Strategic Economics

Market Power and the Lerner Index

Turn the vague question "do we have pricing power?" into one measurable number, learn the three ways that number lies to you, and find out why a 40% share can mean a 17% markup or a 30% markup depending on something nobody in the room is measuring.

  • Advanced
  • 15 min total
  • 14 chapters

What decision this helps you make: Whether the margin you are earning is a rent you can defend and price against, or an accounting artefact — and how much of your price a rival would have to be able to absorb before your position is worth anything.

What this topic is

Market power is the ability to hold a price above the cost of producing one more unit without losing so much volume that the increase stops being worth it. The Lerner index measures exactly that, as a fraction of the price: price minus marginal cost, divided by price. Sell at $100 with an incremental cost of $60 and the index is 0.40 — forty cents of every dollar you charge is markup that competition has not taken away.

Why it matters

Almost every strategic claim a business makes is really a claim about this number. A moat is a reason the index stays high. A competitive threat is a reason it will fall. A discount request is a customer testing whether it was ever real. Yet the number is routinely proxied by gross margin, which is a different quantity computed from a different cost base, and the substitution changes the answer by tens of percentage points in both directions. Getting the measurement right is what separates a pricing decision from a hope.

Who should learn it

Founders and CFOs deciding whether to hold price into a renewal cycle, strategy and corporate-development teams valuing a competitive position, anyone who has to defend a margin to an investor or a regulator, and readers who want the formal version of what the rest of the library calls a moat.

What you will understand

  • The definition, the inverse-elasticity result behind it, and the exact assumption it rests on
  • How to compute it from your own accounts, and why gross margin is not it
  • Why share is not power: the Landes-Posner decomposition into share, demand elasticity, and rivals' ability to expand
  • The cellophane fallacy — why measuring elasticity at the current price makes a monopolist look competitive

Prerequisites

Common misconception

That a high margin proves market power and a low margin proves its absence. Neither follows. A firm with enormous fixed costs and no market power at all must price well above marginal cost simply to survive — its long-run equilibrium markup is whatever covers the fixed base, and that can be most of the price. Meanwhile a firm earning a thin markup may hold total power over a market that is small and cheap to serve. The Lerner index measures the gap between price and the cost of one more unit. It does not measure profit, it does not measure return on capital, and the accounting rate of return you would reach for instead is a worse proxy still.