The Effectiveness of Debt Insurance As a Valid Signal of Bond Quality The role of private bond insurance in financial markets has generated considerable research (see Cole and Officer, 1981; Braswell, Nosari and Browning, 1982; and Kidwell, Sorensen and Wachowicz, 1987). Previous work focuses mainly on the benefits of insurance to borrowers. This third-party credit enhancement improves the quality of a debt issue and increases its marketability, thereby reducing the borrower's interest cost. However, outside credit assistance is not free. Any interest cost savings due to the purchase of bond insurance must be weighed against the out-of-pocket premium expenses incurred by borrowers. Unfortunately, available empirical studies provide no conclusive evidence as to the net effect of purchasing insurance on bond interest cost. On the one hand, Braswell, Nosari, and Browning (1982) find little or no net benefit to borrowers who use bond insurance. Cole and Officer (1981), on the other hand, report significant net interest cost savings to borrowers as a result of purchasing bond insurance. While the answer to the question of whether a net insurance benefit exist for bond issuers still awaits further empirical investigation, a more fundamental issue deserves attention; What theoretical basis supports the existence of bond insurance? In other words, what induces bond issuers or investment bankers to continue buying debt insurance? In an efficient and competitive market, the premium charged by the insurer should exactly offset any potential interest savings to borrowers from the purcahse of bond insurance. Consequently, no excess or net benefits should exist for borrowers purchasing bond insurance. Thakor (1982) argues that even in an efficient and competitive market, net interest cost savings to borrowers may still exist, due to the signal conveyed by insurance coverage about the default probability of insured debt issues. The signaling hypothesis suggests that, although the direct benefits of bond insurance purchase such as enhanced credit quality and marketability may be offset by the premium incurred, borrowers may enjoy indirect benefits through the signaling function of debt insurance, and this debt insurance signaling provides a basis for a positive net insurance benefit. Kidwell, Sorensen, and Wachowicz (1987) examine the benefits of insurance purchase and attribute the net interest cost savings observed in their data to the signaling function of debt insurance. The signaling argument of debt insurance provides a plausible explanation for why borrowers acquire bond insurance when the market is in equilibrium. Its implication that debt insurance is a valid signal of bond quality in the tax-exempt market, however, is debatable. In the current article, the authors find that complete debt insurance is not an informative signal and the purchase of debt insurance in and of itself cannot provide any signaling benefit to borrowers. (1) Therefore, any net interest cost savings due to the purchase of bond insurance should not be viewed as a result of signaling. Debt Insurance As a Quality Signal Thakor (1982) argues that third-party information producers provide unambiguous signals of seller quality. Such information producers expend resources efficiently to produce information about the quality of a product seller and recover the information production costs by selling their services to the product seller. Investors take the level of services purchased by a seller as a quality signal of the product, and pay a market price that reflects the true quality of the product. Viewing private bond insurers as third-party information producers, Thakor suggests that a property designed premium schedule will induce a borrower to obtain the right amount of insurance coverage which signals exactly the borrower's true probability of default, so that a priori imperfectly informed investors will be well informed in equilibrium. …