Since the formulation of the M & M irrelevance propositions 40 years ago, financial economists have been debating whether there is such a thing as optimal capital structure—a proportion of debt to equity that maximizes current firm value. Some finance scholars have followed M & M by arguing that both capital structure and dividend policy are largely “irrelevant” in the sense that they have no significant, predictable effects on corporate market values. Another school of thought holds that corporate financing choices reflect an attempt by corporate managers to balance the tax shields and disciplinary benefits of greater debt against the increased probability and costs of financial distress. Yet another theory says that companies do not have capital structure targets, but instead follow a financial pecking order in which retained earnings are preferred to outside financing, and debt is preferred to equity when outside funding is required.
In reviewing the evidence that has accumulated since M & M, the authors argue that taxes, bankruptcy (and other “contracting”) costs, and information costs (the main factor in the pecking order theory) all appear to play an important role in corporate financing decisions. While much if not most of the evidence is consistent with the argument that companies set target leverage ratios, there is also considerable support for the pecking order theory's contention that firms are willing to deviate widely from their targets for long periods of time.
According to the authors, the key to reconciling the different theories—and thus to solving the capital structure puzzle—lies in achieving a better understanding of the relation between corporate financing stocks (leverage ratios) and flows (specific choices between debt and equity). Even if companies have target leverage ratios, there will be an optimal deviation from those targets—one that will depend on the transactions and information costs associated with adjusting back to the target relative to the costs of deviating from the target. As the authors argue in closing, a complete theory of capital structure must take account of these adjustment costs and how they affect expected deviations from the target.