There is no need to introduce the hypothesis that the error is due to people not agreeing with the particular political views of the Blue Party (which is what the "political correctness" hypothesis states), when simple math suffices to explain it.
If the Blue Party has the belief that it's evil to ever think that any non-white racial group is on average worse at anything than white people are and if this belief is one of the most important beliefs of the Blue Party then what there is no need to introduce here is Simpson's paradox.
Steve Landsburg (like myself) is a college professor. The President of his university recently attacked Landsburg on politically correctness grounds. Landsburg's post was an implicit attack on blank slate politically correctness.
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: