Significant price changes are not only observed after changes of the rating, but also after changes of the rating outlook. In many cases technical factors represent a major reason for bond price changes after a negative rating action. In particular, large price movements can be observed when investment restrictions of institutional investors are triggered by a rating action.
A good example is the downgrade of Fiat in June 2002. The downgrade from Baa2 to Baa3 induced a large sell-off because investors anticipated
a further downgrade to high yield. But Figure 9.1 also shows that bond prices already fell significantly when Moody’s put Fiat on review for downgrade.
Empirical studies by Weinstein (1977), Hand et al. (1992) and Kliger and Sarig (2000) highlight the following relationships between rating changes
and a corporate issuer’s bond and stock prices:
- Bond prices adjust to a new rating.
- Equity prices also react on rating changes, usually opposite to the bond price movements.
- Surprise upgrades tend to result in a reduced implied equity volatility.
There is no indication that new rating information has an impact on firm value. According to the Asset Substitution Theory equity and bond prices react in opposite directions to rating changes, which ultimately leads to changes in the ratio of the market value of equity to debt.