Atheists score higher on IQ tests than religious people according to some research. Does this mean that people accept religious beliefs because they are dimwitted? Not necessarily.
Atheists are probably more intelligent than religious people because they benefit from many social conditions that happen to be correlated with loss of religious belief. When one looks at this phenomenon from the point of view of comparisons between countries, it is not hard to figure out possible reasons that more intelligent countries have more atheists and that more intelligent states in the U.S. also have more nonbelievers. Here are some. Highly religious countries:
Given that each of these factors are recognized causes of low IQ scores, there is little mystery about why religious countries score lower on IQ tests. Of course, the same phenomena are relevant to comparisons within a country, although within-country differences in these factors are generally smaller. Even so, the wealthier individuals in a country experience life differently than the poorer ones, developing higher IQ scores and greater religious skepticism.
Recent research concluded that part of the reason that people in less religious U.S. states have higher IQs is that they are better educated. According to the authors: "Education enhances rational thinking and provides people with rational, non-mystical mechanisms for understanding the world. In short, education provides people with the opportunity to seek a rational alternative to religious dogma."
This argument is reasonable but it is seriously incomplete. There are a lot more atheists in Europe than in the U.S. and this is not because Europeans are smarter or better educated.
As to the more inflammatory explanations, I doubt that religion causes stupidity if only because some of the most brilliant people of history, such as Isaac Newton, were highly religious like most of their contemporaries.
Whether intelligence causes people to reject religious belief is more complex. It is certainly plausible that highly intelligent people would have a problem accepting some of the more improbable beliefs required by their church. Moreover, modern science offers explanations for phenomena that were previously explained exclusively in terms of religion and intelligent people may prefer the scientific account.
In short, discussing correlations between IQ and religiosity without a grasp of the relevant underlying factors is something of a parlor game. It recalls the long and tiresome debate about the correlation between IQ scores and skin color that got a lot of people very excited but proved a scientific dead end.
The really interesting question buried in all of this is why atheism is sparked uniquely by contemporary conditions in developed countries. I addressed this issue in an earlier post that hurtled around the Internet. The gist of it is that religion helps people confront the terror of uncertainty in their lives. In modern states people get more complacent so that there is less of a market for religion. The inevitable consequence of all this is that religion will decline as human prosperity improves.