S&P 500 INDEX CHANGES AND INVESTOR AWARENESS

Honghui Chen, Gregory Noronha and Vijay Singal

We find that, on average, firms added to the S&P 500 index experience a permanent price increase, while those deleted from it suffer only a temporary price decline. Existing theories, such as a downward sloping demand curve, liquidity, and information, fail to explain the asymmetric response. We propose a new explanation for the observed price patterns: changes in investor awareness. Investors become more aware of a stock upon its addition to the S&P 500 index but do not become similarly unaware of a stock following its deletion. This results in a significant price increase after an addition but not an equivalent decline after a deletion. Consistent with our hypothesis, we find that Merton’s (Journal of Finance, 1987, 42, 483–510) measure of awareness improves after an addition but remains essentially unchanged after a deletion. The price reaction is related to changes in the measure of awareness. From a practical standpoint, our results suggest that index fund managers who are not constrained by tracking error minimization are better off not trading on the effective date. Rather, they may be able to benefit their shareholders by executing purchases of additions upon announcement, but waiting to sell deleted firms until well after the effective date.