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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 fact finders 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.

Despite the longstanding rule that courts should not compartmentalize an antitrust plaintiff’s evidence of conspiracy, courts often inappropriately isolate individual plus factors and incorrectly suggest that if a plus factor does not by itself prove collusion, then it is not a plus factor at all. This approach miscomprehends the entire structure of factor tests in legal analysis. Using a series of case studies, the Article examines the causes and consequences of judges improperly compartmentalizing circumstantial evidence in price-fixing litigation. The Article concludes by showing that the failure to appreciate the probative synergy of plus factors has led courts to make it effectively impossible to prove collusion through circumstantial evidence in price-fixing cases.