In olden days, like before the 2008 financial crisis, it was a given that Treasury investors would demand greater compensation for lending for longer periods. Those so-called term premia -- higher interest rates for long-term borrowers-- reflected the greater risk of, say, inflation rising over 10 or 20 years. But term premia shrank and then disappeared, lurking below zero much of the time between late 2014 and early 2021.