Monetary and Macroprudential Policies under Dollar-Denominated Foreign Debt
What this paper finds — and why it matters
Layer 1: Overview
Research question and motivation: Emerging economies have rapidly accumulated foreign-currency (mostly dollar) debt — the dollar share of 14 emerging economies’ foreign debt rose from 75% in 2010 to 81% in 2018. Such debt is dangerous because sudden stops in capital inflows cause sharp currency depreciation that mechanically raises the domestic-currency value of the debt. The paper asks: when a country holds dollar-denominated foreign debt, does macroprudential policy mitigate depreciation and downturns during sudden stops, how should monetary policy be conducted, and how should the two policies cooperate? Existing sudden-stop models (with loan-to-value/debt-to-income collateral constraints and pecuniary externalities) do not model the channel by which depreciation inflates the value of dollar debt.
Model setup: The author builds a small open economy in the tradition of Bianchi and Mendoza (2018), with three innovations: (1) foreign debt is denominated in foreign currency; (2) home tradable exports face a downward-sloping foreign demand (price elasticity rho > 1); (3) New Keynesian (Rotemberg) price stickiness to give monetary policy a role. The borrowing constraint is occasionally binding and the borrowing limit is denominated in domestic currency, creating a currency mismatch between foreign borrowing and the limit. The author deliberately abstracts from the collateral-asset-price pecuniary externality (assets valued at book value) to isolate a new balance-of-payments (BOP) externality. The model is solved with a global numerical method; each period is a year. Calibration targets the average of the 14 countries: discount factor beta = 0.92 (to hit mean foreign-debt-to-GDP of 40%), R* = 1.04, labor share = 0.66, imported-input share targeting import-to-GDP of 22%, theta = 8, price-adjustment cost psi = 50, export price elasticity rho = 3, tight borrowing limit kappa = 0.2 set so the unconditional crisis probability is 7.2%; productivity and interest-rate processes are from Mendoza (2010, Mexican data).
Key mechanism: When the borrowing constraint binds, large debt repayment with limited new borrowing forces net capital outflows, which require larger net exports and thus real depreciation (because exports face downward-sloping demand). Depreciation raises the domestic-currency value of debt repayment, forcing further outflows and a second-round depreciation — an amplification loop. Because households take the exchange rate as given, they socially overborrow ex ante (“ex ante BOP externality”) and use too many imported inputs during crises (“ex post BOP externality”), both producing inefficiently large depreciation. Social costs are twofold: imported inputs become inefficiently expensive (lowering output, explaining the output drop without working-capital financing), and an inefficiently large share of output is exported (lowering consumption).
Main findings: The optimal discretionary monetary policy (without taxes) is contractionary both when the constraint is slack (to discourage overborrowing via real appreciation raising the effective interest rate) and when it binds (to discourage imported-input use). But anticipation of crisis-time intervention lowers the ex ante effective interest rate and induces larger borrowing, destabilizing the economy. In crisis dynamics, without taxes the real exchange rate depreciates 10% under inflation targeting vs 6% under discretion; output drops 6.2% under targeting vs 14.4% under discretion. With macroprudential taxes, depreciation is 6% (targeting) vs 2% (discretion), and output drops 3.8% (targeting) vs 9.2% (discretion). Under taxes, foreign debt at the stochastic steady state is 6-7% smaller. Welfare (permanent-consumption metric, benchmark = inflation targeting without taxes): discretion without taxes is worse by 0.02%; evaluated at the simulation-mean foreign bond (-0.45), discretion with taxes gives +0.07% and targeting with taxes gives +0.03%. If the simulation starts with a binding constraint, the welfare gain under discretion with taxes can reach about 0.2%. Implication: the optimal mix is an ex ante macroprudential tax on foreign borrowing to correct overborrowing plus ex post monetary intervention to mitigate depreciation; monetary intervention improves welfare only when paired with the macroprudential tax.
Layer 2: Deep Dive
What is the core theoretical mechanism (the “amplification loop”) and why does it require a currency mismatch?
