We examine optimal capital requirements in a quantitative general equilibrium model with banks exposed to nondiversifiable borrower default risk. Contrary to standard models of bank default risk, our framework captures the limited upside, but significant downside risk of loan portfolio returns. This helps to reproduce the frequency and severity of twin defaults : simultaneously high firm and bank defaults. Hence, the optimal bank capital requirement, which trades off a lower frequency of twin defaults against restricting credit provision, is higher than under default risk models which underestimate the impact of borrower default on bank solvency.

We show that lender of last resort (LOLR) policy exacerbates bank interconnectedness. Using novel micro-level data, we analyze LOLR’s haircut gaps: the differences between the private market and central bank haircuts. LOLR policy incentivizes banks to increase pledging and holdings of higher haircut-gap bonds, especially those issued by domestic and systemically important banks. Effects only apply to banks, not to nonbanks without LOLR access. LOLR funding revives bank bond issuance associated with higher haircut gaps and increases the subsequent correlation between pledging and issuing banks’ bond prices, in particular during periods of low-market returns and for domestic, systemically important banks.