Edward L. GlaeserがEconomixに表題のエントリを書いていたので、以下に抄録で紹介する(原題は「What We Don’t Know, and Perhaps Can’t」)。

What have we learned from this recent recession?

We have been reminded that the global economy is fragile, and we know that the banking sector needs reform. But on the fundamental question of how to reduce recessions and unemployment, I’m not sure we’re any more knowledgeable than we were five years ago – and that means we’re not very knowledgeable.

Like a 19th century doctor, we are better at diagnosing the illness than delivering a reliable cure.

No one can still think that we live in an age of “great moderation,” and it is good to have learned that lesson. But while it’s helpful to learn that the world is delicate, it would be better to know how to improve things. In this case, there is the usual split between those problems that are subject to microeconomic analysis, where we have some hope of consensus, and those that require the tools of macroeconomics, where intellectual wars still rage.

Banking reform can be understood with basic microeconomic tools, and the logic of economics pushes quite clearly in the direction of reform.
There is certainly a healthy debate about the appropriate nature of the remedy. Are higher capital requirements enough? Should banks be charged a risk tax based on their portfolios? Should the biggest banks be broken up so that they are no longer too big to fail? But all these interventions move in the same direction, and most economists agree that this is the right course for the ship of state to sail.

There is no equivalent consensus about fighting unemployment and economic downturn. For decades, the economics profession had been moving away from Keynes, but when the recession hit, no one had much of a viable alternative to Keynesian countercyclical spending. We’ve had a $787 billion recovery act — a great burst of Keynesian activity — and unemployment remains at 9.9 percent.

Does that mean that recovery spending was a waste or just that we didn’t do enough of it? Is public spending just crowding out private employment? Or is each public employee spending more on private goods, thereby creating an employment multiplier? We don’t really know.
Our knowledge of microeconomic policies comes from repetition and randomization. We have a lot of examples of rent control, and so economists can give pretty reliable advice about its impact. Plenty of smaller interventions – housing vouchers, giving students incentives to read – can be allocated with randomized trials, which produces far more reliable results. The John Bates Clark Medal, given every other year to a particularly distinguished economist under 40, was just awarded to Esther Duflo, who has been a pioneer at using this experimental approach in the developing world.

The fundamental problem with acquiring certainty about Keynesian intervention is that anti-recessionary spending is just not very amenable to clean, compelling empirical evaluation. Recessions aren’t that common, and there are too many moving parts. Times change, so it isn’t obvious that the lessons of the 1930s – not that we can agree on those, either – are applicable today.

And so we are left wading in ignorance. It is a great tragedy that the most important area of economic decision-making is also the area where we will always know the least.

改善のあるべき姿については確かに健全な議論が交わされている。自己資本の必要額を増やすだけで良いのか? 銀行は自らのポートフォリオのリスクに応じた税金を課せられるべきか? 最大手の銀行は分割され、大きすぎて潰せないという問題がもう起きないようにすべきか? しかしそうした改善案はすべて同じ方向を指し示しており、ほとんどの経済学者は、それが進むべき正しい方向であることに合意している。
このことが意味するのは、景気対策の支出は無駄だったということなのか、それとも不十分だったということなのか? 公的支出が単に民間の雇用をクラウドアウトしただけなのか? それとも公的資金による雇用者は民間の財への消費支出を増加させ、雇用の乗数効果をもたらしたのか? その回答を我々は本当に知らない。