How Leverage Changes Everything: The Modigliani-Miller Framework

The tax shield is the reason debt lowers WACC up to a point. Beyond that point, rising distress costs and higher cost of equity from increased risk offset the tax benefit.

M&M’s genius was showing that capital structure is about tradeoffs, not free lunches. Debt creates a tax shield but adds distress risk. The optimal structure balances these forces — and varies by industry, stability, and market conditions.

Going Deeper — the three frictions that break Modigliani-Miller in practice. M&M Proposition I says capital structure is irrelevant in a world with no taxes, no bankruptcy costs, and no agency costs. The real world has all three. (1) Taxes: interest is deductible, creating a tax shield that scales with leverage and pulls the optimal structure toward more debt. (2) Bankruptcy and distress costs: at high leverage, the probability of distress and the deadweight cost of distress (lost customers, lost suppliers, fire-sale asset values) creates an offsetting drag. (3) Agency costs: high leverage can constrain value-destroying empire-building, but can also force underinvestment in maintenance and R&D. The trade-off theory says the optimum is where the marginal tax shield equals the marginal expected distress cost. AI prompt: "For this ticker, estimate the trade-off-theory optimal leverage given its tax rate, asset volatility, and industry distress costs. Compare to current debt-to-equity. Is management under-levered, over-levered, or about right?"