THAT central banks cannot endlessly reduce unemployment without sparking inflation is economic gospel. It follows from “a substantial body of theory, informed by considerable historical evidence”, according to Janet Yellen, chair of the Federal Reserve. Her conviction explains why, on June 14th, the Fed raised interest rates by a quarter of a percentage point, to a range of 1-1.25%.

央行无法在不引发通货膨胀的情况下持续降低失业率，这是一条经济学真理。用美联储主席珍妮特·耶伦（Janet Yellen）的话来说，这条真理是由“大量历史经验堆积起来的理论产物”。6月14日，美联储宣布将利息提高0.25个百分点，波动幅度为1%到1.25%。珍妮特的观点或许可以解释其中原因。

Excluding food and energy, prices are only 1.5% higher than a year ago; the Fed’s inflation target is 2%. But Ms Yellen thinks unemployment is below its so-called “natural” rate, so inflation should soon rise. Is she right? Or has the relationship between unemployment and inflation, dubbed the Phillips curve, gone missing?

除去食品和能源价格，美国的平均物价仅比2016年上升了1.5%，而美联储的通胀目标为2%。耶伦认为，美国目前的失业率尚未达到所谓的自然失业率（即充分就业下的失业率，译者按），因此，通货膨胀率理应很快上升。那么，耶伦的观点是否正确？或者换言之，反映失业率和通货膨胀间关系的菲利普斯曲线是否失效了呢？

It is not the first time the theory has failed. After the financial crisis unemployment soared to 10%. This surfeit of workers should have sent inflation tumbling. But prices held up well; in October 2009, when unemployment peaked, underlying inflation was 1.3%, only a little lower than it is today. Some economists explained this by saying that the natural rate of unemployment had gone up in tandem—in other words, that some of the rise in joblessness was permanent. In August 2013 Robert Gordon, an economist at Northwestern University, put the natural rate of unemployment at fully 6.5%.

这不是该理论第一次失效。2008年金融危机爆发后，美国失业率飙升至10%，工人的过度下岗按理说应抑制通货膨胀，然而，物价水平却保持在高位。到了2009年10月，失业率达到顶峰时，潜在通胀率为1.3%，仅比今天略低。对此，有经济学家解释称，这是因为自然失业率也随外部环境而提高，换言之，某些失业情况将长期存在。2013年8月，西北大学经济学家罗伯特·戈登（Robert Gordon）将美国的自然失业率定为6.5%。

That explanation has not aged well. Unemployment is now 4.3%, yet inflation remains low. In response, the Fed’s estimates of the natural rate have fallen (see chart). This week they dropped again. The constant catch-up undermines the Fed’s justification for rate rises. (It plans another this year, and also to start shrinking its balance-sheet, which ballooned during and after the recession as it bought assets with newly created money.)

这一解释乏善可陈。美国当前失业率为4.3%5，而通胀率仍保持低位。针对此现象，美联储调低了自然失业率的估值。本周，估值再次下降。但是，这种持续的你追我赶，削弱了美联储对利率上调的辩解力度。（由于增印美钞购买资产，美联储的资产负债表在经济衰退期及随后大肆膨胀，拟计划于2017年开始收缩其资产负债表。）

Yet economists are not about to abandon the Phillips curve, for three reasons. First, the effects of unemployment on inflation can get lost amid temporary economic gyrations. That is most obvious when oil prices fall, as they did in late 2014. More recently, the price of mobile data has dropped. One firm, Verizon, began offering limitless data. At the same time, statisticians have increased the weight they give to such changes. As a result, better mobile-phone deals have reduced consumer-price inflation by over 0.2 percentage points over the past year. Economists at Goldman Sachs, a bank, think unemployment would have to change by fully 1-2 percentage points to have a comparable impact.

尽管如此，经济学家仍不打算放弃菲利普斯曲线，主要原因有三。第一，失业率对通胀产生的影响可能被短期的经济波动所弱化，比如2014年下半年石油价格下跌就是最明显的一个例子。最近，移动流量数据价格有所下降。美国无线运营商Verizon公司甚至开始提供无限量数据套餐。与此同时，统计学家也加大了对这些变化的统计力度。因此，更便捷的移动手机服务使得2016年消费价格通胀率下降了0.2个百分点。高盛银行（Goldman Sachs）的经济学家认为，失业率至少要波动1到2个百分点才会对通胀产生明显影响。

Second, it is possible that inflation will take off sharply when unemployment gets too low, rather than gradually as the economy approaches the threshold. This happened during the period that best mirrors today’s circumstances: the late 1960s. With unemployment under 4%, inflation rose from 1.4% in November 1965 to 3.2% a year later, and almost 5% by the end of the decade. President Lyndon Johnson was partly to blame. He pressed the Fed not to offset tax cuts fully with tighter money. With President Donald Trump promising tax cuts, and able to replace Ms Yellen early next year, history may yet repeat itself.

第二，即使失业率很低时，通胀率也有可能急剧上升，而不是随着经济逐渐接近门槛而渐进。这一现象发生在与当下状况最为相似的上世纪60年代末。当时失业率低于4%，通货膨胀率从1965年11月的1.4%上升到一年后的3.2%，在60年代接近尾声时几乎达到了5%。林登·约翰逊（Lyndon Johnson）总统应当对此负有一定的责任，因为他不允许美联储完全依靠缩紧银根来抵消减税。如今，唐纳德•特朗普(Donald Trump)承诺减税，并或将在2018年初换掉耶伦，历史可能会重演。

The last reason not to throw out the textbook is its emphasis on inflation expectations, as well as unemployment. Inflation expectations have sagged while the labour market has recovered. According to the New York Fed, they took another dive in May. Self-fulfilling expectations could explain low inflation and exonerate the Phillips curve. Instead, they call into question the credibility of the Fed’s promise to hit its inflation target.

最后一个原因是，菲利普斯曲线重视对通胀和失业率的预期。随着劳动力市场的恢复，通胀预期会逐渐下降。5月，纽约联邦储备银行（New York Fed）再次大幅下调利息。预期的自我实现或可解释低通胀率，为菲利普斯曲线正名。相反，这些预期对美联储是否能履行其通胀目标的承诺提出质疑。

编译：张玺元

编辑：翻吧君

英文来源：经济学人