GOVERNMENT debt dynamics, once an esoteric subject of interest only to macroeconomists, are suddenly in vogue. With Greece flirting with default, Italy's bond yields rising fast, and America's government bonds losing their AAA status, public-debt burdens have become dinner-party talk. Our interactive chart above shows current IMF forecasts but also allows you to input some basic economic assumptions to see where general government debt as a percentage of GDP might head.
There are two things that matter in government-debt dynamics. The difference between real interest rates and GDP growth (r-g), and the primary budget balance as a % of GDP (ie, before interest payments). In any given period the debt stock grows by the existing debt stock (d) multiplied by r-g, less the primary budget balance (p).
The simple r-g assumption is one of the most important in debt dynamics: an r-g of greater than zero (when interest rates are greater than GDP growth) means that the debt stock increases over time. An r-g of less than zero causes it to fall.
Our interactive model uses the nominal interest rate (i)—approximately equivalent to the ten-year bond yield—and allows you to input your own inflation rate, ∏. Inflation helps reduce the total debt stock over time, by reducing the real value of debt. In our model and using approximations, r-g becomes i - ∏ - g. The greater the inflation rate, the lower r-g becomes.
The second consideration is the primary budget balance. A primary budget surplus causes the debt stock to fall, by allowing the government to pay off some of the existing debt. A primary deficit needs to be financed by further borrowing...
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