More than 400 years before the birth of Jesus of Nazareth, Greek playwright Euripides wrote in his play Bellerophon, “Doth some one say that there be gods above? There are not; no, there are not. Let no fool, led by the old false fable, thus deceive you.”

Euripides was not an atheist and only used the word “fool” to provoke his audience. But, if you look at the studies conducted over the past century, you will find that those with religious beliefs will, on the whole, score lower on tests of intelligence. That is the conclusion of psychologists Miron Zuckerman and Jordan Silberman of the University of Rochester and Judith Hall of Northeastern University who have published a meta-analysis in Personality and Social Psychology Review.

This is the first systematic meta-analysis of 63 studies conducted between 1928 and 2012. In such an analysis, the authors look at each study’s sample size, quality of data collection, and analysis methods and then account for biases that may have inadvertently crept into the work. This data is next refracted through the prism of statistical theory to draw an overarching conclusion of what scholars in this field find. “Our conclusion,” as Zuckerman puts it, “is not new.”

“If you count the number of studies which find a positive correlation against those that find a negative correlation, you can draw the same conclusion because most studies find a negative correlation,” added Zuckerman. But that conclusion would be qualitative, because the studies’ methods vary. “What we have done is to draw that conclusion more accurately through statistical analysis.”

Setting the boundaries

Out of 63 studies, 53 showed a negative correlation between intelligence and religiosity, while 10 showed a positive one. Significant negative correlations were seen in 35 studies, whereas only two studies showed significant positive correlations.

The three psychologists have defined intelligence as the “ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly, and learn from experience.” In short this is analytic intelligence, not the newly identified forms of creative and emotional intelligence, which are still subjects of dispute. In the various studies being examined, analytic intelligence has been measured in many different ways, including GPA (grade point average), UEE (university entrance exams), Mensa membership, and Intelligence Quotient (IQ) tests, among others.

Religiosity is defined as involvement in some (or all) facets of religion, which includes belief in the supernatural, offering gifts to this supernatural, and performing rituals affirming their beliefs. Other signs of religiosity were measured using surveys, church attendance, and membership in religious organizations.

Among the thousands of people involved in these studies, the authors found that gender or education made no difference to the correlation between religiosity and intelligence; however, age mattered. The negative correlation between religiosity and intelligence was found to be the weakest among the pre-college population. That may be because of the uniqueness of the college experience, where most teenagers leave home for the first time, get exposed to new ideas, and are given a higher degree of freedom to act on them. Instead, in pre-college years, religious beliefs may largely reflect those of the family.

The gifted, the atheists

Is there a chance that higher intelligence makes people less religious? Two sets of large-scale studies tried to answer this question.

The first are based on the Terman cohort of the gifted, started in 1921 by Lewis Terman, a psychologist at Stanford University. (The cohort is still being followed.) In the study, Terman recruited more than 1,500 children whose IQ exceeded 135 at the age of 10. Two studies used this data, one conducted by Robin Sears at Columbia University in 1995 and the other by Michael McCullough at the University of Miami in 2005, and they found that “Termites,” as the gifted are called, were less religious when compared to the general public.

What makes these results remarkable is not just that these gifted folks were less religious, something that is seen among elite scientists as well, but that 60 percent of the Termites reported receiving “very strict” or “considerable” religious training while 33 percent received little training. Thus, almost all of the gifted Termites grew up to be less religious.

The second set of studies is based on students of New York’s Hunter College Elementary School for the intellectually gifted. This school selects its students based on a test given at a young age. To study their religiosity, graduates of this school were queried when they were between the ages of 38 and 50. They all had IQs that exceeded 140, and the study found that only 16 percent of them derived personal satisfaction from religion (about the same number as the Termites).

So while the Hunter study did not control for factors such as socioeconomic status or occupation, it did find that high intelligence at a young age preceded lower belief in religion many years later.

Other studies on the topic have been ambiguous. A 2009 study, led by Richard Lynn of the University of Ulster, compared religious beliefs and average national IQs of 137 countries. In their sample, only 23 countries had more than 20 percent atheists, which constituted, according to Lynn, "virtually all higher IQ countries." The positive correlation between intelligence and atheism was a strong one, but the study came under criticism from Gordon Lynch of Birkbeck College, because it did not account for complex social, economical, and historical factors.

