Let's refer to this combination of IQ, education, and predilection for rational thinking as the Functional Level Quotient, or FLQ.
(I probably should have drawn the "escape delusion" circle as an oval that extends lower than I have indicated, but the point is that there is both overlap and separation according to numerous meta-analyses.)
The studies indicate that the average FLQ of atheists is higher than the average FLQ of theists. Although the believer category undoubtedly does include some highly intelligent individuals, belief is statistically more likely to be correlated with lower FLQ.
"Of 43 studies carried out since 1927 on the relationship between religious belief and one's intelligence and/or educational level, all but four found an inverse connection. That is, the higher one's intelligence or education level, the less one is likely to be religious or hold "beliefs" of any kind." ~ Bell, Paul. "Would you believe it?" Mensa Magazine, Feb. 2002, pp. 12–13
Several posts have appeared lately that are internally contradictory concerning this point. However, I do agree with the chaplain that there is also a basic difference in the "mindset" of atheists versus theists. I think that those who de-convert are more rational than those who remain deluded.
American fundamentalist religionists, in particular, sometimes exhibit quite extraordinary levels of misinformation and illogic when one considers that America is a wealthy nation, which does boast some excellent universities. Religious indoctrination beginning in early childhood and protected by home-schooling, anti-intellectual propaganda and general antagonism toward learning, pseudoscientific polemics, outright 'misinformation', widespread rhetoric employing emotional fallacies of logic, coupled with a beleaguered educational system may explain the recent very poor test results of American 15 year olds.
The intriguing question is which is the chicken, and which the egg. Does indoctrination into religious belief, particularly into fundamentalist religious beliefs, actually discourage logical thinking and encourage conservatism? I strongly suspect so.
Many studies indicate that IQ scores generally correlate with educational levels – kids who are moved into a more stimulating environment display a jump in their IQ score. There is also abundant evidence that a nation's IQ scores correlate with its socioeconomic status – wealthier nations have more resources to devote to education, which ideally stimulates analytical thinking.
The intriguing question is not so much why lower levels of education are more likely to be correlated with higher levels of religiosity, but how the US came to have such inordinately high levels of religiosity for its socioeconomic status.
I have not finished reading Karen Armstrong's The Battle for God yet. However, one thing struck me about Protestantism and evangelical preaching on America's frontier. When Martin Luther emphasized the priority of personal scriptural study rather than reliance on the authority of priests, he set people up to assume that their personal opinion carried weight regardless of their educational training.
The paucity of preachers available to minister to Protestants scattered along America's early frontier forced reliance on personal interpretation of the homesteaders' Bibles. Luther's emphasis on individual exegesis was ideal under these conditions, perhaps adding to American contempt for intellectualism – after all, these people had been instructed to consider personal opinion paramount.
This arrogance concerning personal opinion and fundamentalist indoctrination, coupled with ignorance of science, logic, and critical thinking, may have set America up for obstinate, literalist interpretation not only of the scriptures but of many areas of knowledge. A little knowledge may be a dangerous thing, but obstinate confidence in a very little knowledge is an obnoxious and potentially very dangerous thing.