Markups, Elasticity, and Market Power

A markup is the gap between price and marginal cost. One normalized measure is the Lerner index:

For a profit-maximizing single-product firm facing a smooth demand curve,

Demand must be elastic at the optimum. A lower elasticity magnitude, while still above one, supports a larger percentage markup.

What makes demand less elastic?

Market demand becomes less responsive when buyers have fewer close substitutes, switching is costly, the product is a small or necessary part of spending, or adjustment takes time. Market definition matters: demand for one app may be elastic even when demand for digital communication as a broad category is inelastic.

The formula can be rearranged as

It does not say firms freely choose any elasticity. Product design, advertising, contracts, rivals, and consumer heterogeneity shape demand, often at a cost.

Measure with care

A high price-cost margin can indicate market power, but observed accounting cost may omit risk, research, quality, or capacity. Price and elasticity are jointly determined, and temporary scarcity can raise margins without a durable barrier. Strong claims require a clearly defined market, credible marginal-cost evidence, and attention to dynamics.

Translate elasticity into a price

With MC of 10 and elasticity magnitude 2, the Lerner condition gives a markup share of one-half. Solving $(P-10)/P=0.5$ gives $P=20$. If elasticity rises to 5 because close substitutes appear, the markup share falls to 20% and implied price falls to 12.5.

This comparison explains the mechanism: substitutes make buyers more willing to leave after a price increase. The firm may respond by improving quality, building switching costs, or reducing price, but those actions also have costs and may trigger rival responses.

Diagnose evidence instead of declaring power

Observed margins are noisy. Accounting data may classify research and platform investment as current cost even when they create future value, or omit owner and capacity costs. A temporary shortage can raise price without a durable entry barrier.

A careful diagnosis combines market definition, elasticity estimates, marginal cost, actual substitution, entry evidence, and persistence over time. The Lerner index organizes this evidence; it does not replace it.