2009年以来，央行行长们为了支撑经济增长，采取了很多的措施，比如：低利率、量化宽松、前瞻性指引以及当前的负利率政策。
但行长们的日子比以前艰难多了，2008年金融危机之前的经济关系已经难以适用于当前的时代了。举例来讲，失业率和通胀之间的关系是用菲尔普斯曲线来衡量的。失业率下降，企业对劳动力的竞争以及工资水平都会增加，通胀也会开始增加。这两者的关系是基于“前瞻性指引”的。2013年8月，英格兰银行行长 Mark Carney 表示，失业率跌至7%以下是紧缩政策的必要条件（但不是充分条件）。英国失业率下降至5.1%，但通胀率只有0.3%，所以没有必要采取紧缩政策。
2012年12月，美联储在 Ben Bernanke 的领导下引入了前瞻性指引。美联储认为6.5%的失业率可能是个契机，但直到2015年12月失业率跌至5%时，利率的上行趋势才开始出现。
这两家央行到底发生了什么事？最好的解释是非加速性失业增长率 (NAIRU)。NAIRU 并不是一组数字，它在每个周期都不一样。Paul Krugman 指出，随着失业率的下降，美联储的预期 NAIRU 也会随之下降。
这取决于劳动力市场还有多少剩余产能。最佳的计算方法不是失业率，因为失业率统计的是有保险的人，而不是参与劳动的人。在美国，劳动力参与率由2000年的峰值67%下降至62.9%。值得注意的是，随着失业率的下降，劳动力参与率最好是上升的，但如图所示，最近两组数据皆有下降。如果薪水增加了，这些人会重返劳动力市场吗？这个很难说，有些人失业久了，再找新工作就会变难，另外一些人可能是提前退休的。
劳动力参与率没有绝对的标准，在20世纪50年代至60年代，女人们不参加工作，那时的数据是低于60%的。
所以，想要计算劳动力市场的宽松程度，还有其他的办法吗？经济学家试图用实际产出和潜在产出的“产出缺口”来计算。若要计算产出缺口，就要考虑增长率趋势，但这个概念是捉摸不定的。2008年以来，大多数国家的增长率都呈下降趋势。英国预算责任办公室最近修改了增长预期，因为产出能力过低。
十二月以来最大的预期变动是降低了潜在产出增长率。潜在产出增长率是每工作一小时所获得的持续性产量，这是潜在规模的关键驱动因素。该数据自十二月开始，展示了2015年中期以来的产出增速，与我们假设的产出会回到危机前的水平相一致。随着危机后产出增长疲软期的延续，我们越来越注重对未来预期的引导。在过去的十年中，我们每年对 GDP 增长的预期已经下调了0.3%。
为什么产出增长会趋于疲软？1870至1970年间，美国的转型变革大大延长了人们的寿命，也提高了人们的生活质量。人们可以在家中获得干净的饮用水，降低了发病率，也减少了洗衣服的苦差事。1970年以来，信息技术领域的变革是无比显著的，但其影响力却远低于上一个时代。在过去的40年中，公路运输（拥堵）和航空运输（安检）并没有做到真正的提速。冰箱、洗衣机、空调等重要家电早已在1970年之前完全普及。
显而易见，IT 技术的进步可能会提高产出，我们在分销和零售领域都已见识到了。但为什么最近几年不见产出的增长呢？原因之一是收入增长过慢，雇佣员工比投资新技术要便宜得多。另一原因是当代经济的重心已由制造业转向服务业，而服务业的产出是很难提高的。10分钟理出的头发肯定比不上一个小时做出的发型。在网络方面，频繁的邮件、推特订阅、搞笑视频等都会降低办公室一族的工作效率。换句话说，我们很难预测服务业的产出增长，也许实际的 GDP 数据会高于我们的预期。这是未来要担心的事情，数据公布之后，我们就要想办法解释为什么产出降低了。
此外，各国央行也不知道经济增长会有多快，低失业率何时会到来通胀压力。他们有时也会迷茫。
The central bankers’ dilemma
CENTRAL banks have taken the brunt of supporting the global economy, ever since 2009, the last time that governments made any concerted attempt to indulge in a fiscal status. Low interest rates, quantitative easing, forward guidance and now negative rates have all been wheeled out.
