The study of economics is driven by theories of economic behavior and economic performance, which have developed along the lines of the classical ideas, the Marxist idea, or a combination of both. In the process, various models were developed, each trying to explain such economic phenomena as wealth creation, value, prices, and growth from a separate intellectual and cultural setting, each considering certain variables and relationships more important than others. Within the aforementioned historical framework, economics has followed a trajectory that is characterized by a multiplicity of doctrines and schools of thought, usually identifiable with a thinker or thinkers whose ideas and theories form the foundation of the doctrine.
Classical economics.

Classical economic doctrine descended from Adam Smith and developed in the nineteenth century. It asserts that the power of the market system, if left alone, will ensure full employment of economic resources. Classical economists believed that although occasional deviations from full employment result from economic and political events, automatic adjustments in market prices, wages, and interest rates will restore the economy to full employment. The philosophical foundation of classical economics was provided by John Locke's (1632–1704) conception of the natural order, while the economic foundation was based on Adam Smith's theory of self-interest and Jean-Baptiste Say's (1767–1832) law of the equality of market demand and supply.

Classical economic theory is founded on two maxims. First, it presupposes that each individual maximizes his or her preference function under some constraints, where preferences and constraints are considered as given. Second, it presupposes the existence of interdependencies—expressed in the markets—between the actions of all individuals. Under the assumption of perfect and pure competition, these two features will determine resource allocation and income distribution. That is, they will regulate demand and supply, allocation of production, and the optimization of social organization.

Led by Adam Smith and David Ricardo with the support of Jean-Baptiste Say and Thomas Robert Malthus (1766–1834), the classical economists believed in Smith's invisible hand, self-interest, and a self-regulating economic system, as well as in the development of monetary institutions, capital accumulation based on surplus production, and free trade. They also believed in division of labor, the law of diminishing returns, and the ability of the economy to self-adjust in a laissez-faire system devoid of government intervention. The circular flow of the classical model indicates that wages may deviate, but will eventually return to their natural rate of subsistence.
Marxist economics.

Because of the social cost of capitalism as proposed by classical economics and the industrial revolution, socialist thought emerged within the classical liberal thought. To address the problems of classical capitalist economics, especially what he perceived as the neglect of history, Karl Marx (1818–1883), a German economic, social, and political philosopher, in his famous book titled Das Kapital or Capital (1867–1894) advanced his doctrine of dialectical materialism. Marx's dialectics was a dynamic system in which societies would evolve from primitive society to feudalism to capitalism to socialism and to communism. The basis of Marx's dialectical materialism was the application of history derived from Georg Wilhelm Friedrich Hegel (1770–1831), which maintained that history proceeds linearly by the triad of forces or dialectics called thesis, antithesis, and synthesis. This transition, in Marx's view, will result from changes in the ruling and the oppressed classes and their relationship with each other. He then envisaged conflict between forces of production, organization of production, relations of production, and societal thinking and ideology.

Marx predicts capitalist cycles that will ultimately lead to the collapse of capitalism. According to him, these cycles will be characterized by a reserve army of the unemployed, falling rate of profits, business crises, increasing concentration of industry into a few hands, and mounting misery and alienation of the proletariat. Whereas Adam Smith and David Ricardo had argued that the rational and calculating capitalists in following their self-interest promote social good, Marx argued that in rationally and purposefully pursuing their economic advantage, the capitalists will sow the seeds of their own destruction.

The economic thinking or school of economic thought that originated from Marx became known as Marxism. As the chief theorist of modern socialism and communism, Marx advocated fundamental revolution in society because of what he saw as the inherent exploitation of labor and economic injustice in the capitalist system. Marxist ideas were adopted as the political and economic systems in the former Soviet Union, China, Cuba, North Korea, and other parts of the world.

The neo-Marxist doctrines apply both the Marxist historical dimension and dialectics in their explanation of economic relationships, behavior, and outcome. For instance, the dependency theory articulates the need for the developing regions in Africa, Latin America, and Asia to rid themselves of their endemic dependence on more advanced countries. The dependency school believes that international links between developing (periphery) and industrialized (center) countries constitute a barrier to development through trade and investment.
Neoclassical economics.

The period that followed Ricardo, especially from 1870 to 1900, was full of criticism of classical economic theory and the capitalist system by humanists and socialists. The period was also characterized by the questioning of the classical assumption that laissez-faire was an ideal government policy and the eventual demise of classical economic theory and the transition to neoclassical economics. This transition was neither spontaneous nor automatic, but it was critical for the professionalization of economics.
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Neoclassical economics is attributed with integrating the original classical cost of production theory with utility in a bid to explain commodity and factor prices and the allocation of resources using marginal analysis. Although David Ricardo provided the methodological rudiments of neoclassical economics through his move away from contextual analysis to more abstract deductive analysis, Alfred Marshall (1842–1924) was regarded as the father of neoclassicism and was credited with introducing such concepts as supply and demand, price-elasticity of demand, marginal utility, and costs of production.

