Once the acute phases of the financial and euro crises were over, it was clear that it would take time for advanced economies to recover. The history of past financial crises gave a clear warning that recovery would typically be long and painful.

在金融危机和欧元危机过了严重的阶段后，发达经济体显然需要时间来实现复苏。以往金融危机的历史提供了明确的警示：复苏进程通常漫长而痛苦。

Today, the scars are largely healed but growth is still slow. Before the crisis, any economist would have predicted that an economy with interest rates close to zero and no other major brakes on demand would see high growth rates and quickly overheat. Yet this is not what we have seen. The reason, I believe, must be found in mediocre medium term prospects, which in turn affect current demand and growth.

如今，危机造成的创伤已基本痊愈，但经济增长依然缓慢。在危机爆发前，任何一位经济学家都会这样预测：在利率接近零又不存在其他重大需求抑制因素的背景下，经济无疑将快速增长，并且很快就会过热。然而我们看到的却不是这样。我认为原因一定出在不太好的中期前景，后者反过来影响了当前的需求和增长。

Estimates of long term potential growth in advanced countries have come down by 0.5 to 1 per cent since 2007. Some of the decline is due to ageing, some to lower productivity growth. The effect of an ageing population was largely predictable, and indeed largely predicted. (Whether it was taken into account by firms thinking about their investment plans is unclear.)

自2007年以来，对发达国家长期潜在经济增长率的预估下调了0.5-1%。一部分原因是人口老龄化，还有一部分原因是生产率增速下降。人口老龄化的影响大体上是可预见的，而且基本上也被预测到了。（不清楚企业在制定投资计划时是否会考虑人口老龄化因素。)

Productivity growth has been much lower since 2007, more so in Europe where the rate has declined by over 1 per cent in major countries, than in the US, where it has only declined by 0.5 per cent. This decline reflects in part cyclical factors and the effect of lower capital accumulation. But there is more to it: for the US at least, the evidence points to a slowdown in underlying productivity, starting before the crisis and reflecting the end of a period of successful implementations of IT innovations. The safe assumption is that the high pre-crisis productivity growth rate was unusual, and we should expect lower underlying productivity growth in future.

生产率增速自2007年以来下降了很多，而且在欧洲下降得比在美国厉害——欧洲主要国家的生产率增速下滑超过了1%，而美国只下降了0.5%。生产率增速下降在一定程度上反映了周期性因素以及资本积累减少的影响。但还有更多原因：至少就美国而言，证据表明潜在生产率出现了放缓，从金融危机爆发前就开始了，反映了一段IT创新得到成功应用的时期的结束。可以断定，金融危机前处于高水平的生产率增速并不寻常，我们应该预计未来潜在生产率增速会降低。

Low potential growth is bad news for the medium term. But it may explain what is happening today. A main finding of a paper that Eugenio Cerutti, Lawrence Summers and I wrote in 2015 was that, over the past 40 years, recessions in advanced countries have been associated surprisingly often with lower growth following the recession.

潜在经济增长率较低对中期来说是坏消息。但它或许可以解释当前的情形。我与欧金尼奥•切鲁蒂(Eugenio Cerutti)、劳伦斯•萨默斯(Lawrence Summers)2015年合写的一篇论文的一个主要结论是，在过去40年中，发达国家的经济衰退出奇频繁地与衰退结束后增速较低有关。

One interpretation is the presence of hysteresis, namely that the recession affects potential growth. Another is that the causality runs in reverse: bad news about future potential growth leads to a recession. So, for example, when companies realise that sales prospects are worse than they thought, they cut down on investment. And consumers, realising that their income prospects have worsened, cut on consumption.

一种解释是存在滞后现象，即经济衰退影响潜在增速。另一种解释是二者的因果关系正好倒过来：关于未来潜在增速的坏消息导致了经济衰退。比如说，当企业意识到销售前景比他们认为的更糟糕时，他们会减少投资。而意识到自己收入前景恶化的消费者会减少消费。

Monetary policy can limit the drop, but not eliminate it. In a 2013 paper I co-wrote, we showed formally that this interpretation fits postwar US data well. I believe that it also explains what we observe today. The result in this case is not a recession, but a weak recovery.

货币政策可以限制这种下滑，但无法消除。在我2013年与人合写的一篇论文中，我们严谨地提出，这种解释与美国战后的数据相当契合。我相信它也可以解释我们今日所看到的情况。只不过在这里结果并非衰退，而是疲弱复苏。

This weak recovery in advanced countries also goes a long way in explaining the slowdown in emerging markets. While domestic factors play a role, and the evolution of China is largely sui generis, lower exports, less demand for commodities, and a resulting drop in commodity prices, have led to lower output.

发达国家的疲弱复苏对于解释新兴市场增长放缓也大有帮助。虽然有国内因素的作用，而且中国的情况很大程度上具有独特性，但出口下滑、大宗商品需求减少以及由此带来的大宗商品价格下跌，已经导致产出下滑。

If this new narrative is right, the baseline forecast is for a slow but continued recovery, as the adjustment to the reality of lower potential growth plays out in advanced economies, and the adjustment to the commodity bust works itself out in emerging market and developing economies. It suggests that some of the scary talk, and some of the exotic policy measures being discussed, from helicopter money to large negative nominal rates, are not the order of the day. They should be kept in reserve in case bad things happen, but the expectation should be that they will not be needed.

如果这种新的分析正确的话，基准预测就是经济将会缓慢而持续地复苏，发达经济体将逐渐适应潜在增长率下降的现实，新兴市场和发展中国家将逐渐适应大宗商品泡沫破裂。这表明，一些引发恐慌的言论及部分正在讨论的非常规政策措施——从“直升机撒钱”到较大负值的名义利率——并不是当下最重要的事。这些政策应该留待形势严峻的时候使用，但我们最好期待不需要使用到这些措施。

The focus should be increasingly on medium term growth, and the redefinition of normal fiscal and monetary policies in an environment of lower growth and lower interest rates.

我们应该更多地着眼于中期增长，以及在低增长、低利率的环境下如何重新定义正常的财政和货币政策。