Here's a cool piece of research to be published in the journal INTELLIGENCE. Richard Lynn, John Harvey and Helmuth Nyborg from universities in the UK and Denmark have tried to investigate whether there is a negative relationship between IQ (measured in psychometric terms) and religious belief. They cite existing evidence 'pointing to a negative relationship between intelligence and religious belief (coming) from four sources. These are (1) negative correlations between intelligence and religious belief; 2) lower percentages holding religious beliefs among intelligence elites compared with the general population; 3) a decline of religious belief with age among children and adolescents as their cognitive abilities increase; 4) a decline in religious belief during the course of the 20th century as the intelligence of the populations has increased.' There has been plenty of empirical research to establish that case from within nations, eg a representative study in the Netherlands found that non-believers scored in the average a statistically significantly higher IQ than the God squad.
Anyway, Lynn, Harvey and Nyborg decided to investigate the relationship between average IQ and religious belief between nations. They then compared the average IQs of most of the world's nations against the percentage of believers in those nations. Their finding is that 'the negative correlation between IQ and religious belief that has been found in numerous studies within nations is also present between nations.' Explaining the surprisingly high percentage of religious believers in the USA, they observe that 'religious belief has a significant heritability of around 0.4-0.5, so it could be that a number of religious emigrants from Europe had the genetic predisposition for religious belief and this has been transmitted to much of the present population.'
No surprise then that nations with an average IQ that's not overly high tend to be found among those countries with a particularly low percentage of non-believers. Check out the country-by-country statistics presented in the paper. Kinda fun reading if you're suitably prejudiced:).
Disclaimer: I am aware of the academic work highly critical of the concept(s) of IQ. Enjoyed the paper anyway. Here's the doi:10.1016/j.intell.2008.03.004
Thanks to my good friend Michael Ristow (now a medical prof in Germany, back in our student days a fellow leftwing activist) who kindly passed this reference on to me!