Lerner index¶

Collapse a firm's market power into one dimensionless number, the markup of price over marginal cost as a fraction of price, L = (P − MC) / P, which under profit maximization also equals the reciprocal of the demand elasticity the firm faces.

Version: v2 · 2026-09-19 · History
Domain-specific #: 629
Domain group: Social Sciences
Origin domain: Economics & Finance
Subdomain: Industrial Organization → Economics & Finance

Core Idea¶

The Lerner index is a scalar measure of a firm's market power, L = (P − MC) / P — the markup of price over marginal cost as a fraction of price. It ranges from zero (perfect competition, price equals marginal cost) toward one (pure monopoly facing inelastic demand). For a profit-maximizer it equals the reciprocal of the absolute demand elasticity the firm faces, L = 1/|ε|, so an observed margin reveals the elasticity, and elasticity predicts the markup. It measures power as exercised, not structural concentration.

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The Extra-Charge Meter

A lemonade stand spends some money to make each cup and sells it for more. If lots of other stands are nearby, it can't charge much extra, or kids will buy somewhere else. The Lerner index is a number that shows how much extra a seller can charge above what the next cup costs to make. A bigger number means the seller has more power to set prices.

The Price Power Score

The Lerner index measures how much power a company has over its prices. You take the price, subtract the cost of making one more item, and divide by the price. If it's 0, the company charges exactly its cost, which is what happens with lots of competition. Values closer to 1 mean the company can charge far more than its cost. Because it's a fraction, you can compare a bank and a hospital even though their prices are very different.

Markup Market-Power Index

The Lerner index, introduced by Abba Lerner in 1934, is L = (P − MC)/P. Here P is price and MC is marginal cost, the cost of producing one more unit. It ranges from 0, meaning perfect competition where price equals marginal cost, up toward 1 for a pure monopoly. For a profit-maximizing firm, L equals 1 divided by the absolute value of the price elasticity of demand it faces. Elasticity is how sensitive buyers are to price, so the less sensitive they are, the bigger the markup. The index measures actual pricing behavior, not market share. A big firm with close rivals can score near zero, while a small firm with a unique product can score high. It has no units, so values can be compared across industries.

The Lerner index L = (P − MC)/P, due to Abba Lerner (1934), measures market power as the price–cost markup relative to price. L = 0 under perfect competition (P = MC). L approaches 1 in the pure-monopoly limit with perfectly inelastic demand. From the profit-maximizing first-order condition, L = 1/|ε|, where ε is the price elasticity of demand the firm faces. Observed margins therefore reveal that elasticity, and a known elasticity predicts the markup. The index captures behavioral market power, not structural concentration: market share and the Lerner index can diverge. It is dimensionless and scale-free, so a value of 0.3 means the same relative markup in banking or hospitals. Empirically, it is computed from accounting or econometrically estimated costs. It also supports New Empirical Industrial Organization inference of conduct from prices and quantities. Industry averages above roughly 0.3–0.4 are typically read as materially imperfect competition.

Scope of Application¶

Computable, and meaningful, wherever a profit-maximising firm prices against downward-sloping residual demand with price and marginal cost measurable in money.

Industrial organization and antitrust — the home: price-cost margins flag imperfect competition (~0.3–0.4).
Regulated-industry rate-setting — gauging how far a regulated firm prices above its floor.
Banking-competition literature — industry-aggregated indices from financial statements.
Health-care pricing studies — regional inpatient indices, often correlated with market HHI.
New Empirical Industrial Organization — backing out conduct from prices and quantities.

Clarity¶

The index makes legible the difference between power as exercised (the actual price-cost deviation) and structure as concentrated (share, HHI), naming cases the proxies misread — a dominant-share firm priced near cost, or a small differentiated firm pricing high. Being dimensionless, it makes power comparable where dollar prices do not. And the identity L = 1/|ε| turns an observable margin into a window on the unobservable demand elasticity.

Manages Complexity¶

A thick competitive dossier — share, rivals, substitutes, differentiation, entry barriers, price scale — compresses into one scale-free scalar anchored at two reference points, read directly off cost data for first-cut screening. Two features sharpen it: the index isolates power as exercised rather than concentration, and the identity L = 1/|ε| collapses two quantities into one, so computing the markup and inferring competitive conduct become a single act.

Abstract Reasoning¶

The index licenses measuring power as exercised rather than inferring it from structure, bidirectional inference through L = 1/|ε| (a markup reads as inelastic demand; an elasticity predicts a markup), scale-free cross-industry comparison, boundary-drawing against structural measures with a known complementarity, and a first-cut screening move that lets one number stand in for the market-structure file at the triage stage.

Knowledge Transfer¶

The index is a constructed measure, so a precondition governs travel: it transfers literally wherever a profit-maximising firm faces residual demand with measurable price and marginal cost — across antitrust, rate-setting, banking, and health-care studies, all co-instances. Beyond that setup a look-alike normalized-slack ratio (a CPU's thermal headroom) is a formal coincidence, not the Lerner mechanism. What genuinely generalizes is the parent move — measure deviation from a benchmark as a normalized scale-free ratio — carried by efficiency_frontier-style gap-to-benchmark normalization, not by the Lerner index.

Relationships to Other Abstractions¶

Current abstraction Lerner index Domain-specific

Parents (3) — outgoing typed relations

Lerner index presupposes Market power Domain-specific

The Lerner Index presupposes market power as the target attribute whose exercised price-cost wedge it maps onto a scale.
Lerner index includes, conditional Elasticity Prime

Under interior profit maximization, the index contains the reciprocal residual-demand elasticity identity that makes markup and responsiveness two readings of one parameter.
Lerner index is a decomposition of Measurement Prime

Removing firm and price vocabulary leaves an attribute mapped by a stated procedure onto a dimensionless scale with fixed anchors and interpretation conditions.

Hierarchy paths (4) — routes to 3 parentless roots

Lerner index → presupposes Market power → is a decomposition of Bargaining Power → is a kind of Asymmetry

Show alternative paths (3)

Neighborhood in Abstraction Space¶

Lerner index sits in a crowded region of the domain-specific corpus (35^th percentile for distinctiveness): several abstractions share nearly its structure, so a description that fits it tends to fit its neighbors too.

Family — Market Structure & Competitive Dynamics (31 abstractions)

Nearest neighbors

Computed from structural-signature embeddings · 2026-10-25