This paper applies an intuitive approach based on stock market data to a unique dataset of large concentrations during the period 1990-2002 to assess the effectiveness of European merger control. The basic idea is to relate announcement and decision abnormal returns. Under a set of four maintained assumptions, merger control might be interpreted to be effective if rents accruing due to the increased market power observed around the merger announcement are reversed by the antitrust decision, i.e. if there is a negative relation between announcement and decision abnormal returns. To clearly identify the events' competitive effects, we explicitly control for the market expectation about the outcome of the merger control procedure and run several robustness checks to assess the role of our maintained assumptions. We find that only outright prohibitions completely reverse the rents measured around a merger's announcement. On average, remedies seem to be only partially capable of reverting announcement abnormal returns. Yet they seem to be more effective when applied during the first rather than the second investigation phase and in subsamples where our assumptions are more likely to hold. Moreover, the European Commission appears to learn over time. (authors' abstract)

This paper presents empirical evidence about the ability of event studies to capture mergers' ex-post profitability as measured by accounting data. We use a sample of large horizontal concentrations during the period 1990-2002 involving 482 firms either as merging firms or competitors, and contrast a measure of the mergers' profitability based on stock market event studies with one based on balance sheet profit data. We show that using a long window around the announcement date (25 or 50 days before the event) increases the ability to capture the ex-post merger effect: the pairwise correlation coefficient is positive and highly significant.