Public debt is discussed using household analogies that mislead in specific ways, and the actual constraints are different from those a household faces.
Why the analogy fails
A household has a finite lifespan and must repay. A state does not, and rolls debt over indefinitely.
A household cannot influence its own income through its spending. Government spending affects the economy that generates tax revenue.
A household cannot issue the currency its debt is denominated in. A government borrowing in its own currency can.
Which are substantive differences rather than quibbles, and they change what the numbers mean.
The ratio
Debt is generally expressed relative to economic output rather than in absolute terms.
Which means the ratio falls if output grows faster than debt, without any repayment.
Historical episodes of large debt reduction have generally occurred through growth and inflation rather than through repayment.
The debt dynamics
Whether the ratio rises or falls depends on the relationship between the interest rate on debt and the growth rate of the economy, plus the primary balance.
Which means that when growth exceeds the interest rate, the ratio falls even with continued borrowing.
That condition held in many countries for an extended period and has been reversing as rates rose, which changes the arithmetic substantially.
Currency denomination
The most consequential distinction.
Debt in a currency the government issues cannot force default in the way foreign currency debt can.
Which does not make it costless — the constraint becomes inflation and exchange rate rather than default.
Countries borrowing in foreign currency face a genuinely different situation, and most historical sovereign defaults involved it.
Who holds it
Domestically held debt is an obligation from one part of a society to another.
Externally held debt transfers resources abroad when serviced.
Which is a real distinction and it is frequently omitted from headline figures.
Central bank holdings, acquired through asset purchases, are a further category where the interest paid returns to the treasury in most arrangements.
Sustainability
Assessed through debt sustainability analysis, projecting the ratio under assumptions about growth, rates and fiscal balance.
Which is sensitive to those assumptions, and small changes produce large differences over long horizons.
No threshold has been established at which debt becomes unsustainable, and attempts to identify one have been contested methodologically.
What actually constrains
Willingness of markets to lend at acceptable rates.
Inflation, where borrowing exceeds the economy's capacity.
And exchange rate pressure for countries with external deficits.
Which are real constraints operating through different mechanisms from a household budget constraint.
Intergenerational framing
Debt is described as a burden on future generations, and the accounting is more complicated.
Future generations inherit the debt and the assets it financed, and the bonds themselves as assets where held domestically.
Which means what was borrowed for matters — infrastructure and education produce future returns, current consumption does not.
Off balance sheet obligations
Commitments not counted in headline debt figures.
Which include public sector pension liabilities, guarantees, private finance arrangements and contingent commitments.
These are frequently substantial relative to recorded debt, and accounting standards for them vary.
Whole of government accounts, produced in some countries, include them and give a fuller picture.
Maturity structure
How quickly debt must be refinanced affects vulnerability to rate changes.
Which means a country with long-dated debt is insulated from rate rises for longer than one with short maturities.
Average maturity varies enormously between countries and is a meaningful risk indicator that headline ratios do not capture.
Fiscal rules
Numerical constraints on deficits or debt adopted in many countries.
Which are intended to constrain and have been suspended or revised frequently when they bind.
Evidence on whether they change outcomes is mixed, with design features including escape clauses and independent monitoring appearing to matter.
Ratings
Sovereign credit ratings assess default probability and affect borrowing costs.
Which involves substantial qualitative judgement about institutions and political stability alongside fiscal metrics.
Downgrades during crises have been criticised as procyclical, worsening the conditions being assessed.
Independent fiscal institutions
Bodies producing independent forecasts and assessing fiscal policy against stated rules.
Which exist in many countries and improve the informational basis for debate.
Their independence depends on appointment, funding and access to data, which vary.
Following it
Debt management offices publish issuance calendars, holdings breakdowns and maturity profiles.
Which is detailed public information that is more informative than headline debt figures.
Inflation and debt
Unexpected inflation reduces the real value of fixed-rate debt, transferring value from holders to the issuer.
Which has been a substantial historical mechanism of debt reduction.
Index-linked debt removes this, which is why the proportion of it affects how much inflation helps or does not.
Reading the figures
Gross and net debt differ, as do general government and central government measures.
Which means comparisons require using the same definition, and headline figures frequently do not state which they use.
Statistical agencies publish the definitions and the reconciliations between measures.