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<p>A big part of the reason is differences by race, income, and parental education in who takes a college prep curriculum, who graduates from college, and other measures of academic achievement. If you agree with books like Murray and Herrnstein’s “The Bell Curve” and Murray’s “Real Education” you are not surprised by these differences. If you don’t you may regard these differences as evidence of an unjust society and the existence of large amounts of untapped potential.</p>