For an economy with underutilized resources or too low a rate of inflation the traditional prescription for monetary policy is to lower the interest rate. Central banks around the world tried to do that in response to stubbornly weak economies, bringing the overnight interest rate in many countries all the way to zero. But when that didn’t seem to be getting the job done, the Bank of Japan last week decided to go negative, charging banks 0.1% interest for excess reserves. With this step Japan now joins the Euro system, Switzerland, Denmark, and Sweden, all of whom have had negative interest rate policies in place for over a year. Here I describe how negative interest rates work, what they are intended to accomplish, and some of the limitations of using this policy to try to stimulate the economy.

I’ll illustrate how negative interest rates can work by taking the European Central Bank as an example. Traditionally the ECB would set two interest rates: a deposit interest rate that the ECB pays to banks on excess reserves held overnight in the banks’ accounts with the ECB, and a marginal lending rate at which banks could borrow overnight from the ECB. The deposit rate would usually set a lower bound on the interest rate at which banks would offer to lend to each other– why would I lend to another bank at 2.5% if I can get 3% on my ECB deposits, which are in effect an overnight loan from my bank to the ECB? The lending rate likewise sets a ceiling– why borrow from another bank at 5.5% if the ECB will give me all I want at 5% through their lending facility? The graph below shows how this system worked historically, with the interest rate on 3-month loans between banks moving within the corridor specified by the ECB.

Average interest rate over the month on 3-month interbank loans (in gray) and end-of-month values for ECB deposit rate (in blue) and lending rate (in orange), January 2001 to January 2016.

The ECB brought its deposit rate all the way to zero in July 2012. When that didn’t seem to be enough, they went to -0.1% in June of 2014, in other words, charging banks a fee (corresponding to a 0.1% annual rate) on their deposits held in excess of requirements. By last December the ECB had brought the deposit rate down to -0.3%. Here’s a more close-up view of the most recent data. It has functioned just like the historical system, with banks lending to each other at a rate that is somewhere between the deposit rate (currently -0.3%) and lending rate (currently +0.3%). The average rate on interbank loans for January turned out to be -0.15%.