The yield on a 10-year Treasury bond — that is, the interest rate the government pays when it borrows money for 10 years — is below 2 percent. Interest rates tend to move together, so this also means that the interest rate you pay to buy a house — as well as the interest rates you can get from a savings account — are also at record lows. That freaks out a lot of people because they remember the much higher interest rates of the late 20th century and worry that today's very low rates are a sign that something has gone badly wrong with the economy.

But taking a much longer view suggests a different interpretation. From the nation's founding until about 1950, interest rates were on a clear downward trend. This makes sense: As society gets richer, there's more money available to lend, and so the price to borrow money naturally goes down. From this perspective, the high interest rates of the 1970s, '80s, and '90s look like an aberration driven by the unusually high inflation of that era. And today's low rates look more like the resumption of a 200-year trend toward ever-lower borrowing costs. So don't panic — enjoy your 3.5 percent mortgage.