という論文をエガートソンらが書きクルーグマンMark Thomaデロングが取り上げた。論文の原題は「A Model of Secular Stagnation」で、著者はGauti Eggertsson、Neil Mehrotra(共にブラウン大)。

In this paper we propose a simple overlapping generations New Keynesian model in which a permanent (or very persistent) slump is possible without any self-correcting force to full employment. The trigger for the slump is a deleveraging shock which can create an oversupply of savings. Other forces that work in the same direction and can both create or exacerbate the problem are a drop in population growth and an increase in income inequality. High savings, in turn, may require a permanently negative real interest rate. In contrast to earlier work on deleveraging, our model does not feature a strong self-correcting force back to full employment in the long-run, absent policy actions. Successful policy actions include, among others, a permanent increase in inflation and a permanent increase in government spending. We also establish conditions under which an income redistribution can increase demand. Policies such as committing to keep nominal interest rates low or temporary government spending, however, are less powerful than in models with temporary slumps. Our model sheds light on the long persistence of the Japanese crisis, the Great Depression, and the slow recovery out of the Great Recession.


We find that a high enough inflation target can always do away with the slump all together, as it accommodates a negative natural interest rate. Importantly, however, an inflation target which is below what is needed has no effect in this context. This formalizes what Krugman (2014) has referred to as the ”timidity trap”- an inflation target that is too low will simply do nothing in an economy experiencing a secular stagnation. We show this trap explicitly in the context of our model which only arises if the shock is permanent. Similarly, we illustrate that, in a secular stagnation environment, there are strong limitations of forward guidance with nominal interest rates. Forward guidance relies on manipulating expectations after the zero lower bound shock has subsided, but as the shock in our model is permanent, manipulating these types of expectations is of much more limited value. Moving to fiscal policy, we show that either a permanent increase in government spending can eliminate the output gap, or a redistribution of income from savers to borrowers, although this latter result depends on the details of the distribution of income (we provide examples in which income redistribution is ineffective).

ここで参照されたクルーグマン(2014)とは3/21付けNYTコラム「The Timidity Trap」であるが、そこでクルーグマンは、(本ブログではここで言及した)ブルッキングス研究所でのHausman=Wieland研究発表の討論者をバーナンキと共に務めた直近の経験を基に、以下のように書いている。

And I'd argue that an important source of failure was what I've taken to calling the timidity trap — the consistent tendency of policy makers who have the right ideas in principle to go for half-measures in practice, and the way this timidity ends up backfiring, politically and even economically.
In other words, Yeats had it right: the best lack all conviction, while the worst are full of passionate intensity.
Japan has made a radical break with past policies, finally adopting the kind of aggressive monetary stimulus Western economists have been urging for 15 years and more . Yet there's still a diffidence about the whole business, a tendency to set things like inflation targets lower than the situation really demands. And this increases the risk that Japan will fail to achieve “liftoff” — that the boost it gets from the new policies won't be enough to really break free from deflation.


On one side we have a hypothetical but I think realistic Phillips curve, in which the rate of inflation depends on output and the relationship gets steep at high levels of utilization. On the other we have an aggregate demand curve that depends positively on expected inflation, because this reduces real interest rates at the zero lower bound. I've drawn the picture so that if the central bank announces a 2 percent inflation target, the actual rate of inflation will fall short of 2 percent, even if everyone believes the bank's promise – which they won't do for very long.

So you see my problem. Suppose that the economy really needs a 4 percent inflation target, but the central bank says, “That seems kind of radical, so let's be more cautious and only do 2 percent.” This sounds prudent – but may actually guarantee failure.

ちなみにEggertsson=Mehrotra論文を取り上げたブログエントリでクルーグマンは、これは自分の臆病の罠説の正しさを証明したと同時に、It's Baaack論文の欠点も明らかにした、と述べている。即ち論文では、議論の本質には影響しないと考え、数学的に簡便な無限に生きる主体を仮定したのだが、その仮定からは、世代重複モデルでは出てくる長期停滞が出てこない、とのことである。これについてクルーグマンは次のように書いている*3

So this is kind of an object lesson in how strategic simplifications in modeling — which are necessary! — can lead you to overlook important possibilities.


*2:ここでクルーグマンは自分のIt's Baaack論文にリンクしている。