Federal Reserve Chairwoman Janet Yellen faced what may have seemed like an absurd line of questioning on Capitol Hill this week: Has the Fed considered negative interest rates? It's not a fantasy. At central banks around the world, negative benchmark interest rates have become the new normal, leading to some unusual paradoxes. In Denmark, instead of paying interest on their mortgages, many borrowers have been receiving payments from banks on their home loans. On the flip side, some Swiss bank customers have had to pay banks to keep cash in savings accounts. Yellen told Congress Thursday, following negative moves by central banks in Europe and elsewhere, the Fed has...