What Is the Modigliani-Miller Theorem?
Franco Modigliani and Merton Miller published the theorem in 1958, arguing that under idealized conditions the way a firm finances itself has no effect on its total value. If two companies own identical assets generating identical cash flows, they must be worth the same amount regardless of how much debt either one carries. Otherwise investors could earn riskless profits by borrowing on their own account to replicate the leveraged firm, and arbitrage would force the two values back together.
The result depends on strong assumptions: a world with corporate taxes stripped out, costless bankruptcy, symmetric information between managers and investors, and frictionless trading. Both economists later won Nobel Prizes in part for this work—Modigliani in 1985 and Miller in 1990. The theorem's power comes less from describing reality than from isolating exactly which real-world frictions cause capital structure decisions to matter in the first place.
Propositions I and II Explained
Proposition I is the irrelevance result: the value of a levered firm equals the value of an identical unlevered firm, so total enterprise value is fixed by the assets rather than the financing. Proposition II explains why cheaper debt does not lower the overall cost of capital—as leverage rises, equity holders bear more risk and demand higher returns, with the cost of equity increasing linearly with the debt-to-equity ratio. The two effects exactly offset, leaving WACC constant.
In their 1963 revision, Modigliani and Miller reintroduced corporate taxes. Because interest is deductible while dividends are not, the levered firm's value now exceeds the unlevered firm's value by the present value of the interest tax shield, which for perpetual debt equals the tax rate multiplied by the debt balance. Taken literally, this version implies firms should finance themselves almost entirely with debt, a prediction that real-world financial distress costs clearly contradict.
Why Modigliani-Miller Matters in Practice
The theorem gives practitioners a disciplined starting point. Trade-off theory, which balances tax shields against expected bankruptcy costs, builds directly on the MM framework, and so do the levering and unlevering beta formulas used every day in WACC calculations. When an analyst unlevers a peer group's betas and relevers them at a target capital structure, the underlying math traces straight back to Proposition II and its assumptions about how risk shifts between debt and equity holders.
It is also a reliable interview topic. A strong answer explains that capital structure is irrelevant only under perfect-market assumptions, then names the frictions—taxes, financial distress, agency costs, and information asymmetry—that make it highly relevant in practice. Candidates who can walk from MM's clean theoretical baseline to the messy real world demonstrate the kind of structured thinking that banks and investment firms screen for in technical interviews.
