Modigliani was awarded the 1985 Nobel Prize in Economics for this and other contributions.
Miller was awarded the 1990 Nobel Prize in Economics, along with Harry Markowitz and William Sharpe, for their "work in the theory of financial economics," with Miller specifically cited for "fundamental contributions to the theory of corporate finance."
Miller and Modigliani derived the theorem and wrote their groundbreaking article when they were both professors at the Graduate School of Industrial Administration (GSIA) of Carnegie Mellon University. In contrast to most other business schools, GSIA put an emphasis on an academic approach to business questions. The story goes that Miller and Modigliani were set to teach corporate finance for business students despite the fact that they had no prior experience in corporate finance. When they read the material that existed they found it inconsistent so they sat down together to try to figure it out. The result of this was the article in the American Economic Review and what has later been known as the M&M theorem.
The theorem was originally proven under the assumption of no taxes. It is made up of two propositions which can also be extended to a situation with taxes.
Consider two firms which are identical except for their financial structures. The first (Firm U) is unlevered: that is, it is financed by equity only. The other (Firm L) is levered: it is financed partly by equity, and partly by debt. The Modigliani-Miller theorem states that the value of the two firms is the same.
Proposition I: where is the value of an unlevered firm = price of buying a firm composed only of equity, and is the value of a levered firm = price of buying a firm that is composed of some mix of debt and equity.
To see why this should be true, suppose an investor is considering buying one of the two firms U or L. Instead of purchasing the shares of the levered firm L, he could purchase the shares of firm U and borrow the same amount of money B that firm L does. The eventual returns to either of these investments would be the same. Therefore the price of L must be the same as the price of U minus the money borrowed B, which is the value of L's debt.
This discussion also clarifies the role of some of the theorem's assumptions. We have implicitly assumed that the investor's cost of borrowing money is the same as that of the firm, which need not be true in the presence of asymmetric information or in the absence of efficient markets.
A higher debt-to-equity ratio leads to a higher required return on equity, because of the higher risk involved for equity-holders in a company with debt. The formula is derived from the theory of weighted average cost of capital.
These propositions are true assuming the following assumptions:
These results might seem irrelevant (after all, none of the conditions are met in the real world), but the theorem is still taught and studied because it tells us something very important. That is, capital structure matters precisely because one or more of these assumptions are violated. It tells us where to look for determinants of optimal capital structure and how those factors might affect optimal capital structure.
The same relationship as earlier described stating that the cost of equity rises with leverage, because the risk to equity rises, still holds. The formula however has implications for the difference with the WACC. Their second attempt on capital structure included taxes and identified that as the level of gearing increases by replacing equity with cheap debt the level of the WACC drops and an optimal capital structure does indeed exist at a point where debt is 100%
The following assumptions are made in the propositions with taxes:
Miller and Modigliani published a number of follow-up papers discussing some of these issues.
The theorem was first proposed by F. Modigliani and M. Miller in 1958.
The Modigliani-Miller theorem, which justifies near limitless financial leverage, has largely boosted economic and financial activities. But it also brought increased complexity, lack of transparency, higher risk and uncertainty in those activities.