Mr. James also found math imparted career gains to students who did not go onto college. “The more math one takes, the more one earns on average, and the more likely one is to have a job,” he writes.
Either Mr. James or the author of the article is confusing correlation and causation in a big way here. It's even more plausible to me (and many of the WSJ readers leaving comments) that those students with involved and/or wealthy parents, higher IQs, and more diligence are making it through higher levels of high school math. Such kids would most likely do better in their careers anyway and not because they took a couple more math classes.
I would love for it to turn out to be true that more math classes => career gains. But the research discussed in this article doesn't prove that at all.