珍妮特•耶伦(Janet Yellen)已经完成了她作为美联储(Fed)主席的任期，此时的失业率比她就任时低得多、通胀保持较低水平且更接近目标、金融体系的资本状况更好且流动性更充足。人们还能对美联储主席有更多的要求吗?
Ms Yellen’s success is a tribute to her judgment and thoughtfulness. Importantly, though, like Alan Greenspan in the 1990s, she recognised quickly a major structural change in the economy and adjusted policy away from where traditional models would guide it. In Mr Greenspan’s case the structural change was the acceleration of productivity growth. In Ms Yellen’s, it was the decline in the neutral rate of interest — the rate of interest at which saving and investment would balance without either a major acceleration or deceleration in growth.
耶伦的成功充分体现了她的判断力和缜密思维。但很重要的是，正如上世纪90年代的艾伦•格林斯潘(Alan Greenspan)一样，她很快意识到了经济的重大结构性变化并相应调整了政策，而不再遵循传统模型。格林斯潘察觉到的结构性变化是生产率增长加速，而耶伦发现的是“中性利率”(neutral rate of interest)下降。所谓中性利率，是指储蓄和投资达到平衡而且经济增长既没有大幅加速也没有大幅减速的利率水平。
The fashionable view at the Fed and elsewhere when Ms Yellen took office in 2014 was that growth was slow despite very low interest rates because of “headwinds” — transitory factors associated with the financial crisis that would soon recede. When the headwinds receded, it would be possible for the economy to enjoy sustained growth with the “normal” 4 per cent federal funds rate. On this view, the near zero rate policy in place was highly expansionary and risked dangerous inflation.
2014年，当耶伦就任美联储主席时，不管是在美联储内部还是其他地方，普遍观点是，尽管利率非常低，但经济增长缓慢，这是因为存在一些“逆风”——与金融危机有关的将很快消退的暂时性因素。当逆风退去，经济有可能在4%的“正常”联邦基金利率的情况下实现持续增长。按照这种观点，利率接近于零的政策是高度扩张性的，可能带来危险的通货膨胀。
By 2014, after five years of financial repair, the headwinds theory was losing credibility. Estimates of the neutral rate were starting to come in suggesting that it had been trending down for a long time. More straightforwardly, despite near zero rates and the completion of financial repair as measured by credit spreads in 2009, growth remained very slow. That is why I sought to resurrect the secular stagnation theory — the idea that the economy, except at moments of financial excess, was likely to suffer from an excess of saving over investment and be prone to sluggishness and insufficient inflation.
2014年，在金融体系经历了五年的修复期后，上述逆风理论失去了可信性。对中性利率的估计开始表明它很长时间以来处于下行趋势。更简单地说，尽管利率维持在接近于零的水平且根据2009年的信用息差衡量已经完成了金融修复过程，但经济增长仍然非常缓慢。于是我重新提出了“长期经济停滞理论”：除了金融领域存在过度行为的时候以外，经济很可能受到储蓄大于投资的打击，容易出现增长停滞和通胀低迷的情况。
Without endorsing the idea of secular stagnation, Ms Yellen led the Fed gradually but firmly to the recognition that the neutral rate had declined significantly, and to the corollary conclusion that policy was not as expansionary as was generally supposed. Her instincts were corroborated, and even proved to have been if anything too cautious as growth and inflation generally fell short of the Fed’s expectations during her tenure — even as interest rates were kept lower than expected and federal deficits increased more than expected.
耶伦不认同长期停滞理论，在她领导下，美联储逐步地得出一个坚定的看法，认为中性利率已经大幅下降，并推断政策的扩张性不像人们普遍认为的那样强烈。她的直觉得到了证实，甚至可能显得有点过于谨慎，因为在她的任期内，经济增长和通胀水平大体上没有达到美联储的预期——即使是在利率低于预期而联邦赤字增长超出预期的情况下。
It is fortunate for the American economy that Ms Yellen recognised changes in its structure and deviated from models and policy rules derived from historical experience. Had she followed such models we quite likely would be in recession right now. Yet it must be acknowledged that growth in recent years associated with low interest rates would not have been as great as it was without the stock market increasing at a manifestly abnormal rate and without increases in borrowing that far outstripped growth in incomes.
对于美国经济来说幸运的是，耶伦看出了经济结构变化，没有遵循基于历史经验的模型和政策规则。如果她遵循此类模型，我们现在很可能已经处于衰退之中。但必须承认的是，如果股市没有以明显反常的速度上涨，如果借贷增长没有远超收入增长，近年来低利率环境下的经济增长也不会如此强劲。
Thus the first challenge facing the estimable Jay Powell as Fed chairman is working out how to achieve growth that is both adequate and financially sustainable. Even with very low interest rates, the normal level of private saving consistently and substantially exceeds the normal level of private investment in the US. And the differential is magnified by inflows of foreign capital. This creates a deflationary tendency that can be offset only by budget deficits or financial conditions that artificially depress saving and increase investment.
因此，令人尊敬的杰伊•鲍威尔(Jay Powell)出任美联储主席面临的第一个挑战就是，搞清楚要如何实现足够强劲且在财政上可持续的经济增长。即使利率维持在非常低的水平，美国私人储蓄的正常水平仍然持续、大幅地超过私人投资的正常水平。而外国资本流入会放大这种差距。这会造成通货紧缩的趋势——只能通过预算赤字或者人为抑制储蓄、促进投资的金融状况来抵消。
Asset values and levels of borrowing cannot indefinitely grow faster than gross domestic product, even though their ability to do so for a time has contributed to economic success over the past few years. If the Fed raises rates sufficiently to assure financial stability, there is the risk that the economy will slow too much. If it focuses on maintaining the growth necessary to meet its inflation target, there is the risk of further increases in leverage and asset prices setting the stage for trouble down the road .
资产价值和借贷水平的上升不可能无限期地超过国内生产总值(GDP)增速，尽管它们在一段时间内保持这种状况的能力促成了过去几年的经济成功。如果美联储以足够大的力度加息以确保金融稳定，经济增长有可能放缓太多。如果美联储着重于维持实现通胀目标所需的经济增长，就有可能造成杠杆率和资产价格进一步上升，给未来埋下隐患。
There is a very difficult balance to be struck. Except in the aftermath of recessions it has been a long time since the US economy grew well with a stable financial foundation. History will judge how stable the financial conditions of recent years have been. Prior to that we were in recovery from the 2008-09 recession. That in turn was preceded by a period of financial excess in housing and other markets. Prior to that came the 2001 recession and recovery, which in turn was preceded by the internet and stock market bubbles of the late 1990s.
这是一个非常难以实现的平衡。除了历次衰退过后那段时期，美国经济在金融基础稳定的情况下实现良好增长已经是很久以前的事情了。历史将对近年来金融状况的稳定程度做出评判。在那之前，我们处于从2008-09年的衰退中复苏的时期。再往前溯是住房及其他市场存在金融过度行为的时期。再往前是2001年的衰退和复苏，更早则是上世纪90年代末的互联网和股市泡沫。
So it has been a generation since the US economy enjoyed stable, financially sustainable growth from a position of strength. Good luck Mr Chairman.
因此，算起来，美国经济获得那种稳定、在财政上可持续的强有力增长距今已经过去了一代人的时间了。祝新主席好运。
The writer is Charles W Eliot university professor at Harvard and a former US Treasury Secretary
本文作者为哈佛大学(Harvard)查尔斯•W•艾略特大学教授，曾任美国财政部长
