To summarise briefly, a recent review of 63 studies showed that there is a moderate negative relationship between intelligence and religiosity (Zuckerman, Silberman, & Hall, 2013). The review found that religious beliefs, such as belief in God, are somewhat more strongly related to lower intelligence than religious behavior, such as church attendance. The authors estimated that the average difference in IQ points between believers and nonbelievers ranged from 6.2 for non-college samples to 7.8 for college samples. This difference is roughly half a standard deviation in size, so this represents a reasonably substantial effect rather than something trivial.
Studies like this are correlational in nature, so it is not possible to decide for sure what is causing the relationship. That is, we do not know whether intelligence causes people to be less religious, whether religion dampens a person’s intelligence, or whether there is some third variable underlying both. Dr Chamorro-Premuzic proposed that an underlying factor that might link intelligence and religiosity could be the personality trait openness to experience. This trait refers to the breadth and complexity of a person’s mental life. Openness to experience is positively correlated with general intelligence. Additionally, studies have found that non-religious people tend to be higher in openness to experience than the religious (Galen & Kloet, 2011), and that greater openness to experience is associated with more disbelief in God (Shenhav, Rand, & Greene, 2011). (I have written about this in a previous post here.)
Openness to experience, along with intelligence, is also associated with greater general knowledge of the world. This may be because people who are high in openness to experience are intellectually curious and therefore motivated to learn new things about the world. I think this is interesting because a Pew Forum survey on the religious knowledge of Americans found that atheists and agnostics had substantially more knowledge of religion than Christians on average. (A summary of the survey results can be viewed here, while the full report is here. You can take the quiz yourself here.) In fact, atheists and agnostics scored higher on religious knowledge than any other group surveyed, including those who were “nothing in particular”, although Jews and Mormons also scored higher than the remaining groups interestingly enough. A breakdown of the results showed that Mormons had the most knowledge about Christianity, although atheists/agnostics and Jews knew more about Christianity than mainstream Christians on average. Atheists/agnostics, closely followed by Jews, had the most knowledge of world religions, such as Islam, Hinduism and Buddhism. Atheists/agnostics and Jews tend to be more educated than the other groups, and more education is associated with greater religious knowledge. However, even after taking education into account, atheists/agnostics and Jews still maintained their knowledge advantage over Christians generally. The Pew survey also included a short test of general knowledge for comparative purposes. Atheists and agnostics also scored higher on this test than any other group, although Jews were again a close second, Mormons did somewhat less well, and other Christians were even further behind. Additionally, those who did well on the test of religious knowledge also tended to score well on general knowledge too, suggesting that those who know a lot about religion tend to be more knowledgeable in general.
The Pew survey report did not offer an explanation of why atheists/agnostics showed greater religious knowledge than most religious people. A number of possible explanations come to mind. As noted previously, people who are not religious tend to be more intelligent than the religious, and there is a positive association between intelligence and knowledge. An additional consideration is that atheists/agnostics, being higher in openness to experience, may have more interest in acquiring knowledge in general than the religious. This raises a question about the direction of causality. Does rejection of religion motivate people to gain more knowledge or does gaining knowledge lead to rejection of religion? Arguments can be made for each of these, although the actual answer might involve a combination of both, or even some third factor.
Dave Silverman, president of the American Atheists, interpreted the results of the Pew survey to mean that the more someone learns about religion the more likely they are to reject it as mythology. This may well be true for some people, but the results of the survey suggest more nuanced possibilities. For example, the survey showed that higher religious commitment (referring to personal importance of religion and frequency of attendance of religious services) was associated with greater knowledge of the Bible but not of non-Christian religions. As noted earlier, Mormons actually showed the most knowledge of Christianity, but also were not particularly knowledgeable about world religions. This suggests that being knowledgeable about one’s own religion at least does not necessarily lead a person to reject it. On the other hand, the survey also found that people with greater religious commitment scored less well on the general knowledge questions than those who were less committed. It might be that people who are religiously committed mainly seek out information that confirms their existing views, and are less interested in information about alternative belief systems or about secular matters. Atheists/agnostics were more knowledgeable about world religions, so perhaps being aware of alternative belief systems might facilitate the realization that they are all basically arbitrary. On the other hand, atheists/agnostics also scored higher on general knowledge, so it might be that a person’s desire to gain knowledge about a wide range of subjects increases the likelihood of their becoming an unbeliever. Alternatively, it may be that people who have a particular thirst for knowledge find religious belief incompatible with their natural curiosity. Recently, Pope Francis actually stated that “the spirit of curiosity distances one from God.” Many atheists and agnostics may well agree.
Furthermore, some scholars have argued that conservative and fundamentalist religious beliefs can discourage learning (Sherkat, 2010). The Pew survey found that respondents who believed that the Bible was the literal word of God tended to have less religious knowledge than those who endorsed a less literal interpretation, particularly those who said it was a collection of fables. Other studies have found that conservative Christians and fundamentalists tend to have poorer knowledge of science (Sherkat, 2011) and to have a poorer vocabulary (Sherkat, 2010) compared to other religious groups and the religiously unaffiliated. Sherkat (2010) has argued that conservative Christians actually shun information from external sources and scorn the search for knowledge as sinful in that it is equated with pridefulness and self-love. Additionally, they try to “purify” information sources they attend to, e.g. only viewing media sources that are loyal to Christian doctrine. Such close-minded attitudes may then prove a barrier to learning, resulting in less knowledge.
