@SatchelSF: The OECD PISA survey shows that 19% of students surveyed in the US did not attain what PISA calls “level 2” in reading, and defines as “a baseline level – proficiency Level 2, on a scale with 6 as the highest level and 1b the lowest – at which readers begin to demonstrate the competencies that will enable them to participate effectively and productively in life as continuing students, workers and citizens.”
But that is attainment. It would tell you something about the underlying cognitive capacity of this percentage of the student body if, and only if, everyone attended the same high quality schools under the same circumstances. But not everyone does, and in fact chances are much higher that these students attended low quality schools, in poor circumstances. Now equalise schools and other circumstances (“nurture”) and only then scores like these can tell you something about nature.