That isn't a bad heuristic either! However in practice the aggregate claims about scores are often used to make an argument relying upon internal homogeneity, and I don't see the corresponding "scores are up among all groups so aggregate scores must be higher" often, which would require more information and space.
ETA: The table linked by Landsburg has been called into serious question by Evan Soltas [H.T. CronoDAS]. I edited the post to leave only the table to provide context for the comment discussion of its status.
Economist Steve Landsburg has a post [H.T. David Henderson] about the supposed stagnation of median wages in the United States in recent decades. In the linked table median wages have risen for: