Gresham's law

The classic statement concerns two kinds of money that are legally exchangeable at the same face value even though their market or metal values differ. If a full-weight coin and a debased coin both settle the same nominal payment, a person has an incentive to use the cheaper coin for ordinary transactions and retain or export the coin worth more as metal. The more valuable coin can consequently become scarce in domestic circulation. The outcome is not simply that lower-quality money always wins: it depends on the fixed legal exchange rate and on people being able to distinguish and choose between the monies.

A familiar historical setting is bimetallic coinage, in which a government fixed an exchange ratio between gold and silver. If that official ratio differed from the ratio in other markets, traders could profit by moving or exporting the metal that was undervalued domestically, leaving the overvalued metal to circulate. In the United States, for example, the legal gold-to-silver ratio from 1792 to 1834 differed from ratios in Europe; gold was then withdrawn from domestic circulation. The principle is named for Sir Thomas Gresham, a financial agent of Elizabeth I, but he was not its first observer. The term “Gresham's law” was proposed by economist H. D. Macleod in the nineteenth century, following Gresham's explanation of the principle in 1558.

The slogan is a useful description of some coinage systems, not an unconditional law of all monetary behaviour. If exchange rates can adjust, users may price or accept the different monies differently, changing the incentive to spend one and hoard another. Research by economists at the Federal Reserve Bank of Minneapolis has challenged the conventional account's status as a universal law and argues that historical cases need to be examined in their specific monetary arrangements. The Federal Reserve Bank of Cleveland likewise stresses that the principle has limits and that explanations differ. Thus, applying the term requires identifying the relevant legal valuation, market values, transaction rules and alternatives available to users.

The phrase is sometimes applied by analogy to settings beyond metal coinage, including payment systems in which one option displaces another. Such comparisons can be suggestive but are not automatically instances of the original monetary mechanism. The Australian Reserve Bank's discussion of retail payments, for example, uses the phrase to frame a case where a more expensive payment method gained share over cheaper methods, while explaining that the incentives and institutional details differ from the historical coin example. Careful use distinguishes the memorable shorthand from the specific economic explanation.