A Pigouvian Tax on BorrowingIn this set-up, Jeanne and Korinek (2010a) show that efficiency can be restored in the decentral-ized economy by imposing a Pigouvian tax on borrowing in period 0, namelyb1(1t), which isrebated with transfers (TR) in a lump-sum fashion. The optimal tax is given by:t=E0lspp0(c1)u0(c1);(15)This equation states that whenever the borrowing constraint binds in period 1 with positive prob-ability, the policy-maker imposes a positive tax on borrowing in period 0, prompting privateagents to issue less debt in period 0 than under decentralized equilibrium. This is because boththe shadow value of the collateral constraint (lsp) and the derivativep01(c1)are positive.An Interest Rate PolicyA Pigouvian tax on borrowing may be difficult to implement. But the constrained efficient al-location can also be decentralized with the interest rate. The policy-maker can equally curtailhouseholds’ borrowing by increasing lending interest rates. For instance, the policy-maker (e.g.,a central bank in this specific case) can increase the interest rate at the beginning of period 0, af-fecting banks marginal cost and, therefore, consumers’ borrowing and consumption decisions.This increase in interest rates —if rebated with lump sum transfers (TR)— has the same effectof the Pigouvian tax analyzed above. To see this, assume for simplicity that the central bank canaffect the interest rate by an additive factory, so that the marginal cost for banks would be givenbyR+y(see Stein, 2012). The consumers’ maximization problem becomes:maxb1;b28>>>:u(b1)+E0ue+b2+p1b1M(R+y)+TR++yb2RL2lsp(b2p1)9>>=>>;:By equalizing the first order condition with respect tob1of the decentralized equilibrium and thesocial planner equilibrium, we can derive the level ofywhich closes the wedge:(u0(c0) =RL1E0u0(c1)+lspp0(c1);u0(c0) =M(R+y)u0(c1);Solving foryyields:y=E0lspp0(c1)u0(c1)R:(16