哈佛教授：用一年全球GDP损失20%的方法拯救人类|全球经济|萧条|投资|宏观经济|金融

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作者|罗伯特·巴罗（美国经济学家、哈佛大学经济学教授）

罗伯特·巴罗（Robert Barro），美国经济学家，目前为哈佛大学保罗·瓦尔堡经济学讲座教授、斯坦福大学胡佛研究所资深研究员。与小罗伯特·卢卡斯、托马斯·萨金特等人，皆为新兴古典经济学派代表人物。

对新冠病毒流行情况估计的最佳参考范本可能是1918-1920年的大流感。从48个国家/地区的数据中研究发现，流感死亡率平均为2.1％，相当于全球4000万人死亡。死亡率在贫穷国家中最高，包括撒哈拉以南非洲的几个国家，印度、危地马拉和印度尼西亚。这段历史经验表明，随着疾病进一步蔓延到那些公共卫生体系较差的低收入国家，疫情的死亡率高点即将到来。

1918-1920年的大流感还造成了许多国家的经济衰退。典型的情况是一国GDP减少约6％，持续了大约两年时间。也就是说，产出损失约为一年GDP的12％。

由于缺乏有效的治疗方法和疫苗，目前世界没有应对新冠病毒流行的有效措施。一旦能够找到广泛且快速的检测方法，那么就有一种可能性，即隔离受感染人群。在此之前，大多数国家和地区现在正在采用的主要措施是限制经济活动，以减少传染。这一政策相当于短期内世界GDP将减少约20％。本质上，这是自愿实施的负供给冲击，类似于每个国家生产力的突然下降。

如果GDP下降20％持续整整一年或更长时间，将构成罕见的宏观经济灾难，类似于1930年代的大萧条。从历史上看，大萧条持续了2-4年。但是，在当前的环境中，一个合理的希望是，急剧减少的产出损失仅持续几个月，之后病毒传播将被遏制。也就是说，经济将实现快速的“V”形反弹。

当前全球每年的GDP约为100万亿美元，因此，如果20％的产出减少持续一年，则意味着产出损失了20万亿美元。每年GDP下降20％的幅度相当于1918-1920年大流感期间所经历的经济收缩（大约一年GDP的12％）的水平。相比之下，持续三个月的经济收缩意味着损失5万亿美元的产出（占一年GDP的5％），而持续三年的经济收缩则意味着损失产出60万亿美元（占一年GDP的60％）。因此，经济收缩的持续时间是关键变量。目前，我认为即使GDP持续下降一整年，损失GDP的20％也是合理的政策选择，尽管GDP持续下降的趋势已经是现实。

应对GDP下降，合理的货币和财政政策是什么？我们已经确定GDP的损失对于抗击新冠病毒来说是值得的，因此，通过提高总需求来增加实际GDP的刺激政策是不合理的。如果激进的货币政策——降低短期名义利率、提高中央银行购买资产的力度——成功地提高了GDP，那么在当前这个不寻常的情况下，我们不会认为这是成功的。如果我们不希望GDP下降20％，首先我们可以通过减少对经济活动的限制来实现目标。

同样的论点也适用于总体财政扩张。例如，美国的政策是联邦政府向大多数成年人分发1200美元的支票。在某种程度上，这种政策成功地提高了总需求，从而提高了GDP，与积极的货币政策一样。也就是说，我们不会认为GDP的抵消性增长是有价值的。

政策响应一部分针对个人，一部分针对企业。政策的一个方面是加强现有的社会保障。在这种情况下，增加失业保险，食物券和医疗补助（为穷人提供的医疗资金）等，这些做法的可行性和带来的福利水平是有意义的。这些扩张政策比向所有人分发1200美元的支票更具针对性。

从美国最近的一揽子计划中可以看出，旨在避免企业破产和维持企业与其员工之间联系的政策也很有意义。这种计划在很大程度上适用于受损特别严重的行业，例如航空公司和其他旅游业相关的公司。包括美联储在内的中央银行，避免金融市场遭受重大破坏也至关重要。这些措施已经在积极地实施，使人们相信2008-2009年经济衰退期间的金融混乱将不会重演。

