learning German boosts earnings more than learning French or Spanish
This may be true overall, but it doesn't take into account how language will affect earnings
by field. They averaged everything together. In the world of finance or international trade, German might boost your earnings. But if you want to go into anything human-services related, Spanish (in the U.S.) will make you a much more desirable hire, while German will get you squat.
The paper attempted to control for this, but failed in two ways. First (and less serious), they used college major as a proxy for career choice, which is a very inaccurate measure. Second and more important, they merely used that information to adjust the statistics to eliminate it as a source of bias in their results, but they still were lumping everyone together. When you average everyone together, German is buying
somebody enough more that it pulls the whole statistic in favor of German.
Instead, they should have been looking by career category. (In the language of my biz, they should have been looking for simple effects instead of a main effect, because there are probably some serious cross-over interactions going on here.)
TL;DR -- If you want to practice medicine in New Mexico, take Spanish not German.
