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In economics, effective demand (ED) in a market is the demand for a product or service which occurs when purchasers are constrained in a different market. It contrasts with notional demand, which is the demand that occurs when purchasers are not constrained in any other market. In the aggregated market for goods in general, demand, notional or effective, is referred to as aggregate demand. The concept of effective supply parallels the concept of effective demand. The concept of effective demand or supply becomes relevant when markets do not continuously maintain equilibrium prices.   
One example involves spillovers from the labor market to the goods market. If there is labour market disequilibrium such that individuals cannot supply all the labor they want to supply, then the amount that they are able to supply will influence their demand for goods; the demand for goods, contingent on the constraint on the amount of labor that can be supplied, is their effective demand for goods. In contrast, if there were no labor market disequilibrium, individuals would simultaneously choose both their quantity of labor to supply and the quantity of goods to purchase, and the latter would be their notional demand for goods. In this example, the effective demand for goods would be less than the notional demand for goods.
Conversely, if there are goods market shortages, individuals may choose to supply less labor (and enjoy more leisure) than they would in the absence of goods market disequilibrium. The amount of labor they choose to supply, contingent on the constraint on the number of goods they can buy, is the effective supply of labor.
Another example involves spillovers from credit markets to the goods market. If there is credit rationing, some individuals are constrained in the number of funds they can borrow to finance goods purchases (including consumer durables and houses), so their effective demand for goods, as a function of this constraint, is less than their notional demand for goods (the amount they would buy if they could borrow all they want to).
Firms can also exhibit effective demands or supplies that differ from notional demands or supplies. They too can be credit constrained, resulting in their effective demand for goods such as physical capital differing from their notional demand. In addition, in a time of labor shortage, they are constrained in how much labor they can employ; therefore the number of goods they choose to supply at any potential goods price—their effective supply of goods—will be less than their notional supply. And if firms are constrained by excess supply in the goods market, limiting how much goods they can sell, then their effective demand for labor will be less than their notional demand for labor.
The excess demands in different markets can influence each other. The presence of excess demand in one market influences effective demand or supply in another market, which may influence the degree of disequilibrium in the latter market; in turn, the constraints imposed on participants in that market influence their effective demand or supply in the former market.
Classical economist David Ricardo embraced Say's Law, suggesting, in Keynes's formulation, that "supply creates its own demand". According to Say's Law, for every excess supply (glut) of goods in one market, there is a corresponding excess demand (shortage) in another. This theory suggests that a general glut can never be accompanied by inadequate demand for products on a macroeconomic level.  In the challenge of Say's Law, Thomas Malthus, Jean Charles Leonard de Sismondi and other 19th century economists argued that "effective demand" is the foundation of a stable economy.  Responding to the Great Depression of the 20th century, in the 1930s Michał Kalecki and John Maynard Keynes concurred with the latter theory, suggesting that "demand creates its own supply" and developing a comprehensive theory of effective demand.
According to Keynesian economics, weak demand results in unplanned accumulation of inventories, leading to diminished production and income, and increased unemployment. This triggers a multiplier effect which draws the economy toward underemployment equilibrium. By the same token, strong demand results in unplanned reduction of inventories, which tends to increase production, employment, and incomes. If entrepreneurs consider such trends sustainable, investments typically increase, thereby improving potential levels of production.
In the 1960s, Robert Clower and Axel Leijonhufvud did further work on effective demand, and in the 1970s Robert Barro and Herschel Grossman published a well-known model of spillover effects upon effective demand. 
Keynesian economics are the various macroeconomic theories and models of how aggregate demand strongly influences economic output and inflation. In the Keynesian view, aggregate demand does not necessarily equal the productive capacity of the economy. Instead, it is influenced by a host of factors – sometimes behaving erratically – affecting production, employment, and inflation.
