A budget deficit is what the government is short in one year. The national debt is the running total of every deficit it has ever borrowed to cover, minus any surpluses. Run a hypothetical country's budget for ten years and watch each year's deficit pile up into the debt.
The country starts with nominal GDP of $1,000 billion. Spending and taxes are set as a share of GDP. Interest on the debt is paid on top of regular spending, so it counts toward the deficit too.
This table runs the same simulation. Each row obeys one rule: Debt this year = Debt last year + Deficit this year. A surplus is a negative deficit, so it subtracts from the debt.
| Year | Nominal GDP | Tax revenue | Spending | Interest | Surplus (+) / Deficit (−) | Debt at year end | Debt % GDP |
|---|
The amount by which government spending (purchases plus transfer payments) exceeds tax revenue in a single year. The government borrows to cover it by selling bonds.
The total amount the government owes at a point in time. It equals the sum of all past deficits minus all past surpluses.
Tax revenue is greater than spending for the year. The government can use the surplus to pay back bondholders, which reduces the debt.
A cyclical deficit comes from the business cycle: recessions shrink tax revenue and raise transfers. A structural deficit is the part that would remain even at full employment.
Progressive income taxes and transfer programs like unemployment insurance change on their own as the economy moves, with no new law. They push the budget toward deficit in recessions and toward surplus in expansions.
Anyone who owns a government bond: households, banks, pension funds, foreign investors and governments, the central bank, and other government agencies.
A bigger debt means bigger interest payments. Interest is spending too, so it adds to future deficits and leaves less room for other programs.
Government borrowing increases the demand for loanable funds, which raises the real interest rate and reduces private investment. Less investment means slower growth of the capital stock.
Future taxpayers inherit the interest payments. If crowding out lowers investment, they also inherit a smaller capital stock and less productive capacity.
The deficit is a speed. The debt is the distance traveled. Slowing down is not the same as going backward.
| Event | Budget deficit (flow) | National debt (stock) |
|---|---|---|
| Government spending rises, taxes unchanged (expansionary fiscal policy) | Increases (or the surplus shrinks) | Grows faster |
| Recession: tax revenue falls and transfers rise automatically | Increases (cyclical deficit) | Grows faster |
| Expansion: tax revenue rises and transfers fall automatically | Decreases | Grows more slowly (or falls, if the budget moves into surplus) |
| Deficit shrinks from $80B to $50B | Smaller, but still a deficit | Still rises, by $50B |
| Budget surplus | None (revenue exceeds spending) | Falls by the amount of the surplus |
| Interest rate on the debt rises | Increases because interest payments rise | Grows faster |
| Nominal GDP grows faster than the debt | No direct change | Debt-to-GDP ratio falls, even if the debt itself rises |
AP-style multiple choice. Pick an answer to see the explanation.
The vocabulary for topic 5.4. Read the list, flip through flashcards, or quiz yourself. For every term in the unit, see the Unit 5 terminology guide.