(Referenced link is http://www.halfsigma.com/2006/07/higher_intellig.html )
(Referenced link is http://www.halfsigma.com/2006/07/higher_intellig.html )
As an alternate hypothesis, the reason why students who study what they love earn less than students who study something practical is because of the intrinsic value the student finds being employed to do what they love. As a result of this, they don't need or demand to be paid as much to perform the same work, and so salaries tend to be depressed.
But maybe it doesn't bother them; maybe there is something to the advice more intelligent people are given. Perhaps the needs most people fulfill by earning and spending money have already been fulfilled by these people through other means. If you do what you love by trade, perhaps there is less need to spend money in exchange for spare time to work on it.