One of the principal subfields of contemporary economics concerns money, which should not be surprising since one of the oldest, most widely accepted functions of government is control over this basic medium of exchange. The dramatic effects of changes in the quantity of money on the level of prices and the volume of economic activity were recognized and thoroughly analyzed in the 18th century. In the 19th century a tradition developed known as the “quantity theory of money,” which held that any change in the supply of money can only be absorbed by variations in the general level of prices (the purchasing power of money). In consequence, prices will tend to change proportionately with the quantity of money in circulation. Simply put, the quantity theory of money stated that inflation or deflation could be controlled by varying the quantity of money in circulation inversely with the level of prices.

One of the targets of Keynes’s attack on traditional thinking in his General Theory of Employment, Interest and Money (1935–36) was this quantity theory of money. Keynes asserted that the link between the money stock and the level of national income was weak and that the effect of the money supply on prices was virtually nil—at least in economies with heavy unemployment, such as those of the 1930s. He emphasized instead the importance of government budgetary and tax policy and direct control of investment. As a consequence of Keynes’s theory, economists came to regard monetary policy as more or less ineffective in controlling the volume of economic activity.

In the 1960s, however, there was a remarkable revival of the older view, at least among a small but growing school of American monetary economists led by Friedman. They argued that the effects of fiscal policy are unreliable unless the quantity of money is regulated at the same time. Basing their work on the old quantity theory of money, they tested the new version on a variety of data for different countries and time periods. They concluded that the quantity of money does matter. A Monetary History of the United States, 1867–1960, by Milton Friedman and Anna Schwartz (1963), which became the benchmark work of monetarism, criticized Keynesian fiscal measures along with all other attempts at fine-tuning the economy. With its emphasis on money supply, monetarism enjoyed an enormous vogue in the 1970s but faded by the 1990s as economists increasingly adopted an approach that combined the old Keynesian emphasis on fiscal policy with a new understanding of monetary policy.

The study of economic growth and development is not a single branch of economics but falls, in fact, into two quite different fields. The two fields—growth and development—employ different methods of analysis and address two distinct types of inquiry.

Development economics is easy to characterize as one of the three major subfields of economics, along with microeconomics and macroeconomics. More specifically, development economics resembles economic history in that it seeks to explain the changes that occur in economic systems over time.

The subject of economic growth is not so easy to characterize. Indeed, it is the most technically demanding field in the whole of modern economics, impossible to grasp for anyone who lacks a command of differential calculus. Its focus is the properties of equilibrium paths, rather than equilibrium states. In applying economic growth theory, one makes a model of the economy and puts it into motion, requiring that the time paths described by the variables be self-sustaining in the sense that they continue to be related to each other in certain characteristic ways. Then one can investigate the way economics might approach and reach these steady-state growth paths from given starting points. Beautiful and frequently surprising theorems have emerged from this experience, but as yet there are no really testable implications nor even definite insights into how economies grow.

Growth theory began with the investigations by Roy Harrod in England and Evsey Domar in the United States. Their independent work, joined in the Harrod-Domar model, is based on natural rates of growth and warranted rates of growth. Keynes had shown that new investment has a multiplier effect on income and that the increased income generates extra savings to match the extra investment, without which the higher income level could not be sustained. One may think of this as being repeated from period to period, remembering that investment, apart from raising income disproportionately, also generates the capacity to produce more output. This results in products that cannot be sold unless there is more demand—that is, more consumption and more investment. This is all there is to the model. It contains one behavioral condition: that people tend to save a certain proportion of extra income, a tendency that can be measured. It also contains one technical condition: that investment generates additional output, a fact that can be established. And it contains one equilibrium condition: that planned saving must equal planned investment in every period if the income level of the period is to be sustained. Given these three conditions, the model generates a time path of income and even indicates what will happen if income falls off the path.

More complex models have since been built, incorporating different saving ratios for different groups in the population, technical conditions for each industry, definite assumptions about the character of technical progress in the economy, monetary and financial equations, and much more. The new growth theory of the 1990s was labeled “endogenous growth theory” because it attempted to explain technical change as the result of profit-motivated research and development (R&D) expenditure by private firms. This was driven by competition along the lines of what Schumpeter called product innovations (as distinct from process innovations). In contrast to the Harrod-Domar model, which viewed growth as exogenous, or coming from outside variables, the endogenous theory emphasizes growth from within the system. This approach enjoyed, and still enjoys, an enormous vogue, partly because it seemed to offer governments a new means of promoting economic growth—namely, national innovation policies designed to stimulate more private and public R&D spending.

Taxation has been a concern of economists since the time of Ricardo. Much interest centres on determining who really pays a tax. If a corporation faced with a profits tax reacts by raising the prices it charges for goods and services, it might succeed in passing the tax on to the consumer. If, however, sales decline as a result of the rise in price, the firm may have to reduce production and lay off some of its workers, meaning that the tax burden has been passed along not only to consumers but to wage earners and shareholders as well.

This simple example shows how complex the so-called “tax incidence” may be. The literature of public finance in the 19th century was devoted to such problems, but Keynesian economics replaced the older emphasis on tax incidence with the analysis of the impact of government expenditures on the level of income and employment. It was some time, however, before economists realized that they lacked a theory of government expenditures—that is, a set of criteria for determining what activities should be supported by governments and what the relative expenditure on each should be. The field of public finance has since attempted to devise such criteria. Decisions on public expenditures have proved to be susceptible to much of the traditional analysis of microeconomics. New developments in the 1960s expanded on a technique known as cost-benefit analysis, which tries to appraise all of the economic costs and benefits, direct and indirect, of a particular activity so as to decide how to distribute a given public budget most effectively between different activities. This technique, first put forth by Jules Dupuit in the 19th century, has been applied to everything from the construction of hydroelectric dams to the control of tuberculosis. Its exponents hoped that the same type of analysis that had proved so fruitful in the past in analyzing individual choice would also succeed with problems of social choice.

Building upon 18th- and 19th-century mathematical studies of the voting process, Scottish economist Duncan Black brought a political dimension to cost-benefit studies. His book The Theory of Committees and Elections (1958) became the basis of public choice theory. As expressed in the book Calculus of Consent (1962) by American economists James Buchanan and Gordon Tullock, public choice theory applies the cost-benefit analysis seen in private decision making to political decision making. Politicians are conceived of as maximizing electoral votes in the same way that firms seek to maximize profits, while political parties are conceived of as organizing electoral support in the same way that firms organize themselves as cartels or power blocs to lobby governments on their behalf. Public choice challenged the notion, implicit in early public finance theory, that politicians always identify their own interest with that of the country as a whole.