衰退的定义其实是一个非常技术性的问题。NBER关于衰退的定义是比较复杂和主观的 (NBER是有一个委员会，通过投票来决定经济周期的拐点，并不是基于一套严格的算法)，并不是大家一般理解的那样清晰。一般我们用的GDP连续两个季度为负增长只是一个拇指法则，当然这个法则跟委员会的决策有高度的相关性。这是其一，第二衰退的确认是严重滞后的，通常要走入衰退6个月甚至更长时间以后，NBER和白宫才会宣布。也就是说，在宣布之前，市场还是可以有不同的声音在争论的。下面是NBER关于经济衰退的定义。The NBER’s traditional definition of a recession is that it is a significant decline in economic activity that is spread across the economy and that lasts more than a few months. The committee’s view is that while each of the three criteria—depth, diffusion, and duration—needs to be met individually to some degree, extreme conditions revealed by one criterion may partially offset weaker indications from another. For example, in the case of the February 2020 peak in economic activity, we concluded that the drop in activity had been so great and so widely diffused throughout the economy that the downturn should be classified as a recession even if it proved to be quite brief. The committee subsequently determined that the trough occurred two months after the peak, in April 2020. An expansion is a period when the economy is not in a recession. Expansion is the normal state of the economy; most recessions are brief. However, the time that it takes for the economy to return to its previous peak level of activity may be quite extended.那么，委员会具体观察和讨论什么指标呢？首先明确一下，委员会并不是监测GDP数据，而是观察月度的经济数据。我们会发现，这些数据主要是就业、收入和消费数据，而且都是经过通胀调整后的真实数据。这是跟美国的基本观念挂钩的，二战后，美国逐步建立了全球最为庞大和细致的对劳动，收入，消费的观察数据，而当时的苏联体系则建立了详尽的生产数据（服务于计划经济中的变量参数需求）。也就是说，美国经济周期监测委员会的工作目标是观察美国总体的收入和消费周期是否达到衰退的标准。The determination of the months of peaks and troughs is based on a range of monthly measures of aggregate real economic activity published by the federal statistical agencies. These include real personal income less transfers (PILT), nonfarm payroll employment, real personal consumption expenditures, wholesale-retail sales adjusted for price changes, employment as measured by the household survey, and industrial production. There is no fixed rule about what measures contribute information to the process or how they are weighted in our decisions.这里有一个重要启发，那就是NBER划定经济周期拐点的时候非常倚重就业-收入-消费数据。这其实也隐含了一个假设，跟美联储的目标是一样的，那就是就业和物价。就业指标中，如果很多人退出劳动力市场，那么，他们就不再是政府决策的主要目标了，尤其是主动退出的那部分人，美国政策的目标的确有一部分是帮助被动退出劳动市场的人重返市场（比如照顾年幼子女的女性）。而这一次劳动力市场的结构改变很可能会让NBER在划定经济周期时候面临困难。我给出了NBER考察的所有数据列表链接，感兴趣的读者可以自行查阅：https://fredaccount.stlouisfed.org/dashboard/92059我们来看一个图形，这个图形揭示了，在过去几次经济衰退中，我们都要看到美国真实居民支出的同比负增长，而目前的水平是正增长2%。我们再看真实收入增长（剔除转移支付）。这个数据目前是零。对比之前的衰退场景，依然可以说目前还无法确认经济是否进入衰退。