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Underconsumption is a theory in economics that recessions and stagnation arise from an inadequate consumer demand, relative to the amount produced. In other words, there is a problem of overproduction and overinvestment during a demand crisis. The theory formed the basis for the development of Keynesian economics and the theory of aggregate demand after the 1930s. Underconsumption theory narrowly refers to heterodox economists in Britain in the 19th century, particularly from 1815 onwards, who advanced the theory of underconsumption and rejected classical economics in the form of Ricardian economics. The economists did not form a unified school, and their theories were rejected by mainstream economics of the time. Underconsumption is an old concept in economics that goes back to the 1598 French mercantilist text Les Trésors et richesses pour mettre l'Estat en splendeur (The Treasures and riches to put the State in splendor) by Barthélemy de Laffemas, if not earlier. The concept of underconsumption had been used repeatedly as part of the criticism of Say's Law until underconsumption theory was largely replaced by Keynesian economics which points to a more complete explanation of the failure of aggregate demand to attain potential output, i. e., the level of production corresponding to full employment. One of the early underconsumption theories says that because workers are paid a wage less than they produce, they cannot buy back as much as they produce. Thus, there will always be inadequate demand for the product.

In his book Underconsumption Theories from 1976, Michael Bleaney defined two main elements of classical (pre Keynesian) underconsumption theory. First, the only source of recessions, stagnation, and other aggregate demand failures was inadequate consumer demand. Second, a capitalist economy tends toward a state of persistent depression because of this. Thus, underconsumption is not seen as part of business cycles as much as (perhaps) the general economic environment in which they occur. Compare to the tendency of the rate of profit to fall, which has a similar belief in stagnation as the natural (stable) state, but which is otherwise distinct and in critical opposition to underconsumption theory.

Modern Keynesian economics has largely superseded underconsumption theories. Falling consumer demand need not cause a recession, since other parts of aggregate demand may rise to counteract this effect. These other elements are private fixed investment in factories, machines, and housing, government purchases of goods and services, and exports (net of imports). Further, few economists believe that persistent stagnation is the normal state toward which a capitalist economy tends. But it is possible in Keynesian economics that falling consumption (say, due to low and falling real wages) can cause a recession or deepening stagnation.

Underconsumption theory dates to the earlier economic theory of mercantilism, and an early history of underconsumptionism is given in Mercantilism by Eli Heckscher Underconsumption was a small part of mercantilist theory, in Heckscher's view, but was discussed by a number of authors. The earliest reference given was to Barthélemy de Laffemas, who in 1598 in The Treasures and riches to put the State in splendor "denounced the objectors to the use of French silks on the ground that all purchasers of French luxury goods created a livelihood for the poor, whereas the miser caused them to die in distress," an early form of the paradox of thrift. A number of other 17th century authors, English, German, and French, stated similar sentiments, which Heckscher summarizes as:
"the deep rooted belief in the utility of luxury and the evil of thrift. Thrift, in fact, was regarded as the cause of unemployment, and for two reasons: in the first place, because real income was believed to diminish by the amount of money which did not enter into exchange, and secondly, because saving was believed to withdraw money from circulation." The Fable of The Bees by Bernard Mandeville, of 1714, was credited by Keynes as the most popular exposition of underconsumptionism of its time, but it caused such an uproar, being seen as an attack against Christian virtues, specifically attacking temperance, that underconsumptionism was not mentioned in "respectable circles" for another century, until it was raised in the later Malthus.

Malthus devoted a chapter of Principles (1836) to underconsumption theory, which was rebutted by David Ricardo, in his Notes on Malthus, and which debate continued in private correspondence. Malthus was credited by Keynes as a predecessor for his views on effective demand and, other than Malthus, Keynes did not credit the existence of other proponents of underconsumption, stating instead that Ricardo "conquered" English economics. This is now understood to be false – other British proponents of underconsumption are now well established, but, as Keynes demonstrated, they were poorly documented, and by the 1930s not well known. Further, they did not form a unified school, but rather related heterodox ideas. The Birmingham School of economists argued an underconsumptionist theory from 1815, and some of the writings of the school's leading member Thomas Attwood contained formulations of the multiplier effect and an income expenditure model. In continental Europe, Jean Charles Léonard de Sismondi proposed underconsumption and overproduction as causes of the economic cycle, in his Nouveaux Principes d'économie politique (1819), in one of the earliest systematic treatments of economic cycles. Properly, Sismondi discussed periodic economic crises, while the notion of a cycle was devised by Charles Dunoyer in his reconciliation of Sismondi's work with classical economics. The multiplier dates to work in the 1890s by the Australian economist Alfred De Lissa, the Danish politician Julius Wulff, and the German American economist Nicholas Johannsen, Nicholas Johannsen also proposed a theory of effective demand in the 1890s. The paradox of thrift was stated in 1892 by John M. Robertson in his The Fallacy of Savings, and similar sentiments date to antiquity, in addition to the mercantilist statements cited above:

An underconsumption theory of the economic cycle was given by John A. Hobson in his Industrial System (1910). William Trufant Foster and Waddill Catchings developed a theory of underconsumption in the 1920s that became highly influential among policy makers. The argument was that governmental intervention, especially spending on public works programs, was essential to restore the balance between production and consumption. The theory strongly influenced Herbert Hoover and Franklin D. Roosevelt to engage in massive public works projects.

Today these ideas, regardless of provenance, are grouped in academia under the rubric of "Keynesian economics", due to Keynes's role in consolidating, elaborating, and popularizing them. Keynes himself specifically discussed underconsumption (which he wrote "under consumption") in The General Theory of Employment, Interest and Money.

The theory of underconsumption has been criticized by classical economists such as James Mill, Adam Smith who wrote "What is prudence in the conduct of every private family can scarce be folly in that of a great Kingdom," and on grounds of Christian morality. These criticisms were revised by Austrian economics.