But of course correlation != causation. My guess: the real independent variable is wealth, so wealthier countries (e.g. the secular West, with the exception of

But of course correlation != causation. My guess: the real independent variable is wealth, so wealthier countries (e.g. the secular West, with the exception of USA) will have both higher IQ and lower religiosity. It seems obvious to me that long-time wealthy countries would have higher IQ scores (especially with the immigration boost - wealthiest countries can attract the smartest immigrants). If this is all the case, the deeper question is why increasing wealth reduces religiosity. (Edit: What's funny about this question is that you can answer it either way to fit theist or atheist narratives - e.g. "it's because people in wealthy countries become insulated to our true reliance-on-God nature" vs. "only poor people need to resort to fanciful beliefs to make themselves feel better".)