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📐 '''Herfindahl-Hirschman Index (HHI)''' is a numerical measure of [[Definition:Market concentration (insurance) | market concentration]] calculated by summing the squares of each competitor's market-share percentage within a defined market — and in insurance, those markets are often sliced far more granularly than in other industries, broken down by line of business, state, or even sub-segment such as [[Definition:Excess and surplus lines | surplus lines]] versus admitted markets. An HHI below 1,500 generally indicates a competitive market; between 1,500 and 2,500 suggests moderate concentration; and above 2,500 signals high concentration, though regulators apply these thresholds with judgment rather than as rigid cutoffs.

⚙️ When [[Definition:Insurance carrier | carriers]] or [[Definition:Insurance broker | brokers]] propose a merger, the [[Definition:Department of Justice (DOJ) antitrust review | DOJ]] and [[Definition:Federal Trade Commission (FTC) review | FTC]] calculate the post-transaction HHI and the change in HHI (the "delta") to determine whether the deal warrants deeper investigation. In insurance, the relevant data often comes from [[Definition:Annual statement | statutory filings]] reported to the [[Definition:National Association of Insurance Commissioners (NAIC) | NAIC]], premium databases, and state-level market-share reports. A transaction that raises the HHI by more than 200 points in an already concentrated market — say, [[Definition:Commercial auto insurance | commercial auto]] in a particular state — will almost certainly trigger a second request for information and potentially require [[Definition:Divestiture | divestitures]] or behavioral remedies before approval.

💡 Beyond its role in merger review, the HHI serves as a strategic benchmarking tool for [[Definition:Underwriting | underwriters]], [[Definition:Reinsurance | reinsurers]], and investors evaluating competitive dynamics. A rising HHI in a niche like [[Definition:Cyber insurance | cyber insurance]] may signal consolidation that could tighten capacity and harden [[Definition:Premium rate | rates]], while a declining index in [[Definition:Personal lines | personal lines]] might indicate new entrants — often [[Definition:Insurtech | insurtech]] startups — fragmenting the market. Keeping an eye on HHI trends helps industry participants anticipate pricing shifts, distribution disruptions, and the regulatory environment for future deals.

'''Related concepts:'''
{{Div col|colwidth=20em}}
* [[Definition:Market concentration (insurance)]]
* [[Definition:Competitive impact statement]]
* [[Definition:Federal Trade Commission (FTC) review]]
* [[Definition:Department of Justice (DOJ) antitrust review]]
* [[Definition:Insurance M&A]]
* [[Definition:Antitrust law]]
{{Div col end}}

Definition:Herfindahl-Hirschman Index (HHI) - Revision history

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