But life for a central banker is more difficult than it used to be; the economic relationships that prevailed before the 2008 crisis may not hold good today. Take the relationship between unemployment and inflation, famously linked by the Phillips curve. As unemployment falls, inflation should start to rise as businesses compete for labour and wages go up. The relationship was the basis for the introduction of "forward guidance". In August 2013, Mark Carney, governor of the Bank of England, said that a fall in unemployment below 7% would be a necessary (but not sufficient) condition for tighter policy. UK unemployment has fallen to 5.1% but inflation is only 0.3% and there has been no need to tighten policy.
The Federal Reserve had introduced forward guidance under Ben Bernanke way back in December 2012. That envisioned a US unemployment rate of 6.5% as a possible trigger; in fact, the first upward move in rates did not occur until December 2015, when the jobless rate had fallen all the way down to 5%.
So what has been happening? The economic concept underlying the approach of the two central banks is that of the NAIRU; the non-accelerating inflation rate of unemployment. But it seems clear that the NAIRU is not a set figure; it varies from cycle to cycle. Paul Krugman has shown that the Fed's estimate of the NAIRU has trickled down as unemployment has fallen.
It all depends on how much spare capacity there is in the labour market. The best measure might not be the unemployment rate, which measures those entitled to claim benefits, but the labour force participation rate, which calculates the proportion of those of working age who are in employment. In the US, that has fallen from a peak of 67% reached in 2000 to 62.9%. What is remarkable about the chart is that one would expect the labour force participation rate to rise as unemployment falls; until recently, the two have dropped in tandem. Would some of those people rejoin the labour market if wages were higher? It is hard to know; some may have been jobless for a time and thus may find it difficult to get jobs, others may have retired early. There is no "right" level of labour force participation that can be determined; the rate was below 60% for much of the 1950s and 1960s before more women joined the workforce.
So if you can't know how much slack there is in the labour market, are there other measures? Economists use the idea of the "output gap", the difference between the economy's actual output and its potential production. To figure this out, they need an idea off the trend rate of growth. But that is a slippery concept. It looks as if, in most developed countries, the trend rate of growth has fallen since 2008. In Britain, the Office for Budgetary Responsibility recently revised down its assumed rate of growth because of poor productivity.
The most significant forecast change we have made since November has been to revise down potential productivity growth. This is the amount of output the economy can produce sustainably per hour worked and is a key driver of its potential size. The data available in November showed a pick-up in productivity growth in mid-2015, consistent with our assumption that the receding financial crisis would exert less of a drag and that trend productivity growth would return to its pre-crisis average rate by the end of the forecast. But more recent data suggest that this was another false dawn. With the period of weak productivity growth post-crisis continuing to lengthen, we have placed more weight on that as a guide to future prospects – although this judgement remains highly uncertain. This in turn has prompted us to revise down our GDP growth forecasts by around 0.3 percentage points a year to an average of 2.1 per cent a year over the rest of the decade.
Why would productivity growth be weaker? Mr Gordon displays, in impressive detail, the transformational changes in America between 1870 and 1970 that brought vast improvements to both life expectancy and the quality of life; people have clean drinking water in their houses, for example, reducing the incidence of disease and the drudgery of clothes washing. The changes in the economy since 1970 have been remarkable in the sectors where they have occurred; information technology and communications. But they are nothing like as widespread as in the previous era. Conversely, there has been no improvement in the real speed of road transport (congestion) or air transport (security checks) in the last 40 years. Vital household gadgets such as fridges, washing machines and central heating were all widespread before 1970.
Clearly, improvements in IT have the potential to improve productivity; we have seen it happen in areas such as distribution and retail. So why hasn't it happened in recent years? One answer might be that wage growth has been so low; it is cheaper to employ workers than to invest in new technology. Another answer could be that modern economies are service- rather than manufacturing-dominated and it is simply harder to improve service productivity; a 10-minute haircut is not better (and probably worse) than an hour-long appointment. Perhaps the internet, with the instant distractions of e-mails, Twitter feeds and cat videos, actually reduces productivity for office workers. Alternatively, we may simply be bad at measuring the improvement in service-sector productivity and later revisions may show that GDP is higher (and inflation lower) than we thought. We may discover this is the case in future, although it will need to be a big revision to explain the productivity slowdown.
In the meantime, central banks simply don't know the answer to these questions - how fast can the economy grow and, at what point will lower unemployment cause inflationary pressures? No wonder they sometimes seem adrift.
本文翻译由兄弟财经提供
文章来源：http://www.economist.com/blogs/buttonwood/2016/03/monetary-policy