We build a structural two-factor model of default where the stock market index is one of the stochastic factors. We allow the firm to adjust its leverage ratio in response to changes in the business climate for which the past performance of the stock market index acts as a proxy. We assume that the firm's log-leverage ratio follows a mean-reverting process and that the past performance of the stock index negatively affects the firms target leverage ratio. We show that for most credit ratings our model may explain actual yield spreads better than other well-known structural credit risk models. Also, our model shows that the past performance of the stock index returns and the firm's assets beta have a significant impact on credit spreads. Hence, our model can explain why credit spreads may be different within the same credit rating groups and why spreads are lower during economic expansions and higher during recession
