Sovereign debt is not one thing; it is a portfolio of instruments differing in maturity, currency, holder, and indexation, and those differences determine how risky a given debt level actually is (the composition point of the last module). This module covers the instruments and the structural choices — domestic vs external, short vs long, local- vs foreign-currency — that decide a country's vulnerability.
The instruments
- Treasury bills — short-term (under a year), sold at a discount and redeemed at face value (no coupon). Cheap and liquid, but must be rolled over frequently, creating refinancing risk.
- Treasury bonds — longer-term (2 to 30 years), paying periodic coupons. They lock in financing for longer (less rollover risk) but usually at a higher rate (the term premium).
- The yield curve — the schedule of interest rates across maturities. A government building a deep domestic market issues across the curve to establish benchmark rates; the shape of the curve signals the market's view of future rates and risk (the detailed mechanics are in the Quant Finance Math course).
- Indexed instruments — inflation-linked or FX-linked bonds, which shift specific risks between the government and investors.
Domestic versus external debt
The fundamental structural choice
Where and in what currency a government borrows is the most important structural decision in debt management: • Domestic debt — issued at home, usually in local currency, held by domestic banks, pension funds, and the central bank. Advantages: no currency risk (it's in your own currency, which you control), a captive and growing investor base, and it develops the domestic financial market. Disadvantages: it can crowd out private credit (banks lend to the government instead of firms), the domestic market may be too shallow to absorb large amounts, and short maturities create rollover risk. • External debt — issued abroad, usually in hard currency (dollars, euros), held by foreign investors (Eurobonds) or official creditors. Advantages: access to a vast, deep pool of capital, often at longer maturities and (historically) lower headline rates than thin domestic markets. Disadvantages: currency risk (you owe dollars but earn local currency — a depreciation raises the burden, as the last module's arithmetic showed), exposure to volatile foreign investor sentiment (sudden stops), and rollover risk concentrated in hard currency you cannot print.
Original sin
The inability to borrow long-term in your own currency
Eichengreen and Hausmann named the central vulnerability 'original sin': the historical inability of most developing countries to borrow abroad in their own currency, or even to borrow long-term at home. Foreign investors will not hold long-dated local-currency emerging-market debt (they fear inflation and depreciation eroding their returns), so these countries are forced to borrow externally in hard currency (creating a currency mismatch — assets and revenues in local currency, debt in dollars) or domestically only at short maturities (creating a maturity mismatch — rollover risk). Both mismatches are sources of crisis: a currency mismatch means a depreciation balloons the debt burden (the balance-sheet effect behind third-generation currency crises, the International Macro course); a maturity mismatch means a loss of confidence triggers a rollover crisis. Original sin is not a moral failing but a structural feature of the international financial system that makes developing-country debt inherently more fragile than its level alone suggests — and reducing it (by developing local-currency long-term markets) is a central goal of debt management.
The investor base and rollover risk
Who holds the debt matters as much as how much there is. A debt held by long-term domestic investors (pension funds, insurers) who buy and hold is stable; a debt held heavily by foreign portfolio investors who can sell and exit at the first sign of trouble is flighty (non-resident holdings of local-currency debt are a particular vulnerability — they combine foreign-investor flight risk with the government's hope of local-currency benefits). Rollover/refinancing risk — the danger that maturing debt cannot be refinanced on acceptable terms — is driven by the maturity structure: a debt heavily weighted toward short maturities must be constantly refinanced, so a temporary loss of market access (a spike in rates, a downgrade) becomes an immediate crisis. Lengthening maturities and broadening the investor base toward stable domestic holders are the levers a debt manager uses to reduce this risk (module 8).
Exercise
Two countries each have public debt of 60% of GDP. Country A's debt is mostly long-term, local-currency bonds held by domestic pension funds. Country B's debt is half short-term Treasury bills and half hard-currency Eurobonds held by foreign investors. (1) Explain why these two identical-looking debt levels carry very different risk. (2) Apply 'original sin' to Country B's structure. (3) Explain the specific crisis each structure is vulnerable to. (4) What steps could Country B take over time to move toward Country A's safer structure, and what would it cost?