How would a cash-only central bank conduct monetary policy?

What role do changes in the supply of banknotes play in contributing to a central bank's ability to carry out monetary policy? Put differently, to what degree does "printing," or creating new physical currency and issuing it into the economy, contribute to generating a central bank's desired inflation rate of 2-3%?

In a recent blog post at Econlog, Scott Sumner suggests that printing physical cash and "forcing" or "injecting" it into the economy has been an important part of central banks hitting their inflation targets, albeit less so now than in times past. I'm not so sure.

Having imbibed Scott's blog posts for more than a decade, I think I'm 99% on the same page as he is when it comes to thinking about monetary policy. We both agree that a central bank must either reduce the interest rate that it pays on the monetary base, or inject more monetary base into the economy, in order to push up prices. Using either of these two methods, the central bank sets off a hot potato effect in which a long chain of market participants do their best to unload their excess money balances, a process that only comes to an end when all prices have risen to a new and higher equilibrium such that no one feels any additional urge to spend away their extra money. Scott once described the hot potato effect as the the "sine qua non of monetary economics."

The monetary base is comprised of two central bank financial instruments: physical banknotes (a.k.a. cash) and digital clearing balances, sometimes known as reserves, a type of money used by banks.

Reading through Scott's post, I think the one spot where we may disagree is on the relative role played by the two types of base money in the hot potato process.

For my part, I don't think that cash has ever had much of a primary role to play in setting off a hot potato effect. All of the initial uumph necessary for driving prices towards target has typically been provided by reserves, either via a change in the interest rate on reserves or a change in their quantity. Once that initial uumph has been delivered, a whole host of other money types – physical currency, bank deposits, checks, money market funds, and PayPal balances – helps convey the forces originally unleashed by reserves to all corners of the economy.

An example of the hot potato effect in action may help illustrate.

Let's start with a central bank that needs to push inflation up to target. It reduces the interest rates on reserves. The first reaction to lower rates is a flight out of reserves into other assets, say shares.

As a result, share prices quickly rise. Those existing shareholders who realized their gains by selling at the new and higher price now find themselves with a hot potato on their hands; they have too many monetary balances in their possession and not enough non-monetary things.

Some of these ex-shareholders may choose to spend their excess deposits to go on, say, a vacation. As a result, airline ticket prices rise. Others transfer their extra money to their PayPal account in order to send it to friends and family, who may in turn make purchases, pushing up the prices of whatever they buy. Another group of ex-shareholders decides to buy used cars. They withdraw banknotes, their banks in turn asking the Fed to print new banknotes and ship them over. Used car prices rise.

The point is, the initial uumph is delivered by the change in reserves, and this gets conveyed to all prices by a daisy-chain of spenders offloading an array of different types of excess money.

In this story, note that cash isn't being actively "injected" into the economy by central banks, nor by commercial banks. Rather, people are choosing to draw cash out as their preferred method for getting rid of unwanted money, in response to a set of forces initiated by reserves. Reserves are the central bank's lever for change; cash is merely responsive.

Consider too that in a world where cash no longer exists, and has been replaced by digital payments options, monetary policy is still effective. In this world, the response to a reduction in the interest rate on reserves gets conducted to all the economy's nooks and crannies via non-cash types of monies, like fintech balances and bank deposits. (I'd be curious to hear if Scott is of the same opinion about monetary policy in a world without cash.)

Here's an interesting thought experiment. Would it be possible to redesign cash and reserves in such a way that cash takes over the initiatory role in monetary policy from reserves? That is, can we turn cash into the active part of the monetary base, the one that drives changes in monetary policy, and relegate reserves to the passive role?  

One step we could take is to pay interest on cash. This may sound odd, but it's possible to do so by setting up a serial note lottery to pay, say, 3% per year to holders of cash. (I wrote about this idea here and here.) Simultaneously, reserves would be rendered less important by no longer paying interest on them.

Now when a central bank needs to raise consumer prices in order to hit its targets, it reduces the interest rate on cash from 3% to 2%. This ignites the hot potato process as the entire economy suddenly tries to offload its unwanted $20 and $100 bills, which at 2% just aren't as lucrative as before.

Another change we could enact would be to modify the mechanism by which central banks inject base money into the economy. As it stands now, central banks inject base money by purchasing assets with new reserves. Since reserves are digital, they are a lot more convenient for making billion dollar asset purchases than physical money. These extra reserves become hot potatoes in the hands of asset sellers, which sets off the process of price adjustments described in previous paragraphs. If central banks were to buy assets with cash rather than reserves, that would put cash in the driver's seat, albeit at the expense of convenience.

It's an interesting thought experiment, but in the end I don't think it's very helpful to get bogged down over which type of base money has more monetary significance. As Scott says, the key point is that the central bank controls the price level via its control over base money in general. They can raise prices by either adding to the supply of base money, or by reducing the demand for base money with a cut in the interest rate paid on reserves. "It's basic supply and demand, nothing more."