Probative Synergy of Plus Factors in Price-Fixing Litigation

By Christopher R. Leslie (University of California)

Private plaintiffs alleging that defendants conspired to fix prices in violation of antitrust law must usually prove their claims through circumstantial evidence, generally in the form of “plus factors” – evidence indicating that the defendants’ parallel conduct was caused by collusion, not by independent decision-making. Supreme Court precedent requires factfinders to examine antitrust plaintiffs’ evidence holistically. With increasing frequency, however, federal courts in price-fixing cases improperly isolate each piece of circumstantial evidence presented by the plaintiff and then deprive it of all probative value because that single piece of evidence is insufficient, standing alone, to prove a price-fixing conspiracy. As a result, federal courts routinely grant summary judgment to price-fixing defendants even when plaintiffs have proffered more than enough evidence to prove their case.

This Article develops a typology of plus factors. Using antitrust case law, empirical research, and economic theory, this Article categorizes dozens of plus factors and explains the probative value of individual plus factors, as well as their interrelationships with each other. Plus factors may fall into one of several categories, such as Cartel Susceptibility, Cartel Formation, Cartel Management, Cartel Enforcement, and Cartel Markers. The Article then introduces and develops the concept of probative synergy, which describes how the probative value of each individual plus factor increases as additional plus factors are introduced into the equation.

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