ABSTRACT
For each commodity that has a forward market it is possible to define a ‘commodity’ or ‘natural’ rate of interest as the ratio between the quantities exchanged of the commodity at two different dates, minus one. In a monetary economy, i.e. an economy in which transactions (including loans) are made in terms of money, the natural or commodity rate of interest is then defined as the ratio between the spot and the forward price of the commodity (the latter discounted at the relevant money rate of interest), minus one. Approximately, the natural rate is equal to the money interest rate minus the percentage difference between the forward and the spot price of the commodity.9
According to Sraffa, a divergence between the natural or commodity rate and the money rate of interest, and between the various (one for each different) commodity rates, implies a divergence between the spot price and the forward price of the commodities. In particular, the forward prices will be lower than the spot prices for those commodities whose output is expected to increase (and vice versa the forward prices will be higher than the spot prices for those commodities whose output is expected to decrease). Fundamentally, the divergence is due, according to Sraffa, to a difference between the market price of the commodities and their costs of production. Since in a perfectly competitive equilibrium all rates must be uniform (and spot and forward prices coincide for all commodities), such divergence defines a state of disequilibrium of the economy. On the other hand, as Sraffa states,
