The price–specie flow mechanism is a logical argument by David Hume (1711–1776) against the Mercantilist idea that a nation should strive for a positive balance of trade, or net exports. The argument considers the effects of international transactions in a gold standard, a system in which gold is the official means of international payments and each nation’s currency is in the form of gold itself or of paper currency fully convertible into gold.
Hume argued that when a country with a gold standard had a positive balance of trade, gold would flow into the country in the amount that the value of exports exceeds the value of imports. Conversely, when such a country had a negative balance of trade, gold would flow out of the country in the amount that the value of imports exceeds the value of exports. Consequently, in the absence of any offsetting actions by the central bank on the quantity of money in circulation (called sterilization), the money supply would rise in a country with a positive balance of trade and fall in a country with a negative balance of trade. Using a theory called the quantity theory of money, Hume argued that in countries where the quantity of money increases, inflation would set in and the prices of goods and services would tend to rise while in countries where the money supply decreases, deflation would occur as the prices of goods and services fell.
The higher prices would, in the countries with a positive balance of trade, cause exports to decrease and imports to increase, which will alter the balance of trade downwards towards a neutral balance. Inversely, in countries with a negative balance of trade, the lower prices would cause exports to increase and imports to decrease, which will heighten the balance of trade towards a neutral balance. These adjustments in the balance of trade will continue until the balance of trade equals zero in all countries involved in the exchange.
The price–specie flow mechanism can also be applied to a state's entire balance of payments, which accounts not only for the value of net exports and similar transactions (the current account), but also the financial account, which accounts for flows of financial assets across countries, and the capital account, which accounts for non-market and other special international transactions. But under a gold standard, transactions in the financial account would be conducted in gold or currency convertible into gold, which would also affect the quantity of money in circulation in each country.