14 They show that if one is interested in forecasting excess returns on corporate bonds (relative to Treasury securities) over the next few years, credit spreads are indeed helpful, but another powerful predictive variable is a nonprice measure: the high-yield share, defined as issuance by speculative-grade firms divided by total bond issuance.
When the high-yield share is elevated, future returns on corporate credit tend to be low, holding fixed the credit spread. Exhibit 1 provides an illustration of their finding. One possible interpretation is that the high-yield share acts as a summary statistic for a variety of nonprice credit terms and structural features.