Since the formulation of the Miller and Modigliani propositions over 60 years ago, financial economists have been debating whether there is such a thing as an optimal capital structure—a proportion of debt to equity that can be expected to maximize long-run shareholder value. Some finance scholars have followed M&M in arguing that both capital structure and dividend policy are irrelevant in the sense of having 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 more debt against the costs of financial distress. Still another theory says that companies do not have capital structure targets, but instead follow a financial pecking order in which retained earnings are generally 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 all appear to play important roles in corporate financing decisions. While much, if not most, of the evidence is consistent with the idea that companies set target leverage ratios, there is also considerable support for the pecking order theory’s contention that managements 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 (that is, total amounts of debt and equity) and flows (which security to issue at a particular time). Even when companies have leverage targets, it can make sense to deviate from those targets depending on the costs associated with moving back toward the target. And 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 targets.