["The Journal of Finance, EarlyView. ", "\nABSTRACT\nWe 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."]