Os cursos introdutórios de macroeconomia começam tradicionalmente com uma apresentação sucinta dos modelos IS-LM e Mundell Fleming. Ambos assumem algo que os bancos centrais deixaram de fazer há décadas – controlar a oferta monetária (hoje em dia, quase todos usam um inflation target). Esta questão não é irrelevante, e devia ser seriamente reconsiderada pelos autores de manuais, como explica Simon Wren-Lewis em Why introductory macroeconomics should ditch the LM curve e Why we should stop teaching Mundell Fleming. Ver também as reacções ao post.
Lets take a topical issue: the impact of a temporary increase in government spending. We should be immediately worried that TMF makes no distinction between temporary and permanent increases. It says both have no impact on output. So every student learns that fiscal policy is ineffective under flexible exchange rates. For a temporary increase in spending this is incorrect.The logic of the TMF proposition is usually demonstrated by shifting various curves, but it is in fact trivial. In TMF money demand must equal a fixed money supply. If money demand depends on prices, output and interest rates, and the first is fixed in the short run and the last is tied to world rates, then output cannot change either. This complete crowding is achieved through an appreciation in the real exchange rate.But why should domestic interest rates equal world interest rates? UIP tells us they need not. A temporary increase in government spending will raise output, which given a fixed money supply will raise interest rates.This will lead to an appreciation, but the temporary nature of the shock means that the long run exchange rate is unchanged. So the current appreciation implies an expected depreciation, which offsets the additional return offered by higher interest rates. The result is a short run equilibrium where output and domestic interest rates are higher. There is partial crowding out through an appreciation but not full crowding out.Now you might say what is so great about UIP. But at least UIP is based on something: a simple arbitrage theory. As far as I can see the TMF assumption that domestic and world interest rates are equal has no equivalent foundation.We only get some crowding out in the experiment above because the money supply is fixed. If interest rates are fixed instead then we get none. With fixed interest rates, UIP implies the current exchange rate is unchanged when government spending increases, so there is no crowding out. We get exactly the same result as with fixed exchange rates – the complete opposite of what TMF suggests.Now you might plead in mitigation for TMF that at least it gets the impact of a permanent increase in government spending right. I think this is a very weak defence. A permanent increase in government spending, assuming it increases aggregate demand, is crowded out because in a small open economy the real exchange rate equates the demand and supply of domestic output in the long run, which is a more basic result than anything in TMF. Another weak defence of teaching incorrect theories is that they are simpler than better theories. However it we combine this basic idea about the determination of the medium/long run real exchange rate with UIP, we have a complete theory of the small open economy which is no more complicated than TMF. So why does TMF survive?