interest, charge for the use of credit or money, usually figured as a percentage of the principal and computed annually. Simple interest is computed annually on the principal. Compound interest, paid by some savings banks, computes the interest on the principal as well as on any previous interest that has been added to the principal.

Such charges have been made since ancient times, and they early fell into disrepute. In Greece, Solon forbade selling men into slavery for unpaid interest. The Jews, the Christian Church, and Islam forbade interest charges, or usury, as it was called, among their own groups. The merchant princes of Italy and elsewhere evaded such restrictions, even though the medieval churchmen considered money barren, or unable to produce wealth. Gradually the distinction was made between low interest rates and high ones, which came to be known, and condemned, as usury. England in 1545 removed the prohibition on interest charges and fixed a legal maximum interest; other countries followed.

In modern economics, a number of different theories regarding interest have been influential. The classical theory of interest, developed by Adam Smith and David Ricardo and expanded by others in later years, posited the interest rate as the force which balanced savings with investment. Marxist economic theory argued against the classical view that saw interest rates as a function of natural market forces, contending instead that interest was purely exploitative, because no service was rendered and it benefited only the capitalist class.

Abstinence theory, developed by Nassau Senior and later expanded upon by Eugen Böhm-Bawerk's productivity theory, argued that interest was a reward for saving money (in an interest-earning bank account) rather than spending it on commodities. Greater returns were available to those who saved, and interest rates were the deciding force in saving or spending. Irving Fisher advanced productivity theory by adding human capital to the understanding of interest rates. He explored the willingness (or lack thereof) of individuals to give up their present income for a future income, which may be significantly greater, as an important factor in the decision to invest. John Maynard Keynes took a much different approach, arguing that interest rates were a sort of reward for giving up liquidity, and varying interest rates were the significant force in a decision to invest. This new model was fundamental to the understanding of fluctuating interest rates, stepping beyond the focus of classical economics on equilibrium rates.

In recent years, the problem of inflation has been the paramount issue for interest theory. In the United States, the individual states are responsible for setting a legal rate at which debts may be assessed if they have come due and remain unpaid, and for setting the maximum rate allowed in a contract. In 1981, when rates soared to record highs, many legislatures increased or abolished such maximum rates in order to attract lending and credit card businesses and the potential employment they could offer residents. In Great Britain legal interest rates are not fixed by the government, but courts can determine whether a given rate is injurious.

High interest rates can dampen the economy by making it more difficult for consumers, businesses, and home buyers to secure loans, as happened in 1981 when the prime rate—the rate that banks charge their best customers—climbed past 20%. Economists differed over the causes of such extraordinary rates, but inflationary expectations, federal budget deficits, and the restrictive monetary policies of the Federal Reserve System were important factors. Interest rates fell in the latter half of the 1980s and stayed low into the 2000s.

In 2001–3, during recession and subsequent slow growth, the Federal Reserve lowered its short-term rates to levels (as low as 1%) not seen since the 1960s and late 1950s, but the low rates produced the desired economic growth only gradually. In mid-2004 the Federal Reserve began steadily raising rates until mid-2006, when the short-term rate reached 5.25%. When problems with some securitized mortgages cascaded through the economy, making obtaining loans and credit increasingly difficult and expensive and driving the economy into recession, the Federal Reserve again dramatically lowered its target short-term interest rate, to 0.25%, from Sept., 2007 to Dec., 2008. It held the target rate there as the economy slowly recovered until Dec., 2015, when it raised the rate to 0.5%. Although a year passed before it raised the rate to 0.75%, it raised the rate to 2.5% by Dec., 2018, then lowered it by 0.5% by Sept., 2019.

See D. Dewey, Modern Capital Theory (1965); D. Patinkin, Money, Interest, and Prices (1989); C. Rogers, Money, Interest and Capital (1989).

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