Commercial banks in investment banking - Conflict of interest or certification role?

When commercial banks make loans to firms and also underwrite securities, does this hamper or enhance their role as certifiers of firm value? This paper examines empirically the pricing of bank-underwritten securities as compared to investment-house-underwritten securities over a unique period in the U.S. (pre-Glass-Steagall) when both banks and investment houses were allowed to underwrite securities. The evidence shows that investors were willing to pay higher prices for securities underwritten by banks rather than investment houses. The results support a certification role for banks, which is more valuable for junior and information sensitive securities.