The Trouble with Harrod: The fundamental instability of the warranted rate in the light of the Sraffian Supermultiplier

The paper argues that Harrodian instability is an instance of what Hicks in his book Capital and Growth (1965) called static instability, related to the direction (and not to the intensity) of the disequilibrium adjustment process. We show why such instability obtains in demand‐led growth models in which the ratio of capacity creating private investment to output ratio is given exogenously by the aggregate marginal propensity to save. We also show that Sraffian Supermultiplier model overcomes the Harrodian instability and that its demand‐led equilibrium is statically stable. It is explained that the latter results do not follow from the presence of autonomous non‐capacity creating expenditure component as such but from its presence within a model in which investment is driven by the capital stock adjustment principle (i.e., the flexible accelerator). Finally, we argue that, although being statically stable, the equilibrium growth path of the Sraffian Supermultiplier model can be dynamically stable or unstable depending on the intensity of the reaction of investment to demand. We then provide a discrete time sufficient condition for the dynamic stability of such equilibrium that implies that the marginal propensity to invest remains lower than the marginal propensity to save during the adjustment process, a modified Keynesian stability condition.