Positional effects are a big concern for studies which are basically just a regression on one country’s individuals of their IQ against wealth or income, but you can correct a lot for this by cross-country comparisons and looking at natural experiments like iodization, and the influence of IQ is still positive.
Positional effects are a big concern for studies which are basically just a regression on one country’s individuals of their IQ against wealth or income, but you can correct a lot for this by cross-country comparisons and looking at natural experiments like iodization, and the influence of IQ is still positive.