a growing body of work suggests that corporations, far from being passive observers, are some of the market’s most effective arbitrageurs. In 2000 Malcolm Baker of Harvard University and Jeffrey Wurgler, then of Yale University, found a tight connection between firms’ net equity issuance and subsequent stockmarket returns. Years in which companies issued relatively more stock were typically followed by weaker market performance. More tellingly, companies seemed to issue precisely when valuations were rich, and especially when other frothy signals, such as buoyant consumer sentiment, were drawing attention.