Duffee, Gregory R. "The Relation Between Treasury Yields and Corporate Bond Yield Spreads", Journal of Finance, Vol. 53, No. 6, (December 1998), pp. 2225-2241.
Abstract: Because the option to call a corporate bond should rise in value when bond yields fall, the relation between noncallable Treasury yields and spreads of corporate bond yields over Treasury yields should depend on the callability of the corporate bond. I confirm this hypothesis for investment-grade corporate bonds. Although yield spreads on both callable and noncallable corporate bonds fall when Treasury yields rise, this relation is much stronger for callable bonds. This result has important implications for interpreting the behavior of yields on commonly used corporate bond indexes, which are composed primarily of callable bonds.