On the Existence of an Optimal Capital Structure: Theory and Evidence

Abstract

ONE OF THE MOST contentious issues in the theory of finance during the past quarter century has been the theory of capital structure. The geneses of this controversy were the seminal contributions by Modigliani and Miller [18, 19]. The general academic view by the mid-1970s, although not a consensus, was that the optimal capital structure involves balancing the tax advantage of debt against the present value of bankruptcy costs. No sooner did this general view become prevalent in the profession than Miller [16] presented a new challenge by showing that under certain conditions the tax advantage of debt financing at the firm level is exactly offset by the tax disadvantage of debt at the personal level. Since then there has developed a burgeoning theoretical literature attempting to reconcile Miller's model with the balancing theory of optimal capital structure [e.g., DeAngelo and Masulis [5], Kim [12], and Modigliani [17]. The general result of this work is that if there are significant "leverage-related" costs, such as bankruptcy costs, agency costs of debt, and loss of non-debt tax shields, and if the income from equity is untaxed, then the marginal bondholder's tax rate will be less than the corporate rate and there will be a positive net tax advantage to corporate debt financing. The firm's optimal capital structure will involve the trade off between the tax advantage of debt and various leverage-related costs. The upshot of these extensions of Miller's model is the recognition that the existence of an optimal capital structure is essentially an empirical issue as to whether or not the various leverage-related costs are economically significant enough to influence the costs of corporate borrowing. The Miller model and its theoretical extensions have inspired several timeseries studies which provide evidence on the existence of leverage-related costs. Trczinka [28] reports that from examining differences in average yields between taxable corporate bonds and tax-exempt municipal bonds, one cannot reject the Miller hypothesis that the marginal bondholder's tax rate is not different from the corporate tax rate. However, Trczinka is careful to point out that this finding does not necessarily imply that there is no tax advantage of corporate debt if the personal tax rate on equity is positive. Indeed, Buser and Hess [1], using a longer time series of data and more sophisticated econometric techniques, estimate that the average effective personal tax rate on equity is statistically positive and is not of a trivial magnitude. More importantly, they document evidence that is consistent with the existence of significant leverage-related costs in the economy.