从技术的角度，即GDP持续一年下降20％来考虑这些基本政策，有两件事很重要。第一，估算降低死亡率的潜在可能性有多大？这些政策能成功降低多少死亡率？如果我们使用1918年至1920年大流行期间2.1％的世界死亡率，并用这一数字来估算世界目前的约75亿人口，那么我们可能挽救1.5亿条生命。如果我们使用研究文献中的标准数字来统计生命的价值，大约为100万美元，那么，拯救1.5亿条生命价值150万亿美元，大约是世界一年GDP的1.5倍。这个数字大大超过了每年损失20％GDP的损失。

从西班牙大流感中，我们可以了解到一些限制经济活动的政策将如何阻止大流行的蔓延，并挽救多达1.5亿条生命。有一个政策是，澳大利亚实施了积极的海上检疫措施，限制了经济发展，该检疫措施避免了类似1918年的大流感的蔓延。此外，该政策不仅推迟了疾病的影响，最终还使流感总体死亡率降低了很多，比其他国家甚至南半球的情况都好。

第二，有直接证据表明，非药物公共卫生干预措施降低了1918-1919年大流感死亡人数。这些干预措施主要应用于三个方面：学校停课，取消公共聚会以及隔离。有证据表明，这些政策越早实施，实施起来就越有效，其中一些部门关停的时间长达10周。目前，大多数国家都比1918年更加积极地开展减缓GDP增长的举措。100多年前的结果表明，采取这些干预措施是值得的。

English Version：

Macroeconomic Responses to the Coronavirus Pandemic

Robert J. Barro

April 2020

The best source of a worst-case scenario for the ongoing coronavirus pandemic likely comes from the Great Influenza Pandemic of 1918-1920. Our research[1] found from data on 48 countries that the flu death rate averaged 2.1 percent, corresponding to 40 million worldwide deaths. However, the death rate was highest in poor countries—including several in sub-Saharan Africa, along with India, Guatemala, and Indonesia. This historical experience suggest that the worst mortality from the current pandemic is yet to come as the disease spreads further into low-income countries, typical with poor public-health systems.

The 1918-1920 pandemic was also associated with recessions in many countries. The typical size of the contraction in GDP was about six percent and lasted for around two years; that is, the lost output was about 12 percent of a year’s GDP.

Currently, the absence of effective medical treatments and vaccines leaves the world without attractive choices for countering the coronavirus pandemic. One possibility, which becomes possible once widespread and rapid testing are available, is targeted quarantining of the infected population. Until then, the main option, now being followed in most countries, is to curb economic activity as a way to reduce interactions and contagion. This policy amounts to a decision to reduce world GDP in the short run by roughly 20 percent. In essence, this is a voluntarily implemented negative supply shock, akin to a sudden loss in productivity in each country.

A decline in GDP by 20 percent, if it lasts for a full year or more, would constitute a rare macroeconomic disaster, analogous to the Great Depression of the 1930s. Historically, these depressions have lasted for 2-4 years. However, the reasonable hope in the current environment is that the sharp cut in the flow of output will last for only a few months, after which the virus will be contained. That is, the objective is for a sharp, V-shaped recovery.

The world’s annual GDP today is around $100 trillion, so a 20 percent cut that persists for a year implies lost output of $20 trillion. This contraction by 20 percent of a year’s GDP is in the ballpark of the economic contractions experienced during the 1918-1920 pandemic (about 12 percent of a year’s GDP). In contrast, a contraction that lasts for only three months implies lost output of only $5 trillion (5 percent of a year’s GDP), and one that lasts for three years implies lost output of $60 trillion (60 percent of a year’s GDP). Thus, the duration of the contraction is a crucial variable. For the moment, I assume that it is a good policy choice to engineer a 20 percent reduction in GDP even if that reduction lasts for a full year—though a less persistent decline seems realistic and obviously more attractive.

