A re-examination of the index effect: Gambling on additions to and deletions from the S&P 500's ‘gold seal’

This study examines the abnormal returns, trading activity, volatility and long-term performance of stocks that were added to the S&P 500 index. By using a three-factor pricing model that allows for firm size and value characteristics as well as market risk, we are able to shed new light on the widely observed ‘index effect’. We find that the CAPM tends to overstate the performance of large firms and to understate the performance of small firms. We also find a transitory increase in trading volume between the announcement and a few days after the effective date. In terms of the firm's operating performance, we find a significant increase in earnings per share after inclusion, which combines with the stock price rise to leave the average price-earnings ratio largely unaltered. Examining a unique sample of deletions of international companies and replacements with US companies, we find that deleted stocks experienced a considerable and permanent fall in price, inconsistent with the Investor Recognition Hypothesis. The “seal” of S&P 500 index membership has very long-term effects and inclusion appears not to be an information-free event.