If you take any country that was analysed in this study and look at the distribution of reading comprehension levels in it, the distribution will look roughly the same: those with high socio-economic status (relative to the rest of the population) get high scores, and the poorest tend to get the worst scores.

Not necessarily, it looks like countries fall into three categories: egalitarian outcomes, ones where wealth correlates closely with results and finally where results improve with wealth until you go from upper middle class to upper class, where results fall off.

It’d be interesting to know what the public attitude to reading and education in general in the analysed countries is and how that interacts with economic factors. Students’ in-school performance is often influenced by their parents’ attittude towards eduction (e.g. education as a means to an end vs education having intrinsic value even without any specific ends), and I wonder how big the impact of society’s attitudes is when compared to economic factors.

The article mostly correlates it with state level income

There is a second key insight from this research that is worth highlighting: the average income level of the country is more important for a student’s learning than the income of the particular family within that country.9

For example, look at the test results of the poorest students in Korea or Finland. The poorest Korean or Finnish students are poorer than the rich students in Brazil, but their math scores are much higher.

But the drop off for the upper class would indicate that social factors also influence outcomes.