What are reasonable monetary and fiscal responses to the fall in GDP? Since we have determined that the cost of reduced GDP is worth bearing as a way to combat the pandemic, it would be inconsistent to follow the usual stimulus policies that work by raising aggregate demand and, thereby, increasing real GDP. For example, if aggressive monetary policy—cutting short-term nominal interest rates and raising central bank purchases of assets—succeeds in raising GDP, we would not regard that as success in the current unusual environment. If we did not want GDP to fall by 20 percent, we could have achieved that goal by lessening the constraints on economic activity in the first place.

The same argument applies to a general fiscal expansion; for example, the U.S. policy of having the federal government give most adults a check for $1200. To the extent that this kind of policy succeeds in raising aggregate demand and, thereby, GDP, we have the same situation as with aggressive monetary policy. That is, we would not value the offsetting rise in GDP.

More sensible are targeted policy responses aimed partly at individuals and partly at businesses. One dimension of this policy is a strengthening of the existing social-safety net. In this context, it makes sense to increase accessibility and benefit levels for programs like unemployment insurance, food stamps, and Medicaid (which finances medical expenses for poor persons). These program expansions, some of which are in the recent U.S. package, are much more targeted to the needy than is the passing out of $1200 checks to everyone.

It also makes sense that the recent U.S. package includes policies aimed at limiting the permanent disappearance of businesses and maintaining links between firms and their workers. Much of this response applies to particularly distressed sectors, such as airlines and other travel-related companies. It is also crucial for central banks, including the U.S. Federal Reserve, to avoid major disruptions of financial markets. These actions are already being pursued vigorously, and they provide confidence that the financial disruptions during the Great Recession of 2008-2009 will not be repeated.

Consider now the underlying policy of engineering a 20 percent decline in GDP that lasts for a year. Two things matter here. By how much does one value the potential reduced mortality and by how much does the policy succeed in lowering mortality? If we use the world death rate of 2.1 percent from the 1918-1920 pandemic and apply this number to the world’s current population of around 7.5 billion, we are talking about possibly saving 150 million lives. If we use a standard number from the research literature of about $1 million for the value of a statistical life[2], then a saving of 150 million lives is worth $150 trillion, about 1-1/2 years of world GDP. This number greatly exceeds the costs of losing 20 percent of a year’s GDP.

We can get some idea from the Great Influenza Pandemic about how a policy of constraining economic activity will hold back the spread of a pandemic and lead, thereby, to a saving of up to 150 million lives. One point is that Australia constrained its economy by implementing an aggressive maritime quarantine that managed to avoid the Great Influenza Pandemic in 1918. Moreover, this policy did not just postpone the effects of the disease—it ended up achieving a much lower overall flu death rate than those experienced in other countries, even in the Southern Hemisphere. Second, there is direct evidence that non-pharmaceutical public-health interventions held down deaths from the flu in 1918-1919[3]. These interventions applied in three main areas: school closings, cancellation of public gatherings, and isolation/quarantine. The evidence is that these policies worked and more effectively the earlier they were implemented and the longer they were kept in place, with some of the closures applying for as long as ten weeks. Currently, most countries are pursuing these kinds of GDP-suppressing activities even more aggressively than was done in 1918. The results from over 100 years ago suggest that these kinds of interventions are worth the cost.

Robert Barro is Paul M. Warburg Professor of Economics at Harvard University and a visiting scholar at the American Enterprise Institute.

Citations：

[1]RobertJ. Barro, José F. Ursúa, and Joanna Weng, “The Coronavirus and the GreatInfluenza Pandemic: Lessons from the ‘Spanish Flu’ for the Coronavirus’sPotential Effects on Mortality and Economic Activity,” National Bureau ofEconomic Research, working paper 26866, March 2020.

[2]For asurvey of this literature, see Thomas J. Kniesner and W. Kip Viscusi, “TheValue of a Statistical Life,” forthcoming in Oxford Research Encyclopedia ofEconomics and Finance, 2019.

[3]SeeHoward Markel, Harvey B. Lipman, J. Alexander Navarro, Alexandra Sloan, JosephR. Michalsen, Alexandra Minna Stern, and Martin S. Cetron, “NonpharmaceuticalInterventions Implemented by US Cities During the 1918-1919 Influenza Pandemic,”Journal of the American Medical Association 298 (6): 644-654.

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