The current issue of the Journal of Religion and Society has an interesting article which, if social science were written in English, would bear the title "Does religion make you stupid?" The actual title is "Cross-National Correlations of Quantifiable Societal Health with Popular Religiosity and Secularism in the Prosperous Democracies." It's by Gregory S. Paul, and if he were a sociologist you couldn't fault him for choosing the title he did; an academic needs to make a living. But I think he is the Gregory S Paul who writes popular science books on dinosaurs, in which case the title is inexplicable.
A majority of Americans, and all of our candidates for political office, believe implicitly in the words of Ben Franklin, that “religion will be a powerful regulator of our actions, give us peace and tranquility within our minds, and render us benevolent, useful and beneficial to others.”
I got this quote from Paul's article, and the gist of Paul's thesis is that the facts don't bear out Ben Franklin's claim.
America is the most religious country in the developed world, by far. We also have more social pathologies, by far, than any developed country. Paul has collected the charts and graphs to prove it.
Japan, France, and the Scandinavian countries are the most secular nations of the world. The United States, by various measures, displays levels of religiosity otherwise found only in third world countries. Americans, unlike people in every other developed country, tend to doubt or reject evolution. This is untrue in the Western democracies, and in Japan. Japan has the highest levels of acceptance of evolution.
The United States is the only developed country with a third-world murder rate. American adolescents get infected with syphilis and gonorrhea at third world rates. Adolescent abortion rates go hand in hand with disbelief in evolution, and with "increasing belief and worship of a creator." Early adolescent pregnancy and birth in America is "two to dozens" of times higher than in secular democracies. Teen-age abortion rates in the United States are more than twice that of atheistic Denmark, where only slightly over 10% of the public claims to believe in God.
Religiosity correlates with positively with juvenile and young-adult mortality, i.e., our young people die or get killed more often than in secular countries.
We are not only the most religious country in the first world, we are also the richest, leading Paul to comment that "The U.S. is therefore the least efficient western nation in terms of converting wealth into cultural and physical health."
This is interesting. What Paul means by "religion" is fundamentalist Christianity, as anyone can see by the proxies he uses: strong belief in a creator god, disbelief in evolution, frequent religious service attendance, and scriptural literalism. The same markers would probably work for most kinds of fundamentalism, actually. World-wide.
Now he does not come right out and say religion causes stupidity. He says, rather, we need more research on possible cause and effect linkages. I don't disagree with that.
Obviously, though, an alternative hypothesis for the same data would be that inequality and racial injustice make you stupid. Personally, I think that may be more of a cause of the social disfunctions Paul mentions than fundamentalist religion, and that these disfunctions are good indicators of unhappiness, which would then be the cause impelling our people to grasp at religious straws to palliate an otherwise wretched life. This sounds Marxist, I realize. But when Marx spoke of religion as an opiate, he must have been thinking of the Anglicans around him in the reading room of the British Museum. The religion of Jerry Falwell is more like crack cocaine.
To get back to the question of what causes what, it's certainly possible for inequality and racial injustice to be part of a nasty feedback loop with fundamentalist religion. I'd guess that explicit research about that could get you denied tenure in many public university sociology departments, or endanger the grant-attractiveness of departments where the researcher already has tenure. I expect not to see a definitive answer.