Modigliani and Miller proved funding choices don't change a firm's value

In their 1958 essay 'The Cost of Capital, Corporation Finance and the Theory of Investment', Franco Modigliani and Merton Miller attacked the era's folk wisdom that the mix of debt and equity was itself a source of value. They asked what a firm's market value actually depends on, and showed that with rational investors on both sides, the answer is the firm's earning power — not the paper it is printed on.

The result came as two theorems. Capital structure is irrelevant to market value when investors have the same access to capital markets as firms, because stockholders can strike their own balance between risk and return; managers best serve shareholders by maximizing the firm's net wealth rather than tailoring securities to risk appetites. Dividend policy fails the same test. The authors refined the framework in later essays in 1963 and 1966 — and the 1963 tax correction showed precisely where the first real-world friction bites.

The theorem's practical power is in its violations. Taxes predict leverage; distress costs cap it; information and agency gaps explain dividend surprises and security design. Britannica records that Miller became a leading developer of theoretical and empirical corporate finance analysis, and the 1990 Nobel — shared with Markowitz and Sharpe — marked the moment corporate finance became a theorem-provable discipline rather than a book of maxims.

Funding mix matters only via frictions

Investor arbitrage was the engine: if packaging debt changed value, home-made leverage would arbitrage the difference away.

The irrelevance baseline became a measuring stick — every financing innovation must name the friction it exploits.

The 1963 tax version made leverage predictable instead of mystical, converting a maxim into a calculation.

A provable null model let a young discipline measure exactly how far reality departs from it.

State the frictionless version first: a result that holds when nothing gets in the way turns every real-world deviation into a priced opportunity. Theorems don't compete with practice — they index it.

Corporate finance rebuilt itself around M&M as the null model: taxes, distress costs and agency conflicts each became named, priced reasons for real capital-structure choices. Miller taught at Chicago's Graduate School of Business, directed the Chicago Mercantile Exchange from 1990, and shared the 1990 Nobel memorial prize with Markowitz and Sharpe before his death in 2000.