Motivated by debates surrounding international capital flows during the Great Recession, we conduct a positive and normative analysis of capital flows when a region of the global economy experiences a liquidity trap. Capital flows reduce inefficient output fluctuations in this region by inducing exchange rate movements that reallocate expenditure towards the goods it produces. Restricting capital mobility hampers such an adjustment.From a global perspective, constrained efficiency entails subsidizing capital flows to address an aggregate demand externality associated with exchange rate movements. Absent cooperation, however, dynamic terms-of-trade manipulation motives drive countries to inefficiently restrict capital flows, impeding aggregate demand stabilization.

This paper proposes a theory of foreign reserves as macroprudential policy. We study an open-economy model of financial crises in which pecuniary externalities lead to overborrowing, and show that by accumulating international reserves, the government can achieve the constrained-efficient allocation. The optimal reserve accumulation policy leans against the wind and significantly reduces the exposure to financial crises. The theory is consistent with the joint dynamics of private and official capital flows, both over time and in the cross-section, and can quantitatively account for the recent upward trend in international reserves.

The outreach of macroprudential policies is likely limited in practice by imperfect regulation enforcement, whether due to shadow banking, regulatory arbitrage, or other regulation circumvention schemes. We study how such concerns affect the design of optimal regulatory policy in a workhorse model in which pecuniary externalities call for macroprudential taxes on debt, but with the addition of a novel constraint that financial regulators lack the ability to enforce taxes on a subset of agents. While regulated agents reduce risk taking in response to debt taxes, unregulated agents react to the safer environment by taking on more risk. These leakages undermine the effectiveness of macruprudential taxes but do not necessarily call for weaker interventions. A quantitative analysis of the model suggests that aggregate welfare gains and reductions in the severity and frequency of financial crises remain, on average, largely unaffected by even significant leakages.

We develop a simple model of defaultable debt and rational bubbles in the price of an asset, which can be pledged as collateral in a competitive credit pool. When the asset pledgeability is low, the down payment is high, and bubble investment is unleveraged, as in a standard rational bubble model. When the pledgeability is high, the down payment is low, making it easier for leveraged borrowers to invest in the bubbly asset. As loans are packaged together into a competitive pool, the pricing of individual default risk may facilitate risk-taking. In equilibrium, credit-constrained borrowers may optimally choose a risky leveraged investment strategy -borrow to invest in the bubbly asset and default if the bubble bursts. The model predicts joint boom-bust cycles in asset prices and securitized credit.