Debt and Delusion - By Robert J. Shiller

Economists like to talk about thresholds that, if crossed, spell trouble. Usually there is an element of truth in what they say. But the public often overreacts to such talk.

Consider, for example, the debt-to-GDP ratio, much in the news nowadays in Europe and the United States. It is sometimes said, almost in the same breath, that Greece’s debt equals 153% of its annual GDP, and that Greece is insolvent. Couple these statements with recent television footage of Greeks rioting in the street. Now, what does that look like?

Here in the US, it might seem like an image of our future, as public debt comes perilously close to 100% of annual GDP and continues to rise. But maybe this image is just a bit too vivid in our imaginations. Could it be that people think that a country becomes insolvent when its debt exceeds 100% of GDP?

That would clearly be nonsense. After all, debt (which is measured in currency units) and GDP (which is measured in currency units per unit of time) yields a ratio in units of pure time. There is nothing special about using a year as that unit. A year is the time that it takes for the earth to orbit the sun, which, except for seasonal industries like agriculture, has no particular economic significance.

We should remember this from high school science: always pay attention to units of measurement. Get the units wrong and you are totally befuddled.

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What is really happening in Greece is the operation of a social-feedback mechanism. Something started to cause investors to fear that Greek debt had a slightly higher risk of eventual default. Lower demand for Greek debt caused its price to fall, meaning that its yield in terms of market interest rates rose. The higher rates made it more costly for Greece to refinance its debt, creating a fiscal crisis that has forced the government to impose severe austerity measures, leading to public unrest and an economic collapse that has fueled even greater investor skepticism about Greece’s ability to service its debt.

This feedback has nothing to do with the debt-to-annual-GDP ratio crossing some threshold, unless the people who contribute to the feedback believe in the ratio. To be sure, the ratio is a factor that would help us to assess risks of negative feedback, since the government must refinance short-term debt sooner, and, if the crisis pushes up interest rates, the authorities will face intense pressures for fiscal austerity sooner or later. But the ratio is not the cause of the feedback.