Probably the most controversial form of unconventional policy adopted in recent years was what the Federal Reserve called large-scale asset purchases (LSAPs) but most of the rest of the world persisted in calling “quantitative easing”, or QE. The Federal Reserve engaged in three rounds of QE, during which its balance sheet expanded from less than a trillion dollars to $4.5 trillion. The Bank of England, European Central Bank, Swedish Riksbank, and Bank of Japan (which had pioneered asset purchases as a form of monetary policy well before the crisis) have also undertaken quantitative easing.
Quantitative easing involves central bank purchases of securities in the open market, financed by the creation of bank reserves held at the central bank. By law, the Fed was able to purchase only Treasury securities and mortgage-related securities issued by government-sponsored enterprises. Other central banks, in contrast, have been able to buy a range of private securities, including corporate bonds and equities. The limits on the Fed did not seem to prevent its version of QE from being effective, although it was perhaps fortunate that, following a crisis centered on housing finance, the law did permit Fed purchases of mortgage-related securities.


Research suggests that QE works through two principal channels, the signaling channel and the portfolio balance channel. The signaling channel arises to the extent that asset purchases serve to demonstrate the central bank’s commitment to monetary easing, and in particular to keeping short-term rates lower for longer (Bauer and Rudebusch, 2013). As discussed above, the so-called taper tantrum in 2013 demonstrated the practical relevance of the signaling channel of QE. As noted, because of the importance of the signaling channel, it is essential that asset purchases and interest rate policy be closely integrated, and that in particular the central bank be clear about its planned sequencing of the introduction and withdrawal of its various tools.
The portfolio balance channel depends on the premise that securities are imperfect substitutes in investors’ portfolios, reflecting differences in liquidity, transactions costs, information, regulatory restrictions, and the like. Imperfect substitutability implies that changes in the net supply of a security affect asset prices and yields, as investors must be induced to rebalance their portfolios (Bonis et al., 2017). In principle, the two channels of QE can be distinguished by the fact that the signaling channel operates by affecting expectations of future policy rates while the portfolio balance channel works by changing term and risk premiums.
研究によれば、QEは2つの主要な経路を通じて機能する。シグナリング経路とポートフォリオバランス経路である。シグナリング経路は、資産購入が中央銀行の金融緩和へのコミットメント、とりわけ短期金利を低く長く維持することへのコミットメントを示す程度において生じる(Bauer and Rudebusch, 2013)。前述したように、2013年のいわゆるテーパー・タントラムは、QEシグナリング経路が実際に問題になることを示した。前記した通り、シグナリング経路の重要性に鑑みると、資産購入と金利政策が密接に統合されていることが肝要となる。特に、中銀が各種の政策ツールの出し入れをどのような順番で計画しているかを明確にしておくことが肝要である。
ポートフォリオバランス経路は、投資家のポートフォリオにおいて証券同士が不完全な代替物である、という前提に依拠している。それは、流動性、取引コスト、情報、規制による制限などの差を反映している。不完全な代替性は、証券の純供給が変化すると投資家が自分のポートフォリオをリバランスしようとするため、資産価格と利回りが影響を受けることを意味している(Bonis et al., 2017)。原則としてQEの2つの経路は、シグナリング経路は将来の政策金利予想に影響することによって機能し、ポートフォリオバランス経路は期間ならびにリスクプレミアムを変えることによって機能する、という点で区別できる。


There is not a large sample of QE programs to study, and econometric identification of the unexpected (and thus not fully discounted) components of QE announcements is difficult. Consequently, disagreements remain among researchers about the magnitude and persistence of QE effects and about the relative importance of the two primary channels of effect. Nevertheless, the strong view that QE is ineffective has been pretty decisively rejected. There appears instead to be a broad consensus that QE has proven a useful tool, with demonstrable effects on financial conditions. QE has been found to have significant effects on both rate expectations and term premiums, suggesting that both the signaling and portfolio balance channels are operative (Bauer & Rudebusch, 2013: Huther et al., 2017). And, although showing direct links to macroeconomic outcomes is not straightforward, the experiences of the U.S., U.K. Japan, and Europe all suggest that the use of large-scale QE has been followed, over the subsequent couple of years, by strengthening aggregate demand and improved economic performance (Engen et al., 2015).
Controversies about QE have focused less on whether the medicine works and more on the possible side effects.
研究対象となるQE政策の事例はそれほど多くない。また、QEの発表における予期されていない(従って完全に織り込まれていない)要因を計量経済学的に識別することは困難である。そのため、QEの効果の規模と持続性、および、効果を発揮する上での2つの主要な経路の相対的な重要性については、研究者の間で今も意見が分かれている。しかしながら、QEは無効である、という強い見解はかなり決定的に棄却されている。QEは金融環境に明白な効果を及ぼす有用なツールであることが明らかになった、という幅広いコンセンサスが存在するように思われる。QE金利予想と期間プレミアムのいずれにおいても有意な効果が見出されており、シグナリングとバランスシートの両経路が機能していることを示している(Bauer & Rudebusch, 2013、Huther et al., 2017)。また、マクロ経済の結果への直接的な関連を示すのは簡単ではないものの、米英日欧の経験はすべて、大規模なQEの適用後の数年、総需要が強化され経済のパフォーマンスが改善することを示している(Engen et al., 2015*2


*1:[原注] I also tried, without success, to name the program “credit easing,” to distinguish it from the Bank of Japan’s earlier foray into asset purchases (Bernanke, 2009). I argued that “credit easing” focused on removing duration from bond markets, in contrast to BOJ-style quantitative easing, which had the primary goal and metric of increasing the highpowered money stock.

*2:[原注] It is true that QE has not been sufficient in a number of those cases to return inflation to target. However, the weak link in the causal chain appears to be in the influence of declining slack on inflation, not the effect of monetary policy (including QE) on aggregate demand. The apparent flatness (or downward shift) of the Phillips curve is a problem for any macroeconomic policy aimed at raising inflation..