The national/regional IQ literature is messy, because there are so many possible (and even likely) feedback loops between wealth, schooling, nutrition, IQ and GDP. Not to mention the rather emotional views of many people on the topic, as well as the lousy quality of some popular datasets. Lots of clever statistical methods have been used, and IQ seems to retain a fair chunk of explanatory weight even after other factors have been taken into account. Some papers have even looked at staggered data to see if IQ works as a predictor of future good effects, which it apparently does.

Whether it would be best to improve IQ, health or wealth directly depends not just on which has the biggest effect, but also on how easy it is and how the feedbacks work.