If it's possible to be overqualified for the role of Federal Reserve chairwoman, Janet Yellen would be among the few (probably the only) who could stake that claim. Yet it seems almost inevitable that asset prices -- and stock prices in particular, as represented by the S&P 500 (^GSPC 0.83%) -- will take a hit during her tenure leading the central bank. In the video below, I discuss the simple reason why this is the case.