The point of the CPS “study” is that children with identical academic abilities received different educations. In other words, children received different forms of “nurture”—some went to academically rigorous high schools, and others went to less successful high schools. This is seemingly an ideal opportunity to measure the influence of education on success—and therefore, the influence of nurture against nature. Right away, however, the study assumes that school tests are capable of measuring a student’s ability accurately, so that two students with the same test scores really do have the same ability. This is a somewhat questionable assumption (there are racial biases on many tests, and there are many smart students who don’t learn how to be good “test takers” until they’re in high school), but for the purposes of this case study we’ll assume that it’s true.