Liquidity trap
2008 financial crisis

U.S. Federal Reserve economists assert that the liquidity trap can explain low inflation in periods of vastly increased central bank money supply. Based on experience $3.5 trillion of quantitative easing from 2009–2013, the hypothesis is that investors hoard and do not spend the increased money because the opportunity cost of holding cash (namely the interest forgone) is zero when the nominal interest rate is zero. This hoarding effect is purported to have reduced consequential inflation to half of what would be expected directly from the increase in the money supply, based on statistics from the expansive years. They further assert that the liquidity trap is possible only when the economy is in deep recession.
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