In a 2013 meta-analysis of 63 studies, led by professor Miron Zuckerman, a correlation of -.20 to -.25 between religiosity and IQ was particularly strong when assessing beliefs (which in their view reflects intrinsic religiosity) but the negative effects were less when comparing with behavior (such as church going). They note limitations on this since viewing intrinsic religiosity as being about religious beliefs represents American Protestantism more than Judaism or Catholicism, both of which see behavior as just as important as religious beliefs. They also noted that the available data did not allow adequate consideration of the role of religion type and of culture in assessing the relationship between religion and intelligence. Most of the studies reviewed were American and 87% of participants in those studies were from the United States, Canada, and the United Kingdom. They noted, "Clearly, the present results are limited to Western societies." The meta-analysis discussed three possible explanations: First, intelligent people are less likely to conform and, thus, are more likely to resist religious dogma, however this theory was contradicted in mostly atheist societies such as the Scandinavian populations, where the religiosity-IQ relationship still existed. Second, intelligent people tend to adopt an analytic (as opposed to intuitive) thinking style, which has been shown to undermine religious beliefs. Third, Intelligent people may have less need for religious beliefs and practices, as some of the functions of religiosity can be given by intelligence instead. Such functions include the presentation of a sense that the world is orderly and predictable, a sense of personal control and self-regulation and a sense of enhancing self-esteem and belongingness.
However, a 2016 re-analysis of the Zuckerman et al study, found that the negative intelligence-religiosity associations were weaker and less generalizable across time, space, samples, measures, and levels of analysis, but still robust. For example, the negative intelligence–religiosity association was insignificant with samples using men, pre-college participants, and taking into account grade point average. When other variables like education and quality of human conditions were taken into account, positive relation between IQ and disbelief in God was reduced. According to Dutton and Van der Linden, the re-analysis had controls that were too strict (life quality index and proximity of countries) and also some of the samples used problematic proxies of religiosity, which took away from the variance in the correlations. As such, the reduction of significance in the negative correlation likely reflected a sample anomaly. They also did observe that the "weak but significant" correlation of -.20 on intelligence and religiosity from the Zuckerman study was also found when comparing intelligence with other variables like education and income.
Researcher Helmuth Nyborg and Richard Lynn, emeritus professor of psychology at the University of Ulster, compared belief in God and IQs. Using data from a U.S. study of 6,825 adolescents, the authors found that the average IQ of atheists was 6 points higher than the average IQ of non-atheists. The authors also investigated the link between belief in a god and average national IQs in 137 countries. The authors reported a correlation of 0.60 between atheism rates and level of intelligence, which was determined to be "highly statistically significant". ('Belief in a god' is not identical to 'religiosity.' Some nations have high proportions of people who do not believe in a god, but who may nevertheless be highly religious, following non-theistic belief systems such as Buddhism or Taoism.)
The Lynn et al. paper findings were discussed by Professor Gordon Lynch, from London's Birkbeck College, who expressed concern that the study failed to take into account a complex range of social, economic and historical factors, each of which has been shown to interact with religion and IQ in different ways. Gallup surveys, for example, have found that the world's poorest countries are consistently the most religious, perhaps because religion plays a more functional role (helping people cope) in poorer nations. Even at the scale of the individual, IQ may not directly cause more disbelief in gods. Dr. David Hardman of London Metropolitan University says: "It is very difficult to conduct true experiments that would explicate a causal relationship between IQ and religious belief." He adds that other studies do nevertheless correlate IQ with being willing or able to question beliefs.