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Oskar Lange theory of interest and the ISLM

By Roberto Lampa (Guest blogger)

In two previous posts, dated 2011 and 2013, Matías Vernengo clarified that the ISLM model can accommodate changes that incorporate the criticisms of several heterodox groups. In particular, he stresses that the ISLM can accommodate an investment function in which the level of activity (rather than the rate of interest) is central, so that the accelerator can be incorporated. More importantly, he also states the ISLM does not imply a natural rate of unemployment, thus allowing for relevant discussion of policy issues.

Both these aspects can be found in Oskar Lange's 1938 contribution to the neoclassical synthesis, in which he assumes that investment (mostly) depends on consumption, which in turn is permanently distorted by the “irrational” distribution of income, typical of any capitalist economy. More precisely, Lange outlines the mutual dependence of investment and consumption as a sort of ‘indirect’ relationship.

Firstly, he states that, as in traditional theory, in his model an increase in the propensity to save induces a decrease in the rate of interest. However, his reasoning runs along more unconventional lines than the (Neo)-Classical interaction of both supply (of) and demand (for) capital curves:
"…an increase in the propensity to save [implies that] expenditure on consumption is now lower. This causes (…) a lower quantity of investment (…). Total income decreases (…). The consequence is a fall in the rate of interest." (pp. 17-18)
In other words, in Lange's view the immediate effects of an increase in the propensity to save are a decrease in consumption, investment and total income. Therefore, as recognized by Keynes himself:
"The analysis which I gave in my General Theory of Employment is the same as the ‘general theory’ explained by Dr. Lange on p.18 of his article, except that my analysis is not based (as I think his is in this passage) on the assumption that the quantity of money is constant." (Keynes J.M., 1973a, p.232n)
Following this train of thinking, we deduce that it’s only afterwards that the decreased level of the rate of interest will stimulate investment, consumption and total income. The final result of an increase in the propensity to save will then depend on the ‘specific weight’ of each of these two effects.

Not coincidentally, Lange explicitly assumes in equation (3) – by drawing on Karl Marx's realization crisis – that consumption directly affects investment, as an excessive growth in saving (i.e. an excessive contraction of consumption, investment and total income) cannot be counter-balanced by the subsequent decrease in the rate of interest, as it destroys any incentive to invest, "at least in a capitalist economy where investment is done for profit" (Lange, 1938, p.23). He thus firmly rejects the (Neo)-Classical assumption that any abstinence from consumption implies automatically an increase in investment: according to him, such a direct relationship holds only until a certain limit (i.e. the optimum propensity to consume), beyond which the collapse of the demand for investment goods will drastically diminish investment itself. Therefore, the real issue becomes if and how it is possible to determine (and to maintain) such an optimum propensity to consume, given a market economy. Lange's opinion is definitely non-optimistic:
"In a society where the propensity to save is determined by the individuals, there are no forces at work that keep it automatically at its optimum, and it is well possible, as the under-consumption theorists maintain, that there is a tendency to exceed it." ( p.32)
In other words, the result of Lange's analysis converges with (and radicalizes, as well) Keynes' pivotal idea, that is, the tendency towards a chronic under-consumption crisis.

Recently, I have published a detailed analysis of this rather obscure work in the Cambridge Journal of Economics. I explore in depth Lange's theory of interest and its tortuous relationship with both Keynes’ General Theory (1936) and Hicks' synthesis (1937), developing two graphical models that show the non-linearity of Lange's investment function as well as the consequences of his equilibrium solution. Through an unedited manuscript, I also reconstruct Lange's beliefs about the chronic sub-optimality of the capitalist economy and his scientific endorsement of the socialist economy.

Full paper is available here.

P.S. It is worth noting that Keynes himself was prompted to reflect that Lange's article "follows very closely and accurately my line of thought" (Keynes, 1973a) notwithstanding the analytical differences. Lange was, after all, standing on the same "side of the gulf," as he clearly rejected the notion that capitalism could be a "self-adjusting system" (Keynes, 1973b).

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