Field of Science

How come Intelligence, religion, and fertility are linked?

Here's a new study looking at the connection between religion, fertility, and IQ at a national level. We know from previous studies that countries where people are, on average, more religious also tend to have higher average fertility and lower average IQ.

The problem is that we also know that countries that have lower average IQ also have higher fertility. So teasing out the two factors is not obvious.

This is what Charlie Reeve (University of North Carolina Charlotte) has investigated.

What he found was that, even after correcting for per capita wealth, both religion and IQ were independently related to fertility. What's more there was a fascinating interaction.

As you can see in the graph, countries with low religious belief had low fertility, no matter what their average IQ. What's more, countries with high average IQ had low fertility, no matter what their average level of religious belief.

It's only in countries with low IQ that religiosity pumps up the fertility rate. And the effect is quite dramatic - a 150% increase in fertility rate for countries that have both high average religiosity and low average IQ.

Reeve also took a look at other health-related measures, and found a similar effect. Religiosity and low IQ combine to push up both infant mortality and maternal mortality.

With life expectancy, a somewhat different picture emerged (shown in the second figure). In high IQ nations, high religiosity increased life expectancy. In low IQ nations, high religiosity decreased it.

What could be going on here? Well, it's pretty hard to figure out. A simple explanation is that IQ is a buffer against the effects of religion. However, my gut feeling is that there is some other factor mediating the interaction.

Variations in national average IQ probably reflect other social and cultural factors - access to education, for example, as well as democratic and open societies. It may be that religion has a particularly big effect in nations lacking these structures.

One of these factors is our old friend wealth inequality. There does appear to be a relationship between inequality and IQ, perhaps mediated by education.

That said, we do have to treat these results with caution. They don't show that individuals with low IQ and high religion have more kids (although I wouldn't be surprised if that was the case), only that countries with this mix have high fertility. And we can't be sure how robust the results are - I suspect there are very few countries in the 'high IQ + high religion' category.

What's more, the measures of IQ and religion are non-standardised. IQ especially is very susceptible to cultural bias. Reeves does try to reduce this by redoing the analysis after cutting out the countries with very low IQ, but doesn't present the results (only reports that they are essentially the same.

Still, this is a very provocative paper. What it suggests is that, in wealthy nations, religion is not so important as a determiner of fertility. That certainly seems to be the case in Europe.
Reeve, C. (2009). Expanding the g-nexus: Further evidence regarding the relations among national IQ, religiosity and national health outcomes. Intelligence, 37 (5), 495-505 DOI: 10.1016/j.intell.2009.06.003

Creative Commons License This article by Tom Rees was first published on Epiphenom. It is licensed under Creative Commons.


  1. Well, the No 1 factor of fertility is availability of contraceptives, which is not inlcuded in the study, so I'm not quite convinced on its merits.
    In most countries, fertility is not a question of choice but of education, money and resources - in short, of being privileged (especially if contraception is not readily available).

    In Europe, fertility rates are relatively high where having kids doesn't influence the mother's career negatively, and where men are relatively more emancipated from traditional gender roles. They are down where traditional gender roles are still the norm (eg Italy, Spain, Eastern Europe like Hungary, Slovakia, etc.)

  2. The low IQ + high religiosity => high death rate connection is probably easy to explain: people of high religiosity and low IQ are more likely to believe that prayer, religion-based pseudoscience, and religious folk remedies (even things like trepanning) are the "cures" for diseases or general malaise

  3. Nice post, thanks.

    By the way. Have you ever done a piece on IQ specifically? I'm pretty suspicious of it as a measure of intelligence (re Pinker, Gould, etc.)

  4. Hi Michael: no, but I share your suspicions. There's clearly a major cultural and educational effect contributing to IQ, but for the purposes of looking at the link to religion/fertility, I think that doesn't really matter.

    IQ does measure something related to intelligence. Perhaps it measures the particular kind of abstract reasoning that is encouraged in modern education systems and is prized in industrialized economies. Whatever it is, this aspect of intelligence is inversely related to religion and fertility. The fact that IQ can be modified by your environment is doesn't (of course) change that relationship.

  5. Hello,

    I am the author of the paper you discuss here. First, I'd like to thank you for presenting a relatively accurate summary of the work (not always the case). Second, just as an FYI, I have a follow up paper that largely replicates these effects using the 50 US States as observations (Reeve & Basalik, 2010, in the same journal). Third, I have a forthcoming paper that does investigate the potential mediating role of education within this nexus. Education does in fact mediate a significant portion of the effect of IQ, but not all of it.

  6. it not to say that one causes the other. we do not know that ..

  7. Pseudoscience, pseudoscientists. Another 50 years and a doctorate will be the new high school diploma. There is nothing scientific, medical, or even academic about this blog. Another dude clever at turning phrases around, a writer is all you are, so stick to writing, soap operas, stories for children, and stay away from the scientific community, please. Thank you.


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