In 2008, for the first time since the Great Depression, the Federal Reserve encountered the zero lower bound (ZLB) on the nominal interest rate. The Fed’s target rate, the overnight federal funds rate, fell effectively to zero in the fall as the central bank’s emergency lending programs rapidly increased the supply of bank reserves. Then at its December 2008 meeting, the FederalOpenMarketCommittee (FOMC), the Fed’smonetary policymaking body, formally set the target range for the funds rate at 0 to 25 basis points—effectively zero. With the economy in free fall and little scope for additional rate cuts, the FOMC then turned to nonstandard policy tools, including forward guidance (communication about the likely or intended forward path of the policy rate) and large-scale asset purchases, also known as quantitative easing. Both tools were used actively during the Great Recession and the subsequent recovery. Did the nonstandard tools work? The current consensus of central bankers and economists is that forward guidance and asset purchases eased financial conditions and supported the economy, with fewer adverse side effects than many predicted. Accordingly, these policies appear to have become permanent components of the monetary tool kit, both in the United States and abroad. However, the prevailing view also holds that the overall response of monetary policy was effectively constrained by the ZLB; that is, although nonstandard policies evidentlymitigated the downturn, their use did not fully compensate for the inability of the Fed to cut short-term rates significantly further. This view, and the concern that in a world of low neutral interest rates future encounters with the ZLB may be frequent, has led the Fed and other central banks to actively consider new tools and new policy frameworks for dealing with the ZLB constraint. In contrast to this prevailing view, however, a small literature has adduced indirect evidence to argue that the Fed’s response to the Great

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2008 年，美联储自大萧条以来首次遭遇名义利率下限 (ZLB)。由于中央银行的紧急贷款计划迅速增加了银行准备金的供应，美联储的目标利率，即隔夜联邦基金利率，在秋季有效地降至零。然后在 2008 年 12 月的会议上，美联储的货币政策制定机构联邦公开市场委员会 (FOMC) 正式将基金利率的目标范围设定在 0 到 25 个基点——实际上为零。随着经济自由落体且进一步降息的空间很小，联邦公开市场委员会随后转向了非标准的政策工具，包括前瞻指引（关于政策利率可能或预期的前进路径的沟通）和大规模资产购买，也称为量化宽松。在大萧条和随后的复苏期间，这两种工具都得到了积极使用。非标准工具有用吗？央行行长和经济学家目前的共识是，前瞻性指引和资产购买缓解了金融状况并支持了经济，其不利副作用比许多人预期的要少。因此，这些政策似乎已成为美国和国外货币工具包的永久组成部分。但是，普遍的观点也认为，货币政策的整体反应受到ZLB的有效制约；也就是说，尽管非标准政策明显缓解了经济下滑，但它们的使用并没有完全弥补美联储无法进一步大幅降低短期利率的不足。这种观点，并且担心在低中性利率的世界中未来可能会频繁地遇到 ZLB，这导致美联储和其他中央银行积极考虑新工具和新政策框架来应对 ZLB 约束。然而，与这种流行观点相反，一小部分文献引用了间接证据来论证美联储对大