A few years ago, textbooks had traditionally presented monetary policy as a choice between targeting the quantity of money or the interest rate. It was supposed that control of monetary aggregates could be achieved through control over the quantity of reserves, given a relatively stable “money multiplier”. (Brunner 1968; Balbach 1981) This even led to some real world attempts to hit monetary growth targets—particularly in the US and the UK during the early 1980s. However, the results proved to be so dismal that almost all economists have come to the conclusion that at least in practice, it is not possible to hit money targets. (B. Friedman 1988) These real world results appear to have validated the arguments of those like Goodhart (1989) in the UK and Moore (1988) in the US that central banks have no choice but to set an interest rate target and then accommodate the demand for reserves at that target. Hence, if the central bank can indeed hit a reserve target, it does so only through its decision to raise or lower the interest rate to lower or raise the demand for reserves. Thus, the supply of reserves is best thought of as wholly accommodating the demand, but at the central bank’s interest rate target.
Why does the central bank necessarily accommodate the demand for reserves? There are at least four different answers. In the US, banks are required to hold reserves as a ratio against deposits, according to a fairly complex calculation. In the 1980s, the method used was changed from lagged to contemporaneous reserve accounting on the belief that this would tighten central bank control over loan and deposit expansion. As it turns out, however, both methods result in a backward looking reserve requirement: the reserves that must be held today depend to a greater or lesser degree on deposits held in the fairly distant past. As banks cannot go backward in time, there is nothing they can do about historical deposits. Even if a short settlement period is provided to meet reserve requirements, the required portfolio adjustment could be too great—especially when one considers that many bank assets are not liquid. Hence, in practice, the central bank automatically provides an overdraft—the only question is over the “price”, that is, the discount rate charged on reserves. In many nations, such as Canada and Australia, the promise of an overdraft is explicitly given, hence, there can be no question about central bank accommodation.
A second, less satisfying, answer is often given, which is that the central bank must operate as a lender of last resort, meaning that it provides reserves in order to preserve stability of the financial system. The problem with this explanation is that while it is undoubtedly true, it applies to a different time dimension. The central bank accommodates the demand for reserves day-by-day, even hour-by-hour. It would presumably take some time before refusal to accommodate the demand for reserves would be likely to generate the conditions in which bank runs and financial crises begin to occur. Once these occurred, the central bank would surely enter as a lender of last resort, but this is a different matter from the daily “horizontal” accommodation.
The third explanation is that the central bank accommodates reserve demand in order to ensure an orderly payments system. This might be seen as being closely related to the lender of last resort argument, but I think it can be more plausibly applied to the time frame over which accommodation takes place. Par clearing among banks, and more importantly par clearing with the government, requires that banks have access to reserves for clearing. (Note that deposit insurance ultimately makes the government responsible for check clearing, in any event.)
The final argument is that because the demand for reserves is highly inelastic, and because the private sector cannot increase the supply, the overnight interest rate would be highly unstable without central bank accommodation. Hence, relative stability of overnight rates requires “horizontal” accommodation by the central bank. In practice, empirical evidence of relatively stable overnight interest rates over even very short periods of time supports the belief that the central bank is accommodating horizontally.
We can conclude that the overnight rate is exogenously administered by the central bank. Short-term sovereign debt is a very good substitute asset for overnight reserve lending, hence, its interest rate will closely track the overnight interbank rate. Longer-term sovereign rates will depend on expectations of future short term rates, largely determined by expectations of future monetary policy targets. Thus, we can take those to be mostly controlled by the central bank as well, as it could announce targets far into future and thereby affect the spectrum of rates on sovereign debt.