从菲利普斯曲线说开去

菲利普斯曲线失效，算是经济学界大家心照不宣的一个秘密。芝大经济系每年有个传统，学生主办一场搞笑晚会，师生都来捧场。记得某年晚会，台上的学生戏谑道：『我的毕业论文研究的是菲利普斯曲线，所以大概没希望毕业了！』台下大笑。

几天前，民主党女政客科尔特斯（简称AOC）和联储主席鲍威尔（简称P），针对菲利普斯曲线进行了一场颇具学术意味的交锋。时间有限，不翻译了，原文贴上：

AOC: I’ve been seeing lately that economists are increasingly worried that the idea of a Phillips curve, that links unemployment and inflation, is no longer describing what is happening in today’s economy. What are your thoughts on that?

P: Yes, very much so. We’ve spend a great deal of time on that. The connection between slack in the economy – the level of unemployment and inflation – was very strong if you go back 50 years and it’s gotten weaker and weaker and weaker to the point where it’s a faint heartbeat. It’s still there, as you can see from state level data. But what we really have learned is that the economy can sustain much lower levels of unemployment than we thought without triggering troubling levels of inflation.

AOC: So why do you think we are seeing this decoupling in a relationship that we had seen in the economy decades ago?

P: One reason is just that inflation expectations are so settled. For example, when unemployment went way up, you don’t see inflation go down. Inflation just seems very anchored.

AOC: Do you think that that could have implications in terms of policymaking – that’s there’s perhaps room for increased tolerance in terms of policies that have historically been thought to drive inflation? One of the arguments about policies that directly target middle class Americans is that they could drive inflation. Do you think that that decoupling is something we should consider in modern policy considerations?

P: Again, I wouldn’t want to get into the minimum wage discussion directly. But I think we’ve learned that downward pressure on inflation around the globe and here is stronger than we had thought. You see countries all over the world being below their inflation targets, whereas when I was young they were always above. US has done better than other countries, but we are still below our target.

这件事情有意思之处，在于科尔特斯有意无意试图把话题带到最低工资上，但是被鲍威尔巧妙识破。当然，事件后面的发酵也值得玩味。

科尔特斯想要表达的逻辑是，如果菲利普斯曲线已经失效，那么一些传统意义上可能会引起通胀的政策（比如提高最低工资）实际上并不会给美国经济带来太多负面的影响。鲍威尔没有正面回应关于最低工资的讨论，而是谈到了世界上大多数国家通胀率面临下行而不是上行压力（看看日本就知道了）。换言之，现在全球面临的问题是通货膨胀率太低（而不是太高），让央行的货币政策施展空间不足，而这也恰恰是美联储迟迟不愿降息的原因。弹药得省着点打，名义利率一旦到了ZLB（zero lower bound），到时候再出什么问题就无计可施了。

这场交锋也得到了白宫首席经济顾问库德洛的注意。他在福克斯电视台的访谈里说，愿意『对AOC女士脱帽致敬』（“give hats off to Ms. AOC”）。看来，科尔特斯女士关于菲利普斯曲线的问题让她赢得了一些反对者的尊重。不过，库德洛接下来讲的就是不知所云了。他说，他和川普总统都一直主张"同样"的观点，即经济增长不会带来通胀率增长，所以『看起来联储要准备降息了』（“Looks like the Fed is going to cut their rates."）

现在这个节骨眼降不降息，说实话和菲利普斯曲线关系不大。川普和库德洛想降息，是需要在大选前造成一种经济欣欣向荣的假象，给失败的贸易政策造成的恶果买单。而鲍威尔反对降息，是因为确实没有什么降息的必要，不管从目前的通胀率、失业率还是其他宏观数据来看都是如此。

菲利普斯曲线，从来都与这场讨论无关，更与其他政策问题无关。它是曾经繁荣的宏观经济学大厦遗留下来的残垣，是仅存在故纸堆和人们笑谈中的一个渐渐被遗忘的故事。有不少人，包括科尔特斯和库德洛，甚至我们熟悉的曼昆在内，试图使菲利普斯曲线复活。他们或许应该多读读 John Cochrane 的这篇博客。

福克斯采访的最后，库德洛说，『我希望我和她（AOC）能有时间坐下来聊聊供给侧经济学。』

我要是AOC就说：『还是下次吧，顾问先生。』