As noted by Serrano, the research by Nelson and Plosser (1982) (here; subscription required) and Real Business Cycle (RBC) authors suggests that GDP follows a random walk, and as a result after a productivity shock (which they measure as changes in TFP, in spite of significant problems with that measure; see here) output does not return to its previous trend. The point is that once the output trend is affected there are persistent effects that change the trend itself, that is hysteresis. Fluctuations are variations of the optimal level itself.
Serrano correctly points out that "this means that the long run trend of output is not only partially determined by whatever drives short run output (presumably aggregate demand) but rather that potential output is actually fully determined by the trend of whatever drives actual output. As it is well known, this result of strong hysteresis in the output (GDP) series has been taken to provide evidence in favor of the 'real business cycles' strand of new classical macroeconomics in which the common element driving trend and cycle are factor supplies and their productivity." The natural rate or NAIRU is supply determined, but is variable (something that, in a different context, Robert Gordon would call the Time Varying NAIRU or TV-NAIRU).
Supply shocks imply that in a boom the potential output moves first, and actual output adjusts as individuals readjust to higher productivity. Hence, the output gap, if defined as the difference between actual and potential output, becomes negative. And if you believe in a Phillips curve and some sort of central bank monetary rule, a negative output gap suggests a deflationary pressure (and yes RBC authors do believe in endogenous money). Yes, that's what the RBC theory implies! In fact, according to Kydland and Prescott (1990): "the price level has displayed a clear countercyclical pattern."*
Serrano points out a simpler (Occam's Razor applies) explanation for the favorable evidence on hysteresis, namely that demand (in fact, the autonomous components of demand) determine potential output (the supermultiplier). Note that this approach does not require, as the RBC or the acceleracionist versions of the Phillips Curve, any definite relationship between prices and quantities. As noted here (and here) before, there is no reason to expect an unambiguous or systematic relation between prices and quantities, unless you think that prices are always driven by excess (or lack of) demand.
As Serrano argues the: "trend and the cycle indeed have a common nature as the empirical literature shows but this common nature reflects that both are explained by demand (not supply) factors" and "with full hysteresis in output levels and partial inertia on inflation, 'demand-pull' inflation is just a temporary phenomenon and therefore does not determine 'core' or persistent inflation." And it is hard not to agree on this with New Keynesians, and their dismissal of supply shocks as the main cause of business cycles. It is harder to agree with their insistence on a natural rate, even if it's variable.
* The obvious historical event they would have in mind is the stagflation of the 1970s. Note, however, that more often than not deflationary periods are contractionary, like the 1930s. Of course the oil shocks and the increase in costs can explain, together with wage resistance and price inertia, the inflationary pressures of the 1970s in a model that is perfectly compatible with demand driven recessions.
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