How does hhi work

Overview

The Herfindahl-Hirschman Index (HHI) is a widely used measure of market concentration that helps regulators assess the level of competition within an industry. Developed independently by economists Orris Herfindahl in 1950 and Albert Hirschman in 1945, the index gained prominence when the U.S. Department of Justice incorporated it into their 1982 Merger Guidelines. The HHI provides a more nuanced view of market concentration than simple market share calculations by accounting for the distribution of market power among all firms in an industry. Historically, the index emerged during a period of increasing concern about industrial concentration and antitrust enforcement in the United States, with early applications in the manufacturing and banking sectors. The Federal Trade Commission and Department of Justice have used HHI as a screening tool for mergers and acquisitions since the 1980s, with the most recent guidelines updated in 2023 maintaining its central role in antitrust analysis.

How It Works

The HHI is calculated by taking the market share of each firm in the industry as a percentage, squaring each firm's market share, and then summing these squared values. For example, in a market with four firms having market shares of 40%, 30%, 20%, and 10%, the HHI would be calculated as (40² + 30² + 20² + 10²) = 1,600 + 900 + 400 + 100 = 3,000. The resulting score ranges from near 0 (indicating perfect competition with many tiny firms) to 10,000 (indicating a monopoly with one firm controlling 100% of the market). The squaring process gives greater weight to larger firms, making the index particularly sensitive to changes in market share among dominant players. This mathematical approach means that a market with five equal-sized firms (each with 20% share) would have an HHI of 2,000, while a market with one dominant firm (50% share) and five smaller firms (each with 10% share) would have an HHI of 3,000, clearly showing the difference in concentration levels.

Why It Matters

The HHI matters because it directly influences antitrust decisions and merger approvals worldwide. When companies propose mergers, regulators calculate the post-merger HHI and the change in HHI to determine if the transaction would substantially lessen competition. In the United States, mergers that increase HHI by more than 200 points in already concentrated markets (HHI above 2,500) typically trigger additional scrutiny and may be blocked. Beyond antitrust enforcement, HHI helps economists study market dynamics, informs investment decisions by highlighting competitive landscapes, and assists policymakers in designing regulations that promote healthy competition. The index has been applied across diverse sectors including telecommunications, healthcare, banking, and technology, with recent high-profile cases involving proposed mergers in the airline and pharmaceutical industries where HHI calculations played crucial roles in regulatory decisions.