and coin would be held as reserves—but there would instead be demand deposits equal to the
amount of currency. This can be illustrated in terms of the balance sheet of the banking sector,
which we assume consists of one bank. This bank has assets equal to the reserves it holds and
liabilities equal to the deposits of its customers. Under 100–percent–reserve banking, banks have
no influence on the money supply.
Fractional–Reserve Banking
In the real world, banks also make loans. Since banks do not anticipate that all their depositors
will want to withdraw all their money at once, they do not need to hold reserves equal to the
amount of deposits. Instead, they only hold a fraction of their deposits as reserves and loan out
the rest. This is known as fractional–reserve banking.
Suppose that, as before, the public deposits all its currency in the banking system. Thus,
there is no currency in the hands of the public. But now suppose that banks adopt a reserve–
deposit ratio (rr) of 20 percent, so that for every $1 of deposits, the banks hold $0.20 in reserves
Bank Capital, Leverage, and Capital Requirements
The model of the banking system presented above ignores the need for bank owners to have
some financial resources, known as bank capital, representing their equity stake in the bank. A
more realistic balance sheet for a bank would account for bank capital on the liability side and
would also include the debt issued by the bank in addition to deposits as liability items. The
asset side of the balance sheet would include bank investments in securities of the government
and private sector in addition to reserves and loans. Banks will allocate resources to these assets
require banks to hold sufficient capital, under what are known as capital requirements, to ensure
that they can pay off their depositors. Many banks ended up with too little capital during 2008
and 2009 as a result of losses on investments they had made in mortgage loans and mortgage–
backed securities. This capital shortage in turn led banks to reduce lending, contributing to the
downturn in the economy. To counteract this, the Treasury and Federal Reserve began to put
public funds into banks in an attempt to recapitalize the banking system.
4-3 How Central Banks Influence the Money Supply
Having described the functions of money, how it is measured, and the way in which the banking
system affects the amount of money in the economy, we now analyze how a central bank
influences the money supply. This role of the central bank is the key element of monetary policy.