HHI as a Measure of Concentration in the Antitrust Suits: Analogies and Differences Between the two Sides of Atlantic

In the last years, a growing number of scholars is in favour of a wider use of quantitative tools in the antitrust suits: hence, an increase of safe harbours, safe heavens and critical thresholds may be observed. The traditional arguments justifying this approach are the legal certainty and the possibility for the corporations to self-evaluate in advance their behaviours in order to understand the compliance with the antitrust rules. A more important reason may be found in the screening function of the index, since it should be aimed at reducing the antitrust Agencies workload. In this paper, the application of HHI by US Agencies is analyzed in order to understand if the above quoted filter role has been respected; furthermore, the study of EU Merger Review process - during which, a differentiated use of HHI depending on the kind of the scrutinized case was proposed - allows to identify the analogies and differences with the us approach. The results of this comparison offer the theoretical cue to answer the following questions: what are the reasons justifying the selection of HHI instead of rival measures? Is the undifferentiated use of HHI for different antitrust cases (unilateral effects and collusion) theoretically correct? What are the foundations of the HHI critical values choice?