The observation of parallel behavior by undertakings in the market is not sufficient in itself to characterize a concerted practice, since such behavior may reveal only an intelligent adaptation of the undertakings in question to market conditions. The European authorities use two types of complementary evidence to infer the existence of a cartel from the observation of parallel behavior on the market: either they corroborate the parallelism by establishing the absence of any possible explanation other than concertation; or they resort to positive indications of concertation, which alone are insufficient, but which, together with parallel behavior, make it possible to demonstrate a concurrence of wills.

The European authorities can also prove the existence of concertation by establishing the absence of any other possible explanation for the parallelism of behavior, such as oligopolistic interdependence. The search for another explanation is not necessary when the concertation is established by documentary evidence, which demonstrates the existence of an agreement of wills. In some cases, the supervisory authorities prove concertation by establishing either that it cannot be explained by the structure of the market or, more generally, that it is not in the individual interest of the undertakings. In the Commission’s view, parallelism of behavior with regard to prices cannot result from autonomous decisions when the interests of the operators concerned are divergent or even opposed. However, proof of the agreement is not adduced when the explanation of parallelism of conduct through concerted action is not the only plausible one.

A parallelism of behavior makes it possible to find an anticompetitive practice when it is supported by other elements demonstrating the concerted action, such as contacts or exchanges of information. Evidence must be sufficiently specific and consistent for collusion to be assumed. This is the case when competing service providers sign a price grid, when a manufacturer regularly attends meetings with its competitors, or when contacts are made between representatives of competing undertakings given their level of responsibility.