Investor Confidence, Short-Sales Constraints, and the Behavior of Security Prices ∗

Investor confidence affects financial markets. Information, noise, market frictions cause investor confidence to influence security prices, leading to a price different from the rational expectations value. This paper presents a simple theoretical model of asset prices where investor confidence is allowed to differ across traders, and across time – depending on observed outcomes. The presence of short-sales constraints causes asset prices to behave asymmetrically: short-run returns display reversal after good news, but momentum after bad news. This can change somewhat if investor confidence varies because of biased self-attribution: good news causes returns to exhibit short-run momentum and long-run reversal. We also investigate the extent to which asset supply affects the price paths. In case of zero net supply the equilibrium prices are set unilaterally by either of two investor groups. Yet if we allow for positive asset supply we find an intermediate range of information signals and resulting prices consistent with both trader types being active in the market – this happens if news events are moderate. We further identify the shrinkage effect: the range of signals where both groups of traders have positive asset demands may shrink in subsequent trading due to biased self-attribution.