Ex ante or ex post competition policy? A progress report

When intervening in markets, say to block a merger, competition authorities are constrained by the limited information they have about the social desirability of the available alternatives. Compared to ex ante control, ex post control is based on the more accurate information that becomes available in the intervening period, but entails temporary losses to social welfare and reversal costs incurred to unscramble the eggs. Through a toy model, we identify situations in which the competition authority finds it optimal to commit to forego the option of ex post review in order to avoid chilling ex ante socially beneficial mergers. On the other hand, the case for ex post review is strengthened if post-merger market conducts can signal the merged firm's private information about the consequences of the merger.