Federal Reserve Board Chair Janet Yellen made waves in her Congressional testimony last week when she argued that social media and biotech stocks were over-valued. She also said that the price of junk bonds was out of line with historic experience. By making these assertions in a highly visible public forum, Yellen was using the power of the Fed’s megaphone to stem the growth of incipient bubbles. This is an approach that some of us have advocated for close to twenty years. Before examining the merits of this approach, it is worth noting the remarkable transformation in the Fed’s view on its role in containing bubbles. Just a decade ago, then Fed Chair Alan Greenspan told an adoring audience at the American Economic Association that the best thing the Fed could do with bubbles was to let them run their course and then pick up the pieces after they burst. He argued that the Fed’s approach to the stock bubble vindicated this route. Apparently it did not bother him, or most of the people in the audience, that the economy was at the time experiencing its longest period without net job growth since the Great Depression.Read rest here.