Who Wants to Break Up Big Firms? Harm, Fairness, and the Demand for Antitrust

The rise of superstar firms has made dominant companies central to modern economic life, and antitrust enforcement is one of the main policy tools for regulating their market power. Public opinion can shape the political and regulatory environment in which antitrust enforcement takes place, yet there is little direct evidence on what drives these preferences. We conduct a pre-registered information-provision experiment with 4,000 American households. Respondents were told about one of five real antitrust cases and randomly assigned to information treatments designed to study four potential drivers of support for antitrust enforcement: perceived market share, perceived consumer harm, perceived unfair competition, and perceived negative image. All four treatments moved the beliefs they were designed to affect, but their effects on demand for antitrust differed sharply. Information about consumer harm had the most systematic effects: it increased plaintiff support and support for break-up and conduct remedies, with effects remaining visible one month later, and also spilled over to broader support for antitrust policies. By contrast, and contrary to expert forecasts, information about market share had no meaningful effect on demand for antitrust enforcement. The findings suggest that the public thinks like economists in one key respect: they do not care about market share per se, but respond instead to consumer harm. One factor outside the core economic framework, perceived unfair competition, also matters, though its effects are more limited in scope. We discuss implications for policymakers and regulators.