It’s the beliefs, stupid

Overall, Zuckerman, Silberman, and Hall conclude that, according to their meta-analysis, there is little doubt a significant negative correlation exists (i.e. people who are more religious score worse on varying measures of intelligence). The correlation is more negative when religiosity measures beliefs rather than behavior. That may be because religious behavior may be used to help someone appear to be part of a group even though they may not believe in the supernatural.

So why do more intelligent people appear to be less religious? There are three possible explanations. One possibility is that more intelligent people are less likely to conform and, thus, are more likely to resist religious dogma. A 1992 meta-analysis of seven studies found that intelligent people may be more likely to become atheists when they live in religious societies, because intelligent people tend to be nonconformists.

The most common explanation is that intelligent people don’t like to accept any beliefs that are not subject to empirical tests or logical reasoning. Zuckerman writes in the review that intelligent people may think more analytically, which is “controlled, systematic, and slow”, as opposed to intuitively, which is “heuristic-based, mostly non-conscious, and fast." That analytical thinking leads to lower religiosity.

The final explanation is that intelligence provides whatever functions religion does for believers. There are four such functions as proposed by Zuckerman, Silberman, and Hall.

First, religion provides people a sense of control. This was demonstrated in a series of studies conducted between 2008 and 2010, which showed that threatening volunteers’ sense of personal control increased their belief in God. This may be because people believe that God makes the world more predictable and thus less threatening. Much like believing in God, higher intelligence has been shown to grant people more “self-efficacy,” which is the belief in one’s ability to achieve goals. So, if intelligent people have more control, then perhaps they don’t need religion in the same way that others do.

Second, religion provides self-regulation. In a 2009 study, it was shown that religion was associated with better well-being. This was interpreted as an indication that religious people were more disciplined in pursuing goals and deferring small rewards for large ones. Separately, a 2008 meta-analysis noted that intelligent people were less impulsive. Delayed gratification may require better working memory, which intelligent people have. So, just like before, intelligence is acting as a substitute for religion, helping people delay gratification without needing divine interventions.

Third, religion provides self-enhancement. A 1997 meta-analysis compared the intrinsically religious, who privately believe in the supernatural, to the extrinsically religious, where people are merely part of a religious group without believing in God. The intrinsically religious felt better about themselves than the general public. Similarly, intelligent people have been shown to have a sense of higher self-worth. Again, intelligence may be providing something that religion does.

Last, and possibly the most intriguing, is that religion provides attachment. Religious people often claim to have a personal relationship with God. They use God as an “anchor” when faced with the loss of a loved one or a broken relationship. Turns out intelligent people find their “anchor” in people by building relationships. Studies have found that those who score highly on measures of intelligence are more likely to be married and less likely to get divorced. Thus, intelligent people have less need to seek religion as a substitute for companionship.

Give me the caveats

This meta-analysis only targets analytic intelligence, which surely is not the full measure of human intelligence despite the ongoing debate about how to define the rest of it. Also, although the review encompasses all studies conducted from 1928 to 2012, it only does so for studies written in the English language (two foreign language studies were considered only because a translation was available). The authors believe there are similar studies conducted in Japan and Latin America, but they did not have the time or resources to include them.

Zuckerman also warns that, despite there being thousands of participants overall, ranging among all ages, almost all of them belong to Western society. More than 87 percent of the participants were from the US, the UK, and Canada. So after controlling for other factors, they can only confidently show strong negative correlation between intelligence and religiosity among American Protestants. For Catholicism and Judaism, the correlation may be less negative.

There are some complications to the explanations too. For example, the non-conformist theory of atheism cannot apply to societies where the majority are atheists, such as Scandinavian countries. The possible explanations are also currently just that—possible. They need to be empirically studied.

Finally, not all studies reviewed are of equal quality, and some of them have been criticized by other researchers. But that is exactly why meta-analyses are performed. They help overcome limitations of sample size, poor data, and questionable analyses of individual studies.

As always, the word “correlation” is important. It hasn’t been shown that higher intelligence causes someone to be less religious. So, it wouldn’t be right to call someone a dimwit just because of their religious beliefs. Unless, of course, you are an ancient playwright looking to provoke your audience.

Personality and Social Psychology Review, 2013. DOI: 10.1177/1088868313497266 (About DOIs).