community. The title will clue you in to why it is generating so much discussion: "The relation between intelligence and religiosity: A meta-analysis and some proposed explanations." I have not had the chance to read the paper yet, and I'm cautious about making too much of it based on the summaries I have read. Still, there are a few points that should be considered when trying to understand studies of this sort:I mention these points for two reasons. First, I am seeing quite a few atheists gloating about the results of this study, and I expect few understand the limitations. From what I have read so far, these include but are not limited to the narrow definition of intelligence used by the authors, the reliance on studies conducted in the West and emphasizing U.S. Protestants, and the inclusion of studies that have been criticized by other researchers in the data set. Second, I am seeing too many comments like, "Duh! Was there ever any doubt?" Because we are likely talking about small differences here, I'm not sure such reactions are warranted based on this one study. This stuff tends to be quite complex and far from obvious.