Lerner index


Author Definition



The Lerner index, measured as the percentage markup of price above marginal cost, (P-MC)/P, has often been used as a measure of a firm’s market power. The Lerner index always has a value between zero and one. For a textbook-perfectly competitive firm, P=MC so that L=0. The larger is L, the greater is the degree of market power.



The Lerner index was first developed in Abba Lerner’s 1934 paper, The Concept of Monopoly and the Measurement of Monopoly Power. Elzinga and Mills (2011) offer a historical overview and update. Perhaps the most useful adaption of the Lerner Index comes from the fact that a profit-maximizing firm will price its product inversely to the elasticity of demand facing the firm, L = -1/Ed. Unfortunately, determining a firm-related demand elasticity is much more difficult than determining a market demand elasticity, since a firm must consider how its competitors will react to price changes.

There is a significant limitation to the Lerner Index that has particular relevance in today’s technology-driven world. A firm may price substantially above its marginal cost (at its average rather than marginal cost). The reason: the firm needs to cover its fixed costs (moreover, if those expenditures pay for assets that are not redeployable elsewhere, they create a barrier to entry).

The most common source of the price-cost divergence arises when there are scale economies and the firm prices based on marginal cost. The computer software and pharmaceuticals industries are illustrative. For most software or pharmaceutical companies, marginal cost will be close to zero. Therefore, the presence of substantial fixed costs will lead to a Lerner Index slightly below 1.0, whether the software company is big, small or otherwise, and whether the market is highly competitive or monopolistic.

The Lerner Index has not been used extensively in antitrust enforcement. The primary reason is that market power requires the presence of meaningful barriers to entry, whether structural (e.g., substantial scale economies) or behavioural (e.g., substantial brand equity). For antitrust enforcement, the primary focus is on barriers to entry and the secondary focus is on substantial, sustainable high market shares (which are estimated, for example, through the use of the Herfindahl-Hirschman index (HHI).

In the US, the Lerner Index received quite a bit of attention in the 1995 monopolization case brought by the Department of Justice (DOJ) against Eastman Kodak. The DOJ’s expert used an estimated demand elasticity of 2 to infer a high Lerner Index (.5) and substantial market power. The court correctly rejected this view, citing among other things the presence of substantial fixed costs (p.109).

In the decades following the Kodak opinion, U.S. courts have continued to be critical of reliance on the Lerner Index to prove market power, while not ruling out the possibility that it may nevertheless be informative. In Dial Corp. v. News Corp (2016), for example, the court held that the plaintiff’s expert’s use of the Lerner Index in a critical loss analysis of market definition could not be excluded, noting that the Second Circuit’s Eastman Kodak decision did not prohibit use of the Lerner Index in all circumstances (pp. 41-42).

Furthermore, in In re Solodyn (Minocycline Hydrochloride) Antitrust Litigation, (2018 U.S. Dist. LEXIS 11921) the court found that the presence of high sunk costs does not rule out the price of a branded drug being anticompetitive. The Court noted that high margins indicated by a Lerner Index calculation, without more, are insufficient to demonstrate supra-competitive pricing. A similar conclusion was reached by the court in In re Intuniv Antitrust Litigation 2020 U.S. Dist. LEXIS 187579 (D. Mass. Sep. 21, 2020), with the Court noting that high margins indicated by a Lerner Index calculation, without more, are insufficient to demonstrate supracompetitive pricing.

Explicit use of the Lerner Index in the UK and the European Union has been even less common than in the U.S. Two telecom cases are worth noting. First, the European court found Slovak Telekom in violation of Article 102 (Council Regulation (EC) 1/2003 on Case AT.39523 – Re Slovak Telekom, 2014 O.J. (C 7465)), but also found the use of the Lerner Index to calculate a market demand elasticity (not an individual demand) to be highly implausible as the basis of a relevant market analysis. Along similar lines, the UK Competition Appeals Tribunal agreed with the lower court in British Telecommunications Plc v. Office of Communications (British Telecommunications Plc, Everything Everywhere Ltd. v. Office of Communications, (U.K. Competition Appeal Tribunal (Aug. 1, 2011)) that the Lerner Index is likely to suggest overly elastic demand when products included in Lerner Index are complements.



Elzinga, Kenneth G. and David E. Mills, “The Lerner Index of Monopoly Power: Origins and Uses,” American Economic Review: Papers & Proceedings, 2011. 101:3, 558-564 ;

Pindyck, Robert S. and Daniel L. Rubinfeld, Microeconomics, 9th Edition, Pearson, 2019 ;

Lerner, Abba, P. “The Concept of Monopoly and the Measurement of Monopoly,” Review of Economic Studies, 1: 157-75.

This article is being reviewed by the Editors of the Dictionary.


  • University of California (Berkeley)


Daniel Rubinfeld, Lerner index, Global Dictionary of Competition Law, Concurrences, Art. N° 85405

Visites 88

Publisher Concurrences

Date 1 January 1900

Number of pages 500


Institution Definition

A measure proposed by economist A.P. Lerner to measure monopoly or market power. The Lerner Index (LI) is:

In perfect competition, LI is equal to zero. The index defines monopoly power in terms of the slope of the demand curve. In the case of a profit maximizing firm in equilibrium, marginal revenue equals marginal cost and the LI is equal to the inverse of the elasticity of demand.

The LI is a static measure and does not indicate whether the deviation between price and marginal cost is a worthwhile cost to pay for possible innovation or new plant construction, or whether the disparity between marginal cost and price may reflect superior efficiency rather than the ability of a firm to charge high prices.