U-Shaped Cost of Equity Function? Digging into Modigliani-Miller (1958) Mistake

22 Pages Posted: 3 Oct 2006

See all articles by Hugo Oscar Berlingeri

Hugo Oscar Berlingeri

Pontificia Universidad Católica Argentina

Date Written: September 2006


Modigliani and Miller (1958, The cost of capital, corporation finance and the theory of investment, The American Economic Review, Vol. 48, Nº 3) assert that, beyond some high level of leverage, the cost of equity curve may be decreasing. Despite the lack of rigour of their argument, this proposition received a small amount of criticism. However, it is wrong: in an MM world, the cost of equity function must be increasing, because under the dynamics of MM Proposition II, the marginal cost of debt must be lower than the cost of equity. Should, by any chance, the marginal cost of debt be higher than the cost of equity, arbitrage opportunities, incompatible with market equilibrium, would emerge. In this way, the intuitive idea that the cost of equity may increase unboundedly is recovered. Nevertheless, the cost of equity function will tend to infinity only if the first derivative of the interest cost function also tends to infinity. Otherwise, in the limit, the cost of equity will tend to a finite number. These findings reinforce the theoretical validity of MM propositions.

Keywords: Cost of equity, Marginal cost of debt, Arbitrage opportunities, Modigliani-Miller, MM irrelevance propositions

JEL Classification: G32

Suggested Citation

Berlingeri, Hugo Oscar, U-Shaped Cost of Equity Function? Digging into Modigliani-Miller (1958) Mistake (September 2006). Available at SSRN: https://ssrn.com/abstract=934550 or http://dx.doi.org/10.2139/ssrn.934550

Hugo Oscar Berlingeri (Contact Author)

Pontificia Universidad Católica Argentina ( email )

Buenos Aires

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