This shows the initial deviation of real per-capita GDP from its long run trend, and the average growth rate over the following ten years, for 1925 through 2005. The long run trend is based on the 1925-2005 average growth rate of real per-capita GDP of 2.3%. The points in the upper left are the ten-year periods beginning in 1931 through 1941.
UPDATE: A number of people have objected to this exercise on the grounds that the Depression and World War II period is not relevant for our current situation. I don’t agree with this. But even without them, the picture is not so different. While the postwar period up til now has never seen a persistent deviation from trend as we are experiencing now, or as rapid growth as the Friedman paper projects, the relationship between the two is clearly present. And a decade of growth far above the postwar norm turns out to be just what you would predict on the basis of that relationship. Here’s the same graph as above, but this time using only 1947-2005.
As you can see, the relationship is a fairly strong one. The Friedman growth number does lie a bit above the regression line. But it’s still true that the current exceptionally low level of GDP relative to trend would, on historical evidence, lead us to expect that growth over the next ten years will be around 3.8 percent — well above anything previously seen in the postwar period and close to double the long-term average.
Note that the seven points well below the line in the middle are 1999-2005, whose 10-year growth windows include the Great Recession. Without them, Friedman’s number would be much closer to the line. What do we make of that? Should the exceptionally poor performance of this period make us more pessimistic about medium-term growth prospects (it’s sign of supply-side exhaustion) or more optimistic (it’s a sign of a demand gap that can be filled)? This is not an easy question to answer. But just counting up previous growth rates won’t help answer it.