哈佛教授：用一年全球GDP損失20%的方法拯救人類

摘要：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.。目前，我認爲即使GDP持續下降一整年，損失GDP的20％也是合理的政策選擇，儘管GDP持續下降的趨勢已經是現實。

“大師”由網易研究局出品，本文是網易研究局獨家稿件，轉載請註明來源

·聚焦國際思想市場·解析財經新聞熱點·對話國際經濟學大師

作者|羅伯特·巴羅（美國經濟學家、哈佛大學經濟學教授）

羅伯特·巴羅（Robert Barro），美國經濟學家，目前爲哈佛大學保羅·瓦爾堡經濟學講座教授、斯坦福大學胡佛研究所資深研究員。與小羅伯特·盧卡斯、托馬斯·薩金特等人，皆爲新興古典經濟學派代表人物。

對新冠病毒流行情況估計的最佳參考範本可能是1918-1920年的大流感。從48個國家/地區的數據中研究發現，流感死亡率平均爲2.1％，相當於全球4000萬人死亡。死亡率在貧窮國家中最高，包括撒哈拉以南非洲的幾個國家，印度、危地馬拉和印度尼西亞。這段歷史經驗表明，隨着疾病進一步蔓延到那些公共衛生體系較差的低收入國家，疫情的死亡率高點即將到來。

1918-1920年的大流感還造成了許多國家的經濟衰退。典型的情況是一國GDP減少約6％，持續了大約兩年時間。也就是說，產出損失約爲一年GDP的12％。

由於缺乏有效的治療方法和疫苗，目前世界沒有應對新冠病毒流行的有效措施。一旦能夠找到廣泛且快速的檢測方法，那麼就有一種可能性，即隔離受感染人羣。在此之前，大多數國家和地區現在正在採用的主要措施是限制經濟活動，以減少傳染。這一政策相當於短期內世界GDP將減少約20％。本質上，這是自願實施的負供給衝擊，類似於每個國家生產力的突然下降。

如果GDP下降20％持續整整一年或更長時間，將構成罕見的宏觀經濟災難，類似於1930年代的大蕭條。從歷史上看，大蕭條持續了2-4年。但是，在當前的環境中，一個合理的希望是，急劇減少的產出損失僅持續幾個月，之後病毒傳播將被遏制。也就是說，經濟將實現快速的“V”形反彈。

當前全球每年的GDP約爲100萬億美元，因此，如果20％的產出減少持續一年，則意味着產出損失了20萬億美元。每年GDP下降20％的幅度相當於1918-1920年大流感期間所經歷的經濟收縮（大約一年GDP的12％）的水平。相比之下，持續三個月的經濟收縮意味着損失5萬億美元的產出（佔一年GDP的5％），而持續三年的經濟收縮則意味着損失產出60萬億美元（佔一年GDP的60％）。因此，經濟收縮的持續時間是關鍵變量。目前，我認爲即使GDP持續下降一整年，損失GDP的20％也是合理的政策選擇，儘管GDP持續下降的趨勢已經是現實。

應對GDP下降，合理的貨幣和財政政策是什麼？我們已經確定GDP的損失對於抗擊新冠病毒來說是值得的，因此，通過提高總需求來增加實際GDP的刺激政策是不合理的。如果激進的貨幣政策——降低短期名義利率、提高中央銀行購買資產的力度——成功地提高了GDP，那麼在當前這個不尋常的情況下，我們不會認爲這是成功的。如果我們不希望GDP下降20％，首先我們可以通過減少對經濟活動的限制來實現目標。

同樣的論點也適用於總體財政擴張。例如，美國的政策是聯邦政府向大多數成年人分發1200美元的支票。在某種程度上，這種政策成功地提高了總需求，從而提高了GDP，與積極的貨幣政策一樣。也就是說，我們不會認爲GDP的抵消性增長是有價值的。

政策響應一部分針對個人，一部分針對企業。政策的一個方面是加強現有的社會保障。在這種情況下，增加失業保險，食物券和醫療補助（爲窮人提供的醫療資金）等，這些做法的可行性和帶來的福利水平是有意義的。這些擴張政策比向所有人分發1200美元的支票更具針對性。

從美國最近的一攬子計劃中可以看出，旨在避免企業破產和維持企業與其員工之間聯繫的政策也很有意義。這種計劃在很大程度上適用於受損特別嚴重的行業，例如航空公司和其他旅遊業相關的公司。包括美聯儲在內的中央銀行，避免金融市場遭受重大破壞也至關重要。這些措施已經在積極地實施，使人們相信2008-2009年經濟衰退期間的金融混亂將不會重演。

從技術的角度，即GDP持續一年下降20％來考慮這些基本政策，有兩件事很重要。第一，估算降低死亡率的潛在可能性有多大？這些政策能成功降低多少死亡率？如果我們使用1918年至1920年大流行期間2.1％的世界死亡率，並用這一數字來估算世界目前的約75億人口，那麼我們可能挽救1.5億條生命。如果我們使用研究文獻中的標準數字來統計生命的價值，大約爲100萬美元，那麼，拯救1.5億條生命價值150億美元，大約是世界一年GDP的1.5倍。這個數字大大超過了每年損失20％GDP的損失。

從西班牙大流感中，我們可以瞭解到一些限制經濟活動的政策將如何阻止大流行的蔓延，並挽救多達1.5億條生命。有一個政策是，澳大利亞實施了積極的海上檢疫措施，限制了經濟發展，該檢疫措施避免了類似1918年的大流感的蔓延。此外，該政策不僅推遲了疾病的影響，最終還使流感總體死亡率降低了很多，比其他國家甚至南半球的情況都好。

第二，有直接證據表明，非藥物公共衛生干預措施降低了1918-1919年大流感死亡人數。這些干預措施主要應用於三個方面：學校停課，取消公共聚會以及隔離。有證據表明，這些政策越早實施，實施起來就越有效，其中一些部門關停的時間長達10周。目前，大多數國家都比1918年更加積極地開展減緩GDP增長的舉措。100多年前的結果表明，採取這些干預措施是值得的。

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.

本文爲網易研究局獨家稿件，不構成投資決策。

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