「Rethinking the Fed’s 2 percent inflation target(FRBの2%インフレ目標再考)」と題されたブルッキングス研究所小論*1の中の「The natural rate hypothesis straitjacket(自然失業率仮説という拘束衣)」という節で、サマーズが、自然失業率仮説の登場によってマクロ経済学における考え方がどのように変化したか、について以下のように説明している。

The traditional view of macroeconomists and macroeconomic policymakers was that the most important objective of macroeconomic policy including, in particular, monetary policy was to maximize an economy’s level of output and employment over time. (See Blanchard and Summers (2017) for an elaboration of many of the ideas in this section) The idea was that with better policy, catastrophes like the Depression could be avoided and recessions could be minimized without there being important losses of output or employment in boom times. As reflected in Jim Tobin’s famous quip that “it takes a heap of Harberger triangles to fill an Okun Gap”, maintaining adequate and stable demand was seen as a central requirement of sound economic policy.
All of this dramatically changed with the Friedman and Phelps proclamation of the natural rate hypothesis and with the stagflation of the 1970s. Economists concluded that sustained higher rates of inflation would not in general be associated with sustained higher levels of output and employment—this was the essential content of the natural rate hypothesis. In Friedman’s original formulation the Phillips curve represented not a tradeoff between unemployment and inflation but between unemployment and the acceleration of inflation. Other formulations associated with the New Classical macroeconomics asserted that unemployment could be reduced only when inflation exceeded expectations.


The policy conclusion was similar for all formulations of the natural rate hypothesis. Since monetary policy could not influence the average level of output and employment over time, it should properly be dedicated to achieving price stability however defined and minimizing the volatility of output. It quickly followed from work on dynamic consistency that this could best be done by finding commitment devices that reduced inflationary expectations along with inflation. Central bank independence came to be seen as such a device, as did the inflation targeting frameworks now in widespread use. Crucially central banks and even scholars who called themselves new Keynesians abandoned the goal of using monetary policy to raise the level of output over time. The macroeconometric models large and small on which central banks relied almost without exception assumed the independence of long run average output levels from monetary policies. Given this assumption the case for a low inflation target is indeed secure, though the issue of just what that target should be remains.


However three strands of recent macroeconomic research call into question the premise that monetary policy cannot over long intervals affect output and employment. First, an increasing body of evidence suggests the importance of hysteresis effects whereby recessions reduce subsequent potential output (Blanchard 2018, Yagan 2017, Blanchard Cerutti and Summers 2015, Ball 2014). If such effects are present more aggressive monetary policies that prevent or rapidly mitigate recessions will raise levels of output over time. Hysteresis effects may arise from many different sources including reduced levels of investment in physical capital and R&D, lost human capital as those who fall out of work become habituated to being out of work, reductions in the social stigma associated with nonwork, or changes in wage setting practices as firms’ attached workforces shrink.
Second, recent work by Nakamura and Steinsson building on Milton Friedman’s “plucking” model of business fluctuations suggests that it may be better to think of business fluctuations not as symmetric movements around an average level of output whose amplitude is desirable to minimize, but more like periods of illness when output and employment fall short of desired levels (Dupraz, Nakamura and Steinsson 2017). The evidence for this proposition takes the form of demonstrating that the correlation between the size of downturns and subsequent upturns is much greater than the correlation between upturns and subsequent downturns. If one thought of as recessions as like periodic fevers this is exactly what one would expect. With plucking effects, as with hysteresis effects, the case for minimizing recessions is magnified because there is no reason to expect that output lost in recessions is subsequently made up.
Third, ideas related to secular stagnation suggest that economies may be vulnerable to prolonged output shortfalls if monetary policy is unable because of constraints on the lowering of nominal interest rates to achieve real interest rates necessary for full employment levels of demand. Closely related is the argument of Akerlof, Dickens and Perry (1996) that because of a zero lower bound on nominal wage changes the Phillips curve may not be vertical at low rates of inflation. These arguments make a case that a higher rate of inflation, by relaxing constraints that might otherwise bind, allows more output.


All of this matters for consideration of optimal monetary policy. Almost all discussions of monetary policy assume that it can control the level of inflation over time, but that it can affect only the volatility and not the level of output over time. If this is not the case, then monetary policy choices are more consequential than is commonly supposed and issues relating to the average level of output should likely be central in the determination of monetary policy.


If I had to choose one framework today, I would choose a nominal GDP target of 5 to 6 percent. And I would make that choice for two reasons. First, it would attenuate the issues around explicitly announcing a higher inflation target, which I think are a little bit problematic on political economy grounds. Second, a nominal GDP target has an additional advantage in its implicit response to changing conditions. Arithmetically a nominal GDP target has the property that the expected rate of inflation rises as the expected real growth in GDP declines. This is desirable. If growth in underlying real GDP declines, neutral real interest rates are likely to decline as well. In this case allowing higher inflation to make possible even more negative real rates reduces the risk of policy impotence.




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*9:cf. ここ



*12:ちなみにこれは、「25年ごとに1/3の確率で経済が駄目になる」というデロングの試算よりは、「15年に一回はゼロ金利に到達する事態が起きてしまう」というSchmitt-Grohe and Uribe(2009)をベースにした小生の試算に近い。[9/9追記]ただし小生の試算は「15年に一回はゼロ金利に到達する事態が起きてしまう」という「可能性の存在」を示したものの、その「可能性」の確率自体は計算していないため、この比較は厳密には正しくない。

*13:cf. 当時のサマーズの批判、および最近のイエレンインタビュー

量的緩和=理論的には機能しない、と2014年にバーナンキ述べた(cf. ここ)が、ハミルトンらの研究によれば実際面の効果も怪しく、民間が吸収すべき国債が却って増えているほか、国債価格も上がっていない。

*15:これはサマーズのかねてからの持論で、本ブログでも何度か紹介してきた(cf. ここ)。