Bond Ratings Matter: Evidence from the Lehman Brothers Index Rating Redefinition

The 2005 inclusion of Fitch ratings in the Lehman composite index ratings provides a quasi-natural experiment to identify rating-based market segmentation in the corporate bond market. Split-rated bonds with favorable Fitch rating that were mechanically upgraded to investment-grade status exhibit abnormal returns and order flows, whether or not they enter the Lehman investment-grade index itself. An asymmetric impact of favorable Fitch ratings on bonds around the HY-IG boundary whose index rating did not initially change suggests that mechanical changes in future index rating transition probabilities also affect bond pricing. Our results highlight the importance of rating-based industry norms and practices for market segmentation, in addition to rating-based regulation.