Negative interest rates are simply the latest fruitless effort since the 2008 global financial crisis to revive economies by monetary measures. When cutting interest rates to historically low levels failed to revive growth, central banks took to so-called quantitative easing: injecting liquidity into economies by buying long-term government and other bonds. It did some good, but mostly the sellers sat on the cash instead of spending or investing it. Enter negative interest-rate policy. The central banks of Denmark, Sweden, Switzerland, Japan, and the eurozone have all indulged. The US Federal Reserve and the Bank of England are being tempted.