But they're not "counteracted" at all. White people's wages have actually increased. You don't have to try very hard to put a positive spin on the fact that everyone is making more money, women are becoming more equal with men, and previously poor non-whites are now finding more jobs. What's not to like?
It seems unlikely to me that people discovered this explanation for the data and then covered it up. More likely that they simply hadn't noticed that Simpson's paradox was in effect (maybe because they never even looked at the demographic data).
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: