Post-Keynesians frequently highlight the Kaldor-Verdoorn Law which states that aggregate demand affects the supply side. This was even used by economists who prepared the economic plan for Bernie Sanders’ presidential campaign in the United States, as explained well by John Cassidy for The New Yorker. Although, the law is not generally known, economists roughly understand it as a theory of “hangovers”.
The Kaldor-Verdoorn Law is:
rate of growth of productivity = constant1 + constant2 × rate of growth of production
where constant1 is the exogenous rate of growth of productivity.
These constants will be different for every nation and are to be found empirically.
Of course, productivity is not the only measure of the supply side, but that gives an idea about the general notion of super-hysteresis.
So Post-Keynesians would argue that slowdown, crises and recessions will affect the supply side but fiscal and monetary policy can be used to quickly recover and that delays will affect the supply side.
It’s interesting to note how others see it.
Carmen Reinhart and Kenneth Rogoff do an empirical analysis and claim that the hangover effect is permanent.
John Cogan, Glenn Hubbard, John Taylor and Kevin Warsh claim this all wrong and that policy can be used to recover.
The two ideas contain a mix of ideology and scientific validity. As Joan Robinson says, “I believe that economic analysis, though it cannot help containing an element of propaganda, yet can be scientific as well.”
While Reinhart and Rogoff’s analysis about hangovers is half right, what they wish to say is that nothing can be achieved via policy to change it. Taylor and Co.’s analysis is correct that policy can be used to resolve the slowdown. They however reject the hangover effect or that there is a damage to the supply side because of a slowdown of aggregate demand. They are rejecting super-hysteresis. Also, by policy, what they mean is deregulation.