Macroeconomics is a branch of economics that deals with the performance, structure, behavior, and decision-making of an economy as a whole. For example, using interest rates, taxes, and government spending to regulate an economy's growth and stability. This includes regional, national, and global economies.
In economics, general equilibrium theory attempts to explain the behavior of supply, demand, and prices in a whole economy with several or many interacting markets, by seeking to prove that the interaction of demand and supply will result in an overall general equilibrium. General equilibrium theory contrasts to the theory of partial equilibrium, which analyzes a specific part of an economy while its other factors are held constant. In general equilibrium, constant influences are considered to be noneconomic, therefore, resulting beyond the natural scope of economic analysis. The noneconomic influences is possible to be non-constant when the economic variables change, and the prediction accuracy may depend on the independence of the economic factors.
Post-Keynesian economics is a school of economic thought with its origins in The General Theory of John Maynard Keynes, with subsequent development influenced to a large degree by Michał Kalecki, Joan Robinson, Nicholas Kaldor, Sidney Weintraub, Paul Davidson, Piero Sraffa and Jan Kregel. Historian Robert Skidelsky argues that the post-Keynesian school has remained closest to the spirit of Keynes' original work. It is a heterodox approach to economics.
In economics, economic equilibrium is a situation in which economic forces such as supply and demand are balanced and in the absence of external influences the values of economic variables will not change. For example, in the standard text perfect competition, equilibrium occurs at the point at which quantity demanded and quantity supplied are equal.
In macroeconomics, aggregate demand (AD) or domestic final demand (DFD) is the total demand for final goods and services in an economy at a given time. It is often called effective demand, though at other times this term is distinguished. This is the demand for the gross domestic product of a country. It specifies the amount of goods and services that will be purchased at all possible price levels. Consumer spending, investment, corporate and government expenditure, and net exports make up the aggregate demand.
In classical economics, Say's law, or the law of markets, is the claim that the production of a product creates demand for another product by providing something of value which can be exchanged for that other product. So, production is the source of demand. In his principal work, A Treatise on Political Economy, Jean-Baptiste Say wrote: "A product is no sooner created, than it, from that instant, affords a market for other products to the full extent of its own value." And also, "As each of us can only purchase the productions of others with his own productions – as the value we can buy is equal to the value we can produce, the more men can produce, the more they will purchase."
Walras's law is a principle in general equilibrium theory asserting that budget constraints imply that the values of excess demand must sum to zero regardless of whether the prices are general equilibrium prices. That is:
In economics, aggregate behavior refers to economy-wide sums of individual behavior. It involves relationships between economic aggregates such as national income, government expenditure, and aggregate demand. For example, the consumption function is a relationship between aggregate demand for consumption and aggregate disposable income.
In macroeconomics, a general glut is an excess of supply in relation to demand, specifically, when there is more production in all fields of production in comparison with what resources are available to consume (purchase) said production. This exhibits itself in a general recession or depression, with high and persistent underutilization of resources, notably unemployment and idle factories. The Great Depression is often cited as an archetypal example of a general glut.
The neoclassical synthesis (NCS), neoclassical–Keynesian synthesis, or just neo-Keynesianism was a neoclassical economics academic movement and paradigm in economics that worked towards reconciling the macroeconomic thought of John Maynard Keynes in his book The General Theory of Employment, Interest and Money (1936). It was formulated most notably by John Hicks (1937), Franco Modigliani (1944), and Paul Samuelson (1948), who dominated economics in the post-war period and formed the mainstream of macroeconomic thought in the 1950s, 60s, and 70s.
Axel Leijonhufvud was a Swedish economist and professor emeritus at the University of California, Los Angeles (UCLA), and professor at the University of Trento, Italy. Leijonhufvud focused his studies on macroeconomic monetary theory. In his defining book On Keynesian Economics and the Economics of Keynes (1968) he focuses on a critique of the interpretation of Keynesian economic theory by Keynesian economists. He goes on to call the standard neoclassical synthesis interpretation of the Keynes' General Theory as having misunderstood and misinterpreted Keynes. In one of his papers, "Life Among the Econ" (1973), he takes a comical yet critical look at the inherent clannish nature of economists; the paper was considered a devastating takedown of economics and economists.
