EFF: The empirical evidence says that all the gains from mergers are eaten up in the premiums paid to acquire firms. On average, the acquiring firm gets nothing. This doesn't necessarily imply that the shares of the acquired firm were mispriced since there can be synergies (real business gains) from mergers.
KRF: Takeover premiums do not imply that the target firms were mispriced. Since we do not expect the market to accurately forecast every acquisition that will create value, we should not be surprised that prices rise when tender offers and mergers are announced.
Eugene Fama and Ken French are members of the Board of Directors for and provide consulting services to Dimensional Fund Advisors LP.