When the borrowing constraint binds, the country must repay outstanding foreign debt with only limited new borrowing, producing net capital outflows that must be matched by larger net exports via the balance-of-payments identity. Since exports face downward-sloping foreign demand, this requires real depreciation. Depreciation raises the domestic-currency value of the foreign-currency debt repayment (-e_t b*_{t-1}), but new borrowing is capped by the domestic-currency-denominated limit kappa*k, so the depreciation forces a cut in new borrowing, generating further outflows and a second-round depreciation. The loop continues. The currency mismatch — foreign-currency debt against a domestic-currency borrowing limit — is crucial: the author states explicitly that if the borrowing limit were denominated in foreign currency, the amplification loop would not occur.
What are the two externalities and how are they distinguished?
The “ex ante BOP externality” distorts borrowing in normal times: households do not internalize that reducing foreign debt today would reduce next-period net capital outflows and mitigate depreciation if the constraint binds, so they overborrow. The “ex post BOP externality” distorts imported-input use when the constraint is binding: households do not internalize that cutting imported inputs would improve the trade balance and mitigate depreciation, so they use socially excessive imported inputs. Both are formalized through the planner’s Lagrange multiplier gamma^SP_t (social value of real appreciation through BOP adjustment), which is strictly positive given rho>1 and negative net foreign assets. The ex ante term appears in the foreign-bond Euler equation; the ex post term appears in the imported-input first-order condition and is positive only when the constraint binds (mu^SP_t > 0).
Why is the optimal discretionary monetary policy contractionary in both states, and what does “contractionary” mean here?
The target inflation is zero (Rotemberg cost), so positive inflation is “expansionary” and negative inflation “contractionary.” When the constraint is slack but may bind, contractionary policy causes real appreciation, which raises the effective interest rate on foreign borrowing (via the exchange-rate term in the Euler equation), discouraging borrowing and partially correcting overborrowing. When the constraint binds, contractionary policy discourages production and imported-input use, improving the trade balance and partially correcting the ex post externality. Proposition 1 and Corollary 1 establish that strict inflation targeting is not optimal and that the optimal discretionary policy is contractionary in both states. Crucially, this period-by-period optimality does not imply discretion dominates inflation targeting in welfare, because it ignores how anticipation of future intervention shapes ex ante borrowing.
How does adding a macroprudential tax change the optimal monetary policy?
With an optimal time-consistent macroprudential tax on foreign borrowing available, Proposition 2 / Corollary 2 show the optimal discretionary monetary policy becomes pi_t = 0 when the constraint is not binding (the tax now corrects overborrowing, so the eta^EE term is zero and monetary policy focuses only on minimizing price-adjustment cost) but remains contractionary (pi_t < 0) when the constraint binds — because the ex ante tax cannot correct the ex post externality of excessive imported inputs during a crisis. The macroprudential tax is strictly positive whenever there is positive probability the constraint binds next period, and rises with outstanding debt; it is notably higher under discretion (by about 0.6% before a crisis) to offset the extra overborrowing induced by anticipated intervention.
What is the quantitative crisis-dynamics evidence across the four regimes?
Crisis is defined as the current account exceeding two standard deviations above its long-run mean; crisis events are picked under inflation targeting without taxes. Real exchange rate depreciation: 10% (targeting, no tax), 6% (discretion, no tax), 6% (targeting, with tax), 2% (discretion, with tax). Output drop: 6.2% (targeting, no tax), 14.4% (discretion, no tax), 3.8% (targeting, with tax), 9.2% (discretion, with tax). Macroprudential taxes reduce pre-crisis debt and capital-flow reversals; discretion raises pre-crisis debt through anticipation of intervention. Standard deviations (relative to targeting-no-tax = 100%): under discretion with tax, real exchange rate volatility falls to 37.9% and current-account/GDP to 82.0%, while output is 111.7% and consumption 88.3% — i.e., discretion lowers exchange-rate volatility but raises output/consumption volatility.
What are the welfare results and their scope conditions?
