This paper uses a methodology robust to recent criticisms of standard long-horizon event study tests to show that bidders in mergers underperform while bidders in tender offers overperform in the three years after the acquisition. However, the long-term underperformance of acquiring firms in mergers is predominantly caused by the poor post-acquisition performance of low book-to-market “glamour” firms. We interpret this finding as evidence that both the market and the management overextrapolate the bidder's past performance (as reflected in the bidder's book-to-market ratio) when they assess the desirability of an acquisition.