We present a simple new explanation for the diversification discount in the valuation of firms. We demonstrate that, ceteris paribus, limited liability of equity holders is sufficient to explain a diversification discount. To derive this result, we use a credit risk model based on the value of the firm's assets. We show that a conglomerate can be regarded as an option on a portfolio of assets. By splitting up the conglomerate, the investor receives a portfolio of options on assets. The conglomerate discount arises because the value of a portfolio of options is always equal to or higher than the value of an option on a portfolio. The magnitude of the conglomerate discount depends on the number of business units and their correlation, as well as their volatility, among other factors.
An Analytical Portfolio Credit Risk Model Based on the Extended Binomial Distribution