In economics, Gresham's law is a monetary principle stating that "bad money drives out good". For example, if there are two forms of commodity money in circulation, which are accepted by law as having similar face value, the more valuable commodity will gradually disappear from circulation.
The law was named in 1860 by economist Henry Dunning Macleod after Sir Thomas Gresham (1519–1579), an English financier during the Tudor dynasty. Gresham had urged Queen Elizabeth to restore confidence in then-debased English currency. The concept was thoroughly defined in medieval Europe by Nicolaus Copernicus and known centuries earlier in classical Antiquity, the Middle East and China.
"Good money" and "bad money" Under Gresham's Law, "good money" is money that shows little difference between its nominal value (the face value of the coin) and its commodity value (the value of the metal of which it is made, often precious metals, such as gold or silver).
The price spread between face value and commodity value when it is minted is called seigniorage. As some coins do not circulate, remaining in the possession of coin collectors, this can increase demand for coinage.
On the other hand, "bad money" is money that has a commodity value considerably lower than its face value and is in circulation along with good money, where both forms are required to be accepted at equal value as legal tender.
In Gresham's day, bad money included any coin that had been debased. Debasement was often done by the issuing body, where less than the officially specified amount of precious metal was contained in an issue of coinage, usually by alloying it with a base metal. The public could also debase coins, usually by clipping or scraping off small portions of the precious metal, also known as "stemming" (reeded edges on coins were intended to make clipping evident). Other examples of bad money include counterfeit coins made from base metal. Today virtually all circulating coins are made from base metals, often the cheapest available, durable base metal, collectively these monies are known as fiat money. While virtually all contemporary coinage is composed solely of base metals, there have been periods during the 21st century in which the market value of some base metals, like copper, have been high enough that at least one common coin (the U.S. nickel) still maintained "good money" status.
In the case of clipped, scraped, or counterfeit coins, the commodity value was reduced by fraud, as the face value remains at the previous higher level. On the other hand, with a coinage debased by a government issuer, the commodity value of the coinage was often reduced quite openly, while the face value of the debased coins was held at the higher level by legal tender laws.
The old saying "a bad penny always turns up" is a colloquial recognition of Gresham's Law.
Theory
The law states that any circulating currency consisting of both "good" and "bad" money (both forms required to be accepted at equal value under legal tender law) quickly becomes dominated by the "bad" money. This is because people spending money will hand over the "bad" coins rather than the "good" ones, keeping the "good" ones for themselves. Legal tender laws act as a form of price control. In such a case, the intrinsically less valuable money is preferred in exchange, because people prefer to save the intrinsically more valuable money.
If a customer purchases an item which costs five pence and possesses several silver sixpence coins. Some of these coins are more debased, while others are less so – but legally, they are all mandated to be of equal value. The customer would prefer to retain the better coins, and so offers the shopkeeper the most debased one. In turn, the shopkeeper must give one penny in change, and has every reason to give the most debased penny. Thus, the coins that circulate in the transaction will tend to be of the most debased sort available to the parties.