Collateralized borrowing and risk taking at low interest rates

Empirical evidence suggests financial intermediaries increase risky investments when interest rates are low. We develop a model consistent with this observation and ask whether the risks undertaken exceed the social optimum. Interest rate policy affects risk taking in the model through two opposing channels. First, low policy rates make riskier assets more attractive than safe bonds. Second, low policy rates reduce the amount of safe bonds available for collateralized borrowing in interbank markets. The calibrated model features excessive risk taking at the optimal policy. However, at low policy rates, collateral constraints tighten and risk taking does not exceed the social optimum.