In the history of economic thought, a school of economic thought is a group of economic thinkers who share or shared a common perspective on the way economies work. While economists do not always fit into particular schools, particularly in modern times, classifying economists into schools of thought is common. Economic thought may be roughly divided into three phases: premodern (Greco-Roman, Indian, Persian, Islamic, and Imperial Chinese), early modern (mercantilist, physiocrats) and modern (beginning with Adam Smith and classical economics in the late 18th century, and Karl Marx and Friedrich Engels' Marxian economics in the mid 19th century). Systematic economic theory has been developed mainly since the beginning of what is termed the modern era. Currently, the great majority of economists follow an approach referred to as mainstream economics (sometimes called 'orthodox economics'). Economists generally specialize into either macroeconomics, broadly on the general scope of the economy as a whole, and microeconomics, on specific markets or actors. Within the macroeconomic mainstream in the United States, distinctions can be made between saltwater economists and the more laissez-faire ideas of freshwater economists. However, there is broad agreement on the importance of general equilibrium, the methodology related to models used for certain purposes (e.g. statistical models for forecasting, structural models for counterfactual analysis, etc.), and the importance of partial equilibrium models for analyzing specific factors important to the economy (e.g. banking). Some influential approaches of the past, such as the historical school of economics and institutional economics, have become defunct or have declined in influence, and are now considered heterodox approaches. Other longstanding heterodox schools of economic thought include Austrian economics and Marxian economics. Some more recent developments in economic thought such as feminist economics and ecological economics adapt and critique mainstream approaches with an emphasis on particular issues rather than developing as independent schools. Contemporary economic thought Mainstream economics Mainstream economics is distinguished in general economics from heterodox approaches and schools within economics. It begins with the premise that resources are scarce and that it is necessary to choose between competing alternatives. That is, economics deals with tradeoffs. With scarcity, choosing one alternative implies forgoing another alternative—the opportunity cost. The opportunity cost expresses an implicit relationship between competing alternatives. Such costs, considered as prices in a market economy, are used for analysis of economic efficiency or for predicting responses to disturbances in a market. In a planned economy comparable shadow price relations