It could be that religious fundamentalism inhibits a person ability to acquire knowledge, or it may be that people who lack interest in expanding their worldview have a preference for narrow minded belief systems. Orthodox religious beliefs generally seem to discourage people from questioning core tenets which are supposed to be accepted on faith. However, it is possible that some religions are more supportive of learning than others. The studies in the review by Zuckerman et al. mainly looked at Christian denominations rather than other religions, such as Jewish. As noted earlier, Jews scored very highly not only on religious questions but on general knowledge as well. Jewish culture traditionally places a high value on intellectual activities, such as reading, that facilitate acquiring knowledge (Fejgin, 1995). Perhaps Jews generally do not share the Pope’s view that curiosity distances people from God?
As noted earlier, Mormons did somewhat less well on general knowledge than atheists/agnostics and Jews, but somewhat better than other Christians. Additionally, they were more knowledgeable about Christianity than other Christians. Unfortunately, I currently have no idea why this might be the case. Future research studies might examine whether there is something special about Mormons that would account for this.
Based on the foregoing I am inclined to think that one of the reasons that higher intelligence is associated with less religious belief might have something to do with the desire that intelligent people have to acquire knowledge. Intelligence tends to be associated with openness to experience, and one of the core features of openness to experience is intellectual curiosity. People with high levels of intellectual curiosity might access more information that helps them to question religious beliefs. Alternatively, such people might find religious beliefs unappealing insofar as they discourage intellectual activity. However, it should be noted that the relationship between intelligence and religiosity in non-Christian traditions has not yet been examined. Perhaps this relationship applies more to people from Christian rather than say Jewish backgrounds, because of the more intellectual nature of the Jewish religion. Longitudinal studies, which follow up respondents over extended periods of time, would be needed to determine what role the desire for knowledge might play in shaping a person’s religious identity, and whether this mediates the relationship between intelligence and religious belief.
Finally, I know that this is a sensitive subject, and I want to point out that the statistical trends discussed in the studies cited should not be interpreted as absolute generalizations and that exceptions apply. Some religious people are highly intelligent, some unbelievers are the opposite, and the general findings may not apply to specific individuals.
 Atheists and agnostics were treated as a single group in the Pew survey because the number of respondents in each of these two categories was too small to allow separate analyses of each group.
 Those who identify as “nothing in particular” do not belong to a particular religion. However, this does not necessarily mean they are particularly secular either. A separate Pew survey found that two-thirds of religious “nones” believed in God or a higher power, and a slight majority described themselves as either a religious person or as “spiritual but not religious.” Hence, it is reasonable to treat self-identified atheists/agnostics as a distinct and separate group who reject religion altogether.
 On the other hand, there are many people who consider themselves Jewish due to their heritage but who are not particularly religious. This could be considered in future studies.
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© Scott McGreal. Please do not reproduce without permission. Brief excerpts may be quoted as long as a link to the original article is provided.
Other posts discussing intelligence related topics
The Knowledgeable Personality
What is an Intelligent Personality?
Emotional intelligence is not relevant to understanding psychopaths
Why there are sex differences in general knowledge
Personality, Intelligence and “Race Realism”
Intelligence and Political Orientation have a complex relationship
Think Like a Man? Effects of Gender Priming on Cognition
Cold Winters and the Evolution of Intelligence: A critique of Richard Lynn’s Theory
The Illusory Theory of Multiple Intelligences – a critique of Howard Gardner’s theory
Other posts about the psychology of religion and/or spirituality
Troubled Souls: Spirituality as a Mental Health Hazard
The Spirituality of Psychedelic Drug Users
Reason versus Faith? The Interplay of Intuition and Rationality in Supernatural Belief
Opening the Mind: Where Skepticism and Superstition Meet
Is Insulting Religion "Extremism"?
Dogmatism and Openness to Experience in the Non-Religious
Belief in God Supports Prejudice against Gays and Atheists
What Oprah doesn’t understand about Awe and Atheists
Belief in Hell: Does it Benefit of Harm Society?
Fejgin, N. (1995). Factors Contributing to the Academic Excellence of American Jewish and Asian Students. Sociology of Education, 68(1), 18.
Galen, L. W., & Kloet, J. (2011). Personality and Social Integration Factors Distinguishing Nonreligious from Religious Groups: The Importance of Controlling for Attendance and Demographics. Archive for the Psychology of Religion, 33(2), 205-228. doi: 10.1163/157361211x570047
Shenhav, A., Rand, D., & Greene, J. (2011). Divine intuition: Cognitive style influences belief in God. Journal of Experimental Psychology: General. doi: doi: 10.1037/a0025391
Sherkat, D. E. (2010). Religion and verbal ability. Social Science Research, 39(1), 2-13. doi: http://dx.doi.org/10.1016/j.ssresearch.2009.05.007
Sherkat, D. E. (2011). Religion and Scientific Literacy in the United States. Social Science Quarterly, 92(5), 1134-1150. doi: 10.1111/j.1540-6237.2011.00811.x
Zuckerman, M., Silberman, J., & Hall, J. A. (2013). The Relation Between Intelligence and Religiosity: A Meta-Analysis and Some Proposed Explanations. Personality and Social Psychology Review. doi: 10.1177/1088868313497266