Welfare is measured as permanent-consumption gain/loss relative to inflation targeting without taxes. Without taxes, discretion is slightly worse (-0.02%). Evaluated at the simulation-mean foreign bond (-0.45) with no borrowing-limit shock at the initial period: discretion with tax gives +0.07%, inflation targeting with tax gives +0.03%. When a borrowing-limit shock hits at the initial period (constraint binding): discretion without taxes gives +0.03% and with taxes +0.09%, with larger gains for larger initial debt; the gain can be as high as about 0.2% when the simulation starts with the constraint binding. Scope condition: monetary intervention during a crisis improves welfare ONLY when combined with an ex ante macroprudential tax; absent the tax, anticipation of intervention induces overborrowing and reduces welfare.
How does this paper differ from closely related prior work (Fornaro 2015, Ottonello 2015, Mendoza and Rojas 2019, Devereux et al. 2018, Coulibaly 2018)?
Fornaro (2015) and Ottonello (2015) introduce nominal wage rigidities and emphasize the BENEFIT of depreciation (boosting exports, reducing unemployment); this paper emphasizes the NEGATIVE effect of depreciation through inflating the value of foreign-currency debt. Mendoza and Rojas (2019) model depreciation as REDUCING the debt-repayment burden (depreciation lowers the consumption-composite real interest rate); here depreciation increases the burden. Devereux et al. (2018) and Coulibaly (2018) are closest — both add NK price stickiness and study monetary-macroprudential combinations — but in those the collateral channel/asset price drives the externality; this paper’s contribution is to study optimal policy where depreciation raises the domestic-currency value of foreign debt and causes a severe crisis. The welfare result (inflation targeting dominates discretion without taxes, but discretion preferable with the optimal tax) mirrors Coulibaly (2018).
Why is the optimal policy time-consistent, and how is the planner’s problem set up?
The BOP externalities themselves do not generate time inconsistency (the macroprudential tax in this model is time consistent, unlike pecuniary externalities from collateral asset prices). However, NK price stickiness can create time inconsistency via firms’ forward-looking pricing, so the author assumes no commitment and solves for time-consistent policy in a Markov perfect equilibrium: each period’s planner optimizes taking future planners’ rules as given while internalizing how current policy affects them, and the optimal rules coincide with those expected by past planners. The Ramsey planner maximizes household utility subject to the decentralized equilibrium conditions as implementability constraints. The nominal interest rate R_t is backed out from the Euler equation after other variables are pinned down.
What real-side outcome does the model explain without standard assumptions, and what is the consumption-labor trade-off in welfare?
The model explains the output drop during sudden stops WITHOUT working-capital financing (commonly assumed in the literature): the inefficiently expensive imported inputs caused by real depreciation directly reduce output. On welfare, although contractionary monetary intervention causes output and labor (hence labor disutility) to drop more under discretion, consumption does not drop as much because mitigated depreciation means smaller exports and a larger share of output consumed domestically. Period utility (consumption minus labor disutility) can therefore be slightly higher under discretion when combined with taxes. An appendix (Section F) with fixed labor and no labor disutility shows monetary intervention under discretion actually raises crisis-period consumption above inflation targeting.
What robustness/extensions does the paper note?
Section E of the appendix studies the model WITH the asset-price pecuniary externality (as in Bianchi and Mendoza 2018), which the baseline shuts off via book-value asset valuation. Section A proves the constant tax tau_m = 1/(rho-1) corrects the terms-of-trade externality. Section F examines fixed labor supply with no labor disutility. The conclusion proposes three extensions: foreign-reserve accumulation and reserve interventions (as in Arce et al. 2019), endogenous choice of borrowing currency, and introducing financial intermediaries with currency mismatch (as in Aoki et al. 2018 and Mendoza and Rojas 2019).
What are the main caveats?
This is a theoretical/quantitative DSGE exercise, not an empirical-identification paper, so there is no causal identification strategy in the econometric sense; the model is calibrated (not estimated) to standard literature values and the average of 14 emerging economies. Results depend on parameter choices, notably the export price elasticity rho = 3 (within Simonovska-Waugh’s 2.79-4.46 range) and the domestic-currency denomination of the borrowing limit, which is essential to the amplification loop. The author also notes that introducing imported-input taxes only during crises may be difficult to implement in practice, motivating reliance on monetary policy for ex post intervention.