Animal Spirits, Margin Requirements, and Stock Price Volatility

Abstract

A simple overlapping generations model is used to characterize the effects of initial margin requirements on the volatility of risky asset prices. Investors are assumed to exhibit heterogeneous preferences for risk-bearing, the distribution of which evolves stochastically across generations. This framework is used to show that imposing a binding initial margin requirement may either increase or decrease stock price volatility, depending upon the microeconomic structure behind fluctuations in economy-wide average risk-bearing propensity. The ambiguous effect on volatility similarly arises when the source of heterogeneity is noise trader beliefs. BEYOND THEIR PRUDENTIAL FUNCTIONS, the effects of initial margin requirements on securities markets are not well understood. Many have argued, based upon empirical findings, that the level of margin requirements has no discernable impact on stock price behavior.' Nevertheless, the view that high margin requirements dampen speculative excesses and reduce excess price volatility remains popular. The simultaneous existence of such views highlights the uncertainty surrounding this issue. In fact, there exists no rigorously articulated theory of the interaction between initial margin requirements and stock prices and their returns' characteristics. In this paper we construct a simple dynamic model useful for characterizing the effects of initial margin requirements on security prices and returns. To analyze the impact of margins, we introduce investor heterogeneity into an otherwise simplified overlapping generations (OLG) economy.2 The OLG models are constructed so that each generation displays heterogeneous preferences or beliefs, the distribution of which evolves stochastically across generations. In each of two heterogeneous taste models we consider there are two types of agents, one of which is more risk tolerant than the other. In one model, the