Involuntary unemployment occurs when a person is unemployed despite being willing to work at the prevailing wage. It is distinguished from voluntary unemployment, where a person refuses to work because their reservation wage is higher than the prevailing wage. In an economy with involuntary unemployment, there is a surplus of labor at the current real wage. This occurs when there is some force that prevents the real wage rate from decreasing to the real wage rate that would equilibrate supply and demand. Structural unemployment is also involuntary.
In economics, an excess supply, economic surplus market surplus or briefly surply is a situation in which the quantity of a good or service supplied is more than the quantity demanded, and the price is above the equilibrium level determined by supply and demand. That is, the quantity of the product that producers wish to sell exceeds the quantity that potential buyers are willing to buy at the prevailing price. It is the opposite of an economic shortage.
Macroeconomic theory has its origins in the study of business cycles and monetary theory. In general, early theorists believed monetary factors could not affect real factors such as real output. John Maynard Keynes attacked some of these "classical" theories and produced a general theory that described the whole economy in terms of aggregates rather than individual, microeconomic parts. Attempting to explain unemployment and recessions, he noticed the tendency for people and businesses to hoard cash and avoid investment during a recession. He argued that this invalidated the assumptions of classical economists who thought that markets always clear, leaving no surplus of goods and no willing labor left idle.
"Supply creates its own demand" is the formulation of Say's law. The rejection of this doctrine is a central component of The General Theory of Employment, Interest and Money (1936) and a central tenet of Keynesian economics. See Principle of effective demand, which is an affirmative form of the negation of Say's law.
Robert Wayne Clower was an American economist. He is credited with having largely created the field of stock-flow analysis in economics and with seminal works on the microfoundations of monetary theory and macroeconomics.
In macroeconomic theory, general disequilibrium is a situation in which some or all of the aggregated markets, such as the money market, the goods market, and the labor market, fail to clear because of price rigidities. In the 1960s and 1970s, economists such as Edmond Malinvaud, Robert Barro and Herschel Grossman, Axel Leijonhufvud, Robert Clower, and Jean-Pascal Benassy investigated how economic policy would impact an economy where prices did not adjust quickly to changes in supply and demand. The most notable case occurs when some external factor causes high levels of unemployment in an economy, leading to households consuming less and firms providing less employment, leading to a rationing of both goods and work hours. Studies of general disequilibrium have been considered the "height of the neoclassical synthesis" and an immediate precursor to the new Keynesian economics that followed the decline of the synthesis.
Disequilibrium macroeconomics is a tradition of research centered on the role of disequilibrium in economics. This approach is also known as non-Walrasian theory, equilibrium with rationing, the non-market clearing approach, and non-tâtonnement theory. Early work in the area was done by Don Patinkin, Robert W. Clower, and Axel Leijonhufvud. Their work was formalized into general disequilibrium models, which were very influential in the 1970s. American economists had mostly abandoned these models by the late 1970s, but French economists continued work in the tradition and developed fixprice models.
Marxism and Keynesianism is a method of understanding and comparing the works of influential economists John Maynard Keynes and Karl Marx. Both men's works has fostered respective schools of economic thought that have had significant influence in various academic circles as well as in influencing government policy of various states. Keynes' work found popularity in developed liberal economies following the Great Depression and World War II, most notably Franklin D. Roosevelt's New Deal in the United States in which strong industrial production was backed by strong unions and government support. Marx's work, with varying degrees of faithfulness, led the way to a number of socialist states, notably the Soviet Union and the People's Republic of China. The immense influence of both Marxian and Keynesian schools has led to numerous comparisons of the work of both economists along with synthesis of both schools.