Modigliani and Miller, two professors in the 1950s, studied capital-structure theory intensely. From their analysis, they developed the capital-structure irrelevance proposition. Essentially, they hypothesized that in perfect markets, it does not matter what capital structure a company uses to finance its operations. They theorized that the market value of a firm is determined by its earning power and by the risk of its underlying assets, and that its value is independent of the way it chooses to finance its investments or distribute dividends.
The basic M&M proposition is based on the following key assumptions:

  • No taxes
  • No transaction costs
  • No bankruptcy costs
  • Equivalence in borrowing costs for both companies and investors
  • Symmetry of market information, meaning companies and investors have the same information
  • No effect of debt on a company's earnings before interest and taxes

Of course, in the real world, there are taxes, transaction costs, bankruptcy costs, differences in borrowing costs, information asymmetries and effects of debt on earnings. To understand how the M&M proposition works after factoring in corporate taxes, however, we must first understand the basics of M&M propositions I and II without taxes.

Modigliani and Miller's Capital-Structure Irrelevance Proposition
The M&M capital-structure irrelevance proposition assumes no taxes and no bankruptcy costs. In this simplified view, the weighted average cost of capital (WACC) should remain constant with changes in the company's capital structure. For example, no matter how the firm borrows, there will be no tax benefit from interest payments and thus no changes or benefits to the WACC. Additionally, since there are no changes or benefits from increases in debt, the capital structure does not influence a company's stock price, and the capital structure is therefore irrelevant to a company's stock price.

However, as we have stated, taxes and bankruptcy costs do significantly affect a company's stock price. In additional papers, Modigliani and Miller included both the effect of taxes and bankruptcy costs.

Modigliani and Miller's Tradeoff Theory of Leverage
The tradeoff theory assumes that there are benefits to leverage within a capital structure up until the optimal capital structure is reached. The theory recognizes the tax benefit from interest payments - that is, because interest paid on debt is tax deductible, issuing bonds effectively reduces a company's tax liability. Paying dividends on equity, however, does not. Thought of another way, the actual rate of interest companies pay on the bonds they issue is less than the nominal rate of interest because of the tax savings. Studies suggest, however, that most companies have less leverage than this theory would suggest is optimal. (Learn more about corporate tax liability in

How Big Corporations Avoid Big Tax Bills and Highest Corporate Tax Bills By Sector.)

In comparing the two theories, the main difference between them is the potential benefit from debt in a capital structure, which comes from the tax benefit of the interest payments. Since the MM
capital-structure irrelevance theory assumes no taxes, this benefit is not recognized, unlike the tradeoff theory of leverage, where taxes, and thus the tax benefit of interest payments, are recognized.

In summary, the MM I theory without corporate taxes says that a firm's relative proportions of debt and equity don't matter; MM I with corporate taxes says that the firm with the greater proportion of debt is more valuable because of the interest tax shield.

MM II deals with the WACC. It says that as the proportion of debt in the company's capital structure increases, its return on equity to shareholders increases in a linear fashion. The existence of higher debt levels makes investing in the company more risky, so shareholders demand a higher risk premium on the company's stock. However, because the company's capital structure is irrelevant, changes in the debt-equity ratio do not affect WACC. MM II with corporate taxes acknowledges the corporate tax savings from the interest tax deduction and thus concludes that changes in the debt-equity ratio do affect WACC. Therefore, a greater proportion of debt lowers the company's WACC.



Bankruptcy Costs And Optimal Capital Structure

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