IQ and Religious Belief: What the Correlation Actually Says
Every few years someone reports that intelligent people are less religious, and every few years the report is received in one of two ways: as a trophy by the atheists and as an insult by the believers. Both reactions skip the part that matters, which is what the numbers actually are and what they can and cannot support. I want to write that part down, because the finding is real, it is modest, and almost everything said about it in public is an exaggeration in one direction or the other.
The Bottom Line
There is an inverse correlation between intelligence and religious belief. More intelligent people are, on average, less likely to believe. That much is settled; it has been measured in dozens of studies over several decades and it comes out negative essentially every time.
What is not settled is how big it is, where it comes from, and what, if anything, follows from it.
The Numbers
| Level | Correlation (r) | Variance explained | Source |
|---|---|---|---|
| Individuals | about −0.15 to −0.25 | about 2–6% | Meta-analyses of 63–83 studies (Zuckerman, Silberman & Hall 2013; Zuckerman et al. 2019; Dutton et al. 2020) |
| Countries (national averages) | about −0.6 | about 36% | Lynn, Harvey & Nyborg 2009 |
In plain terms: religious believers score roughly 2 to 6 IQ points lower than non-believers, with enormous overlap between the two groups. If you line up a random believer against a random non-believer, the believer will have the higher IQ a great deal of the time. This is not a finding about individuals. It is a finding about distributions that are slightly shifted relative to each other.
How to Read the Numbers
Effect size benchmarks. The usual Cohen thresholds call 0.1 a small correlation, 0.3 a medium one and 0.5 a large one. By that scale the individual-level figure is small. But Cohen's thresholds were a rule of thumb, not a measurement, and when Gignac and Szodorai (2016) actually looked at the distribution of effect sizes in individual-differences research they found that 0.1 is small, 0.2 is typical and 0.3 is relatively large. By that more honest standard, a correlation of −0.25 is an above-average effect for psychology. It is real and it is not negligible. It is also nowhere near a law.
Individual versus country figures. The −0.6 is the number that gets quoted, because it is impressive, and it is also the number that should be quoted least. It compares country averages, not people. Averaging strips out the individual variation, and the country averages share a long list of common causes: wealth, education, health, political stability. Each of those raises measured IQ and each of them lowers religiosity, so the two averages move together whether or not intelligence has anything to do with belief in any particular person. When you control for development the country-level link weakens substantially. Taking a correlation between national averages and applying it to individuals is the ecological fallacy in its textbook form. And that is before you get to the national IQ datasets themselves, which have serious and well-documented quality problems.
So the honest headline is the individual figure, and the individual figure is modest.
Where the Effect Is Strongest
The correlation is not uniform. It is stronger for religious belief than for religious practice: intelligence predicts whether you believe the doctrine better than it predicts whether you turn up on Sunday. It is clearest in Western samples and weaker or absent in some other cultures. And a large share of the studies behind it were done on Western college students, which is a narrow slice of humanity to generalise from.
Proposed Explanations
None of these is established. They are the candidates.
Nonconformity. More intelligent people may be more willing to question the prevailing norms of their society. If this is the mechanism, the effect should be weakest where secularism is already the norm, since there the nonconformist move is toward religion. This is testable and the cross-cultural pattern is at least consistent with it.
Cognitive style. Religious belief has been linked to intuitive rather than analytic thinking, and intelligence to analytic thinking, so the correlation would be a by-product. The experimental work that was supposed to demonstrate this, the priming studies of Gervais and Norenzayan (2012) in which nudging people into an analytic frame of mind reduced reported belief, failed to replicate. The idea may still be right, but the evidence that was offered for it is gone.
Functional substitution. Religion offers a sense of control, self-worth and security. So do the things that come with intelligence: education, income, self-regulation. If both meet the same needs, people who have more of the second may simply need less of the first.
Caveats
The data are correlational and the direction of causation is unknown. Upbringing, education and social class could easily drive both intelligence scores and religiosity, with no direct link between them at all. I have written before about how reliably religion is inherited from one's parents, and that same inheritance carries a great deal else with it.
"Religiosity" is not one thing. Studies measure it as belief in God, as attendance, as self-rated importance of religion, as fundamentalism, and the correlation depends on which you pick.
And a correlation of this size, as I have argued elsewhere, licenses no inference about any individual. It tells you nothing about the intelligence of the believer in front of you, and it is not an argument about whether any religion is true. Smart people have believed absurd things throughout history; rationality is a separate ability from intelligence and IQ tests do not measure it.
What the finding does do is rule out one claim that believers sometimes make, namely that faith and intelligence go together, that the more one understands the more one believes. The data say the opposite, weakly but consistently. That is all they say, and it is enough.