Capital Structure Theory: Modigliani-Miller and the Pecking Order

What Is Capital Structure?

Capital structure refers to the mix of debt and equity a company uses to finance its assets and operations. Every company must decide how much to borrow versus how much to fund from shareholders. This decision affects the cost of capital, financial risk, and ultimately the value of the firm.

Modigliani-Miller Theorem (Without Tax)

In 1958, Franco Modigliani and Merton Miller proposed that in a perfect capital market (no taxes, no transaction costs, symmetric information), the capital structure of a firm is irrelevant to its value. The value of the firm is determined by its operating cash flows, not by how those cash flows are split between debt and equity holders. This is Proposition I — the firm value proposition.

Modigliani-Miller With Tax

In 1963, Modigliani and Miller modified their theorem to include corporate taxes. Because interest is tax-deductible, debt financing creates a tax shield that increases firm value. In a world with taxes only (still no financial distress costs), the optimal capital structure would be 100% debt — maximising the tax shield. This extreme result is not observed in practice because it ignores the costs of financial distress.

Trade-Off Theory

Trade-off theory reconciles the tax shield with financial distress costs. As a company borrows more, the tax shield increases firm value — but so does the probability and cost of financial distress (bankruptcy costs, agency costs of debt, reduced operational flexibility). The optimal capital structure is where the marginal benefit of additional tax shield equals the marginal cost of additional financial distress risk. This varies by industry — stable, asset-rich businesses can carry more debt than volatile, asset-light ones.

Pecking Order Theory

Proposed by Myers and Majluf (1984), pecking order theory argues that firms prefer financing sources in a specific order due to information asymmetry. Managers prefer retained earnings first (cheapest, no new information released to markets). If external finance is needed, they prefer debt next (signals confidence in future cash flows). New equity is the last resort because issuing shares signals to the market that the firm may be overvalued. This explains why profitable firms often carry less debt — they have more retained earnings available. Capital structure is a core topic in ACCA AFM and CIMA F3.