Information asymmetry around S&P 500 index changes

This study aims to explore information asymmetry (IA) (as measured by the adverse selection component of the bid-ask spread) around S&P 500 revisions.

The authors use adverse selection cost of trading measures to examine the effects of S&P 500 index composition changes on the trading environment from 2001 to 2010.

The authors find that the adverse selection cost of trading significantly decreases post-addition and increases post-deletion. However, the intraday price dynamics of additions to the index seem to be distinct from those of deletions from the index. The event period cumulative abnormal returns (CARs) for additions are significantly associated with the change in the adverse selection cost of trading. However, this association is non-significant for deletions. The CARs for deletion events are found to be significantly associated with the change in realized spreads. Realized spreads are a measure of revenue earned by liquidity providers in the market.

This study helps better understand the dynamics of two types of IA – one from a firm to investor and the other between investors – and presents evidence of the role of adverse selection in index changes. By doing so, it helps better understand the mechanism driving price formation post-addition to and deletion from an index.