Modigliani-Miller theorem

The Modigliani–Miller, or M–M, theorem is a benchmark for thinking about capital structure. Under stringent assumptions, the mix of debt and equity does not change total firm value. Its practical usefulness comes from identifying exactly which real-world departures—taxes, distress, agency, information, transaction costs and financing constraints—make capital structure matter.

When to use it

• To introduce capital-structure reasoning and expose its assumptions.
• To challenge a proposal to add leverage or equity.

This is educational material, not financing, tax, legal or investment advice.

Origins

Franco Modigliani and Merton Miller developed the theory while working at Carnegie Mellon in the late 1950s. Preparing corporate-finance teaching led them to question the accepted belief that every firm had a discoverable ideal debt-equity mix. Their arbitrage argument showed that, in a frictionless market, investors could offset corporate leverage themselves. The initially controversial work became foundational to modern finance, and both authors later received Nobel recognition.

What it is

M–M 1 is the capital-structure irrelevance proposition. In its simplest setting there are no taxes, transaction or distress costs; companies and investors borrow on the same terms; information is shared; and financing does not change operating cash flow. Under those conditions, leverage does not alter total firm value.

Suppose equity investors require 20 per cent when the firm is unlevered and debt costs 12.5 per cent. Cheap debt initially appears to reduce the weighted average cost of capital, but additional leverage makes equity riskier and raises its required return. In the frictionless proposition, these effects offset.

M–M 2 relates leverage to the required return on equity. In 1963 the authors added corporate tax to the analysis. If interest is deductible, debt creates a tax shield. With debt costing 12.5 per cent and a tax rate of 30 per cent, the simplified after-tax cost is 12.5 per cent × (1–30 per cent) = 8.75 per cent, which can pull WACC downward.

The tax version does not imply unlimited borrowing. Expected distress, refinancing, covenant, agency and lost-flexibility costs can eventually outweigh tax benefits.