The Fed’s December 2015 decision to raise interest rates after an unprecedented seven-year stasis offers a chance to assess the link between interest rates and bank profitability. A key determinant of a bank’s profitability is its net interest margin (NIM)—the gap between an institution’s interest income and interest expense, typically normalized by the average size of its interest-earning assets. The aggregate NIM for the largest U.S. banks reached historic lows in the fourth quarter of 2015, coinciding with the “low for long” interest rate environment in place since the financial crisis. When interest rates fall, interest income and interest expenses tend to fall as well, but the relative changes—and the impact on NIM—are less clear. In this post, we explore how NIM fell during the low-interest-rate period, finding that banks mitigated some, but not all, of the impact of lower rates by shifting into less costly types of liabilities. Our analysis also gives insight into how NIM may respond to the new rising interest rate environment.