The price, liquidity and information asymmetry changes associated with new S&P 500 additions

Using S&P 500 additions, the purpose of this paper is to test the permanence of abnormal returns around the index inclusion announcement and effective implementation dates to differentiate among competing explanations for the index inclusion premia puzzle.

The event study methodology is used to examine abnormal returns and volume effects around the announcement dates (ADs) and implementation dates of index additions.

This study documents a twofold increase in trading volume and significant permanent abnormal returns at the ADs that are correlated with subsequent decreases in bid‐ask spreads. There is a fivefold increase in trading volume, but only temporary abnormal returns, around the effective dates (EDs). Taken collectively, the evidence indicates that the permanent return at announcement is best explained by liquidity/information cost explanation, but the temporary return and large trading increases at the ED can best be attributed to the price pressure hypothesis.

These results do not support the well documented long‐run downward‐sloping demand curve as the primary explanation for the abnormal returns observed on these dates.

This study contributes to the body of literature on the index inclusion effect by providing supporting evidence for the liquidity/information cost explanation, and by extending the previously analyzed index additions with an additional five‐year period from 2000‐2004.