昨年から主流派マクロ経済学に痛撃を加えている*1ポール・ローマーが、「The Trouble With Macroeconomics」という論文をネットに掲載して話題を呼んでいる。

Lee Smolin begins The Trouble with Physics (Smolin 2007) by noting that his career spanned the only quarter-century in the history of physics when the field made no progress on its core problems. The trouble with macroeconomics is worse. I have observed more than three decades of intellectual regress. In the 1960s and early 1970s, many macroeconomists were cavalier about the identification problem. They did not recognize how difficult it is to make reliable inferences about causality from observations on variables that are part of a simultaneous system. By the late 1970s, macroeconomists understood how serious this issue is, but as Canova and Sala (2009) signal with the title of a recent paper, we are now "Back to Square One." Macro models now use incredible identifying assumptions to reach bewildering conclusions. To appreciate how strange these conclusions can be, consider this observation, from a paper published in 2010, by a leading macroeconomist:

... although in the interest of disclosure, I must admit that I am myself less than totally convinced of the importance of money outside the case of large inflations.

リー・スモーリンは、「The Trouble with Physics(スモーリン、2007)*2」の冒頭で、物理学がその中心的な問題についてまったく進歩しなかった史上唯一の四半世紀に自分はキャリアを送った、と記している。マクロ経済学の問題はそれより悪い。私は30年以上にわたる知的退歩を目撃した。1960年代と1970年代初め、多くのマクロ経済学者は識別問題に無頓着だった。同時決定体系の一部である変数の観察から因果関係に関する信頼できる推定をするのがどれだけ難しいかを彼らは認識していなかった。1970年代末までに、マクロ経済学者たちはこの問題がどれほど深刻かを理解した。しかし、Canova=Sala(2009)*3が最近の論文のタイトルでいみじくも示唆しているように、我々は今や「振り出しに戻ってしまった」。今のマクロ経済モデルは識別に関する信じ難い仮定を用い、途方に暮れるような結論を導き出している。そうした結論がどれだけ奇妙なものとなっているかについて確認したければ、指導的なマクロ経済学者が2010年に出版した論文*4の以下の記述を見てみるべきである:



  1. 途轍もない自信
  2. 異常なほど一枚岩の共同体
  3. 宗教的信仰や政治要綱への自己同一化と似たような、集団への自己同一化の感覚
  4. 集団と他の専門家との区別に対する強い感覚
  5. 集団の一員でない専門家の考えや意見や研究に対する無視、無関心
  6. 証拠を楽観的に解釈する傾向、結果についての誇張されたもしくは不完全な記述を信じる傾向、理論が間違っているかもしれないという可能性を無視する傾向
  7. 研究計画がどの程度リスクを必然的に伴うものか、という点に関する認識の欠如


The conjecture suggested by the parallel is that developments in both string theory and post-real macroeconomics illustrate a general failure mode of a scientific field that relies on mathematical theory. The conditions for failure are present when a few talented researchers come to be respected for genuine contributions on the cutting edge of mathematical modeling. Admiration evolves into deference to these leaders. Deference leads to effort along the specific lines that the leaders recommend. Because guidance from authority can align the efforts of many researchers, conformity to the facts is no longer needed as a coordinating device. As a result, if facts disconfirm the officially sanctioned theoretical vision, they are subordinated. Eventually, evidence stops being relevant. Progress in the field is judged by the purity of its mathematical theories, as determined by the authorities.

「Back to Square One(振り出しに逆戻り)」と題した論文の8節でローマーは以下のように書いている。

I agree with the harsh judgment by Lucas and Sargent (1979) that the large Keynesian macro models of the day relied on identifying assumptions that were not credible. The situation now is worse. Macro models make assumptions that are no more credible and far more opaque.
I also agree with the harsh judgment that Lucas and Sargent made about the predictions of those Keynesian models, the prediction that an increase in the inflation rate would cause a reduction in the unemployment rate. Lucas (2003) makes an assertion of fact that failed more dramatically:

My thesis in this lecture is that macroeconomics in this original sense has succeeded: Its central problem of depression prevention has been solved, for all practical purposes, and has in fact been solved for many decades. (p. 1)

Using the worldwide loss of output as a metric, the financial crisis of 2008-9 shows that Lucas’s prediction is far more serious failure than the prediction that the Keynesian models got wrong.
So what Lucas and Sargent wrote of Keynesian macro models applies with full force to post-real macro models and the program that generated them:

That these predictions were wildly incorrect, and that the doctrine on which they were based is fundamentally flawed, are now simple matters of fact ...
... the task that faces contemporary students of the business cycle is that of sorting through the wreckage ...(Lucas and Sargent, 1979, p. 49)





*1:cf. ここ




*3:cf. ここ

*4:cf. これ(引用部はp.5の下から4-2行目)。


*6:cf. ここ