Modigliani-Miller Theorem
The Modigliani-Miller Theorem holds that, in a world without taxes, bankruptcy costs, or information asymmetry, a firm's total value is independent of its capital structure — meaning the choice between debt and equity financing does not affect what the enterprise is worth.
Franco Modigliani and Merton Miller published their landmark theorem in 1958, earning both men Nobel Prizes and transforming corporate finance theory. The core insight is elegantly simple: if capital markets are perfect and there are no frictions, investors can replicate any capital structure on their own account (a concept called 'homemade leverage'), making a firm's financing mix irrelevant to its total value. In Proposition I, V_Levered = V_Unlevered, firm value is the same regardless of how it is financed.
The original frictionless world is, of course, fictional. The more useful version of M-M incorporates taxes. Because the US tax code allows corporations to deduct interest payments, debt financing generates a 'tax shield' equal to the corporate tax rate times the outstanding debt balance: PV(Tax Shield) = Tc x D. This tax shield has real value and makes leverage attractive from a pure tax standpoint. Under M-M with taxes, V_Levered = V_Unlevered + Tc x D — meaning firm value rises with debt. The implication taken to its extreme would suggest companies should use nearly 100% debt, which is obviously not observed in practice.
The resolution lies in the costs M-M deliberately excluded. Financial distress costs — the legal fees, management distraction, customer defections, and supplier credit tightening that accompany a bankruptcy or near-bankruptcy — grow as leverage rises. This tension between the tax shield benefit and distress costs forms the basis for the Trade-Off Theory of capital structure. Additionally, agency conflicts between managers, debt holders, and equity holders impose costs that influence optimal leverage.
For US equity analysts, M-M provides a critical intellectual scaffold. When comparing two similar businesses — say, two regional banks or two energy producers — with different capital structures, M-M reminds us to strip out leverage effects to arrive at a leverage-neutral valuation of the underlying business. Enterprise value, which adds net debt to equity market cap, reflects this intuition: it measures the value of the whole firm regardless of how it happens to be financed. Similarly, unlevered free cash flow and unlevered beta are both designed to isolate operating performance from financing decisions, exactly as M-M prescribes.