However, the number of banks willing to lend to one another on such terms has been sharply reduced because of economic turmoil, including the 2008 global financial crisis, the European debt crisis that began in 2010, and the downgrading of large banks’ credit ratings this year.

Banks have shifted toward secured borrowing and, on occasion, borrowing from central banks like the Federal Reserve and the European Central Bank. As Mervyn King, the governor of the Bank of England, said of Libor in 2008: “It is, in many ways, the rate at which banks do not lend to each other.”

These changes in the markets raise questions about the integrity of this important benchmark.

First, why is Libor so different from another benchmark interest rate for borrowing in United States dollars — Euribor, or euro interbank offered rate? Both rates are calculated on the basis of banks’ answers to roughly the same question. For Libor, a bank is asked at what rate it thinks it can borrow, while for Euribor, a bank is asked at what rate it thinks other banks are able to borrow. And yet the Euribor for dollar borrowings is about twice as high as the comparable Libor.

Second, why have Libor and other benchmark rates typically not been aligned, since 2008, with the borrowing rates that would be implied by foreign exchange markets? A long-established financial theory known as interest rate parity says that the difference in interest rates between two countries should be roughly in line with the expected change in exchange rates between the countries’ currencies. (If it isn’t, that opens an opportunity for arbitrage, the practice of taking advantage of price differences.)

Until 2007, as the theory predicted, the difference between the borrowing rate in one currency and the lending rate in another could typically be derived from foreign currency exchange rates. In the last few years, that hasn’t been the case, and this divergence between theory and practice has yet to be adequately explained.