Tacit Collusion The Neglected Experimental Evidence∗

fully acknowledged. 2 I. The Issue Normally, hard scientific evidence on legal issues is a seriously scarce commodity. More than one empirical publication on a precisely defined doctrinal question is a rare event. Against this backdrop, a problem in antitrust is salient. Both in the US and in Europe, antitrust authorities prohibit merger not only if the merged entity, in and of itself, is no longer sufficiently controlled by competition. The authorities also intervene if, post merger, the market structure has changed such that "tacit collusion " becomes disturbingly more likely. Antitrust authorities rely on game theory to assess this risk (Ivaldi, Jullien et al. 2003). Yet on this prognostic question, economists have also, for more than 50 years, been doing experiments. Almost any conceivable determinant of higher or lower collusion has been tested. This solid body of evidence is untapped by the legal community. It seems that the antitrust authorities are not even aware of the fact that one of their thorniest empirical questions has already been thoroughly investigated by a neighbouring disci-pline