Some Negative Results on the Existence of Comparative Statics Results in Portfolio Theory

Abstract

Consider an investor who has a certain amount of wealth to invest in a riskless security and several risky securities. The investor's optimal portfolio will depend on his attitudes towards risk, his wealth and the probability distribution of the security returns. An interesting question to ask is how the investor's optimal portfolio is affected by changes in his wealth, given that all other things remain constant. For example, does the total amount invested in risky securities increase as wealth increases? Does the proportion of wealth invested in risky securities decrease as wealth increases? Questions such as these have been investigated by Arrow [1, Chapter 3] in the case of one riskless security and one risky security. Arrow showed that if the investor's von Neumann-Morgenstern utility function exhibits decreasing absolute risk aversion and increasing relative risk aversion, the amount invested in the risky security is an increasing function of wealth and the proportion of wealth invested in the risky security is a decreasing function of wealth. More recently, Cass and Stiglitz [3] have shown that Arrow's results do not generalize to the case of many risky securities. They give an example where an investor who can purchase one riskless security and two risky securities invests a greater proportion of his wealth in the two risky securities when his wealth increases, even though his utility function exhibits increasing relative risk aversion. Cass and Stiglitz note, however, that Arrow's results do generalize for an important, if highly restrictive, class of utility functions-those for which the mix of risky securities in the investor's optimal portfolio is independent of the investor's wealth for all probability distributions of security returns. Such utility functions are said to possess the separation property. The purpose of this paper is to prove that the separation property is a necessary condition as well as a sufficient condition for the generalization of Arrow's results to the case of many risky securities. We will show that given more than one risky security and a utility function which does not possess the separation property, it is always possible to pick probability distributions for the returns of the risky securities so that the directions of change which Arrow established for the single risky security case are reversed; that is, for some probability distributions of security returns, the total amount invested in risky securities decreases as wealth increases, and for other probability distributions of security returns, the proportion of wealth invested in risky securities increases as wealth increases. In fact, we will show that there always exist probability distributions of security returns such that the amount (proportion of wealth) invested in every risky security decreases (increases) as wealth increases. It should be emphasized, moreover, that this is the case