Neoclassical or marginalist economic theories emphasized use value and demand and supply as determinants of exchange value. Likewise neoclassicals, William Stanley Jevons (1835–1882) in England; Karl Menger (1840–1925) in Austria; and Léon Walras (1834–1910) in Switzerland, independently developed and highlighted the role of marginal utility (and individual utility maximization), as opposed to cost of production, as the key to the problem of exchange valuation. Neoclassical models assume that everyone has free access to information they require for decision making. This assumption made it possible to reduce decision making to a mechanical application of mathematical rules for optimization. Hence, in the neoclassical view, people's initial ability to maximize the value of output will, in turn, affect productivity and determine allocation of resources and income distribution. Neoclassical economics is grounded in the rejection of Marxist economics and on the belief that the market system will ensure a fair and just allocation of resources and income distribution.

Since its emergence, neoclassical economics has become the dominant economic doctrine in the study and teaching of economics in the West, especially in the United States. A host of economic theories have emerged from neoclassical economics: neoclassical growth theory, neoclassical trade theory, neoclassical theory of production, and so on. In the neoclassical growth theory, the determinants of output growth are technology, labor, and capital. The neoclassical growth theory stresses the importance of savings and capital accumulation together with exogenously determined technical progress as the sources of economic growth. If savings are larger, then capital per worker will grow, leading to rising income per capita and vice versa. The neoclassical thinking can be expressed as the Solow-Swan model of the production function type Y F (N, K) which is expanded to ΔY/Y = ΔA/A + ΔN/N + ΔK/K where Y represents total output, N and K represent the inputs of labor and capital, and A represents the productivity of capital and labor, and ΔY/Y, ΔA/A, ΔN/N, and K / K represent changes in these variables, respectively.

The Solow-Swan model asserts that because of the diminishing marginal product of inputs, sustained growth is possible only through technological change. The notion of diminishing marginal product is rooted in the belief that as more inputs are used to produce additional output under a fixed technology and fixed resource base, additional output per unit of input will decline (diminishing marginal product). This belief in the stationary state and diminishing marginal product led neoclassical economics to believe in the possibility of worldwide convergence of growth.

Known also as the neoliberal theory, neoclassical economics asserts that free movement of goods (free trade), services, and capital unimpeded by government regulation will lead to rapid economic growth. This, in the neoclassical view, will increase global output and international efficiency because the gains from division of labor according to comparative advantage and specialization will improve overall welfare. Even modern trade models (such as the Hecksche-Ohlin) are based on the neoclassical trade theory, which assumes perfect competition and concludes that trade generally improves welfare by improving the allocation of factors of production across sectors of the economy.
Rational expectation.

Rational expectation is the economic doctrine that emerged in the 1970s that asserts that people collect relevant information about the economy and behave rationally—that is, they weigh costs and benefits of actions and decisions. Rational expectation economics believes that because people act in response to their expectations, public policy will be offset by their action. Also known as the "new classical economics," the rational expectation doctrine believes that markets are highly competitive and prices adjust to changes in aggregate demand. The extent to which people are actually well informed is questionable and prices tend to be sticky or inflexible in a downward direction because once they go up, prices rarely come down. In the rational expectation doctrine, expansionary policies will increase inflation without increasing employment because economic actors—households and businesses—acting in a rational manner will anticipate inflation and act in a manner that will cause prices and wages to rise.
Monetarism.

Like rational expectations theory, monetarism represents a modern form of classical theory that believes in laissez-faire and in the flexibility of wages and prices. Like the classical theorists before them, they believe that government should stay out of economic stabilization since, in their view, markets are competitive with a high degree of macroeconomic stability. Such policies as expansionary monetary policy will, in their view, only lead to price instability. The U.S. economist Milton Friedman, who received the Nobel Prize in 1976, is widely regarded as the leader of the Chicago school of monetary economics.
Institutionalism.

Institutional economics focuses mainly on how institutions evolve and change and how these changes affect economic systems, economic performance, or outcomes. Both Frederick Hayek and Ronald Coase, major contributors to the Institutionalist School in the tradition of Karl Marx and Joseph Schumpeter, look at how institutions emerge. Hayek examines the temporal evolution and transformation of economic institutions and concludes that institutions result from human action. Hence, he suggests the existence of a spontaneous order in which workable institutions survive while nonworkable ones disappear. Coase believes that institutions are created according to rational economic logic when transaction costs are too high. Other notable contributors to institutionalism include Thorstein Veblen, Clarence Ayers, Gunnar Myrdal, John R. Commons, Wesley Cair Mitchell, and John Kenneth Galbraith.

The New Institutionalism, represented mostly by Douglas North, Gordon Tullock, and Mancur Olson, uses the classical notions of rationality and self-interest to explain the evolution and economic impact of institutions. It considers such issues as property rights, rent-seeking, and distributional coalitions and argues that institutional transformation can be explained in terms of changes in property rights, transaction costs, and information asymmetries.