On the individual level, differences in general intelligence evidence biological differences. Steppan (2010), however, finds that this is not the case on the national level between protestant and catholic countries. (With regards to other national differences, I’d advise HBDers to take pause when drawing inferences.)
Meisenberg (2007) poses the question to the point: if national IQ and PISA-tests measure the same, how can we assess the quality of educational institutions? The conclusions here permit to confirm that IQ and PISA indeed measure the same thing on the international level, namely two big sources of variance: non-cognitive determinants of cognitive test scores (long-term motivation, test-motivation, psychic capacity) on the one hand and the cognitive enrichment of the environment (g-) in a country on the other hand – and not hereditarian intelligence (g+). Both datasets are therefore applicable to measure the ability to think (Rindermann, 2006) on the aggregated (country) level, but just the software (g-) learned throughout lifetime and not the biological hardware (g+). That is why the Protestantism-effect nearly equally affects both kinds of tests, and why it is still reasonable to compare school systems based on PISA results.