During the previous two decades, the four of us have published many studies on shareholder activism and mergers and acquisitions at leading academic outlets. But until this paper, Antitrust, Anti-Activism, neither we nor anyone else had uncovered the intricate interaction between antitrust regulation and shareholder activism.

There have been numerous examples, as recently as 2024 and 2025, of both regulators and activists responding to the antitrust disclosure thresholds set in the Hart-Scott-Rodino (HSR) Act, including both federal civil penalty settlements from regulators and high-profile activist decisions that were impacted by HSR thresholds. We had heard over the years from mostly anecdotal sources that these HSR thresholds have real bite, and recently we decided to study whether they do, using a comprehensive empirical study. Our findings surprised us in their clarity and significance.

Our empirical strategy is based on the fact that the dollar-based HSR disclosure thresholds correspond to an increasingly smaller percentage of shares outstanding as a firm’s market capitalization increases. As a result, the HSR thresholds are lower than the percentage-based 5% Section 13(d) disclosure threshold for some mid-cap firms and much lower than 5% for large-cap firms. (more…)