ニューケインジアンの重鎮マイケル・ウッドフォードが、コロンビア大学の同僚のMariana Garcia-Schmidtと共に新フィッシャー派(フィッシャー式逆さ眼鏡派)の議論を初めて取り上げ、話題を呼んでいる。直近ではDavid Glasnerがその件について書いているが、それによると、これまでに各人が以下のブログ記事で反応している。


Using Nick’s discussion as a starting point, I am going to offer some comments of my own on Neo-Fisherism and the WGS critique. Right off the bat, WGS concede that it is possible that by increasing the setting of its interest-rate instrument, a central bank could, under move the economy from one rational-expectations equilibrium to another, the only difference between the two being that inflation in the second would differ from inflation in the first by an amount exactly equal to the difference in the corresponding settings of the interest-rate instrument. John Cochrane apparently feels pretty good about having extracted this concession from WGS, remarking

My first reaction is relief — if Woodford says it is a prediction of the standard perfect foresight / rational expectations version, that means I didn’t screw up somewhere. And if one has to resort to learning and non-rational expectations to get rid of a result, the battle is half won.

And my first reaction to Cochrane’s first reaction is: why only half? What else is there to worry about besides a comparison of rational-expectations equilibria? Well, let Cochrane read Nick Rowe’s blogpost. If he did, he might realize that if you do no more than compare alternative steady-state equilibria, ignoring the path leading from one equilibrium to the other, you miss just about everything that makes macroeconomics worth studying (by the way I do realize the question-begging nature of that remark). Of course that won’t necessarily bother Cochrane, because, like other practitioners of modern macroeconomics, he has convinced himself that it is precisely by excluding everything but rational-expectations equilibria from consideration that modern macroeconomics has made what its practitioners like to think of as progress, and what its critics regard as the opposite .
But Nick Rowe actually takes the trouble to show what might happen if you try to specify the path by which you could get from rational-expectations equilibrium A with the interest-rate instrument of the central bank set at i to rational-expectations equilibrium B with the interest-rate instrument of the central bank set at i ­+ ε. If you try to specify a process of trial-and-error (tatonnement) that leads from A to B, you will almost certainly fail, your only chance being to get it right on your first try. And, as Nick further points out, the very notion of a tatonnement process leading from one equilibrium to another is a huge stretch, because, in the real world there are “no backs” as there are in tatonnement. If you enter into an exchange, you can’t nullify it, as is the case under tatonnement, just because the price you agreed on turns out not to have been an equilibrium price. For there to be a tatonnement path from the first equilibrium that converges on the second requires that monetary authority set its interest-rate instrument in the conventional, not the Neo-Fisherian, manner, using variations in the real interest rate as a lever by which to nudge the economy onto a path leading to a new equilibrium rather than away from it.
The very notion that you don’t have to worry about the path by which you get from one equilibrium to another is so bizarre that it would be merely laughable if it were not so dangerous. Kenneth Boulding used to tell a story about a physicist, a chemist and an economist stranded on a desert island with nothing to eat except a can of food, but nothing to open the can with. The physicist and the chemist tried to figure out a way to open the can, but the economist just said: “assume a can opener.” But I wonder if even Boulding could have imagined the disconnect from reality embodied in the Neo-Fisherian argument.


そしてコクランの最初の反応への私の最初の反応は、なぜ半ばだけ?、というものだ。合理的期待均衡の比較以外に懸念すべきことが何かあるのか? コクランはまずNick Roweのブログ記事を読むべきだ。もし読んだならば、ある均衡から別の均衡へと至る経路を無視して定常状態均衡の比較だけを行った場合には、マクロ経済学を研究に値するものとする殆ど全てのことを捨象することになる、と気付くはずだ(このコメントが疑問を招くようなものであることは分かっている)。もちろんこのことがコクランを悩ませるとは限らない。というのは、他の現代マクロ経済学者と同様、合理的期待均衡以外のすべてのことを考慮の対象から外すことによって、現代マクロ経済学は、その従事者が進歩と見做したがるもの、そして批判者が正反対に受け止めているもの、を達成した、と彼は自分を納得させているからである。
しかし実際のところ、Nick Roweは、中銀の政策金利がiに設定されている合理的期待均衡Aから、中銀の政策金利がi+εに設定されている合理的期待均衡Bに移行する経路を特定しようとすると何が起きるかをわざわざ示している。もしAからBに至る試行錯誤の過程(模索過程)を特定しようとすると、ほぼ確実に失敗する。成功の唯一の機会は、一発目の試行で上手く行く場合のみである。そして、ニックがさらに指摘しているように、模索過程がある均衡から別の均衡に至るという考え方自体も非常に大胆なものである。というのは、現実世界では模索過程のような「やり直し」は効かないからである。もし取引を行った場合、合意した価格が均衡価格ではなかったからといって模索過程のようにそれを破棄するわけには行かないのである。最初の均衡から第二の均衡に収束するような模索過程の経路が存在するためには、金融当局が新フィッシャー的ではなく従来型の方法で政策金利を設定する必要がある。即ち、実質金利の変化を梃子として利用し、経済が新たな均衡から離れる経路ではなくそれに至る経路に乗るように促す、ということである。