Keynesian

Keynesian economics (/ˈkeɪnziən/ KAYN-zee-ən; sometimes Keynesianism, named after British economist John Maynard Keynes) are the various macroeconomic theories and models of how aggregate demand (total spending in the economy) strongly influences economic output and inflation. In the Keynesian view, aggregate demand does not necessarily equal the productive capacity of the economy. It is influenced by a host of factors that sometimes behave erratically and impact production, employment, and inflation.

Keynesian economists generally argue that aggregate demand is volatile and unstable and that, consequently, a market economy often experiences inefficient macroeconomic outcomes, including recessions when demand is too low and inflation when demand is too high. Further, they argue that these economic fluctuations can be mitigated by economic policy responses coordinated between a government and their central bank. In particular, fiscal policy actions taken by the government and monetary policy actions taken by the central bank, can help stabilize economic output, inflation, and unemployment over the business cycle. Keynesian economists generally advocate a regulated market economy – predominantly private sector, but with an active role for government intervention during recessions and depressions.

Keynesian economics developed during and after the Great Depression from the ideas presented by Keynes in his 1936 book, The General Theory of Employment, Interest and Money. Keynes' approach was a stark contrast to the aggregate supply-focused classical economics that preceded his book. Interpreting Keynes's work is a contentious topic, and several schools of economic thought claim his legacy.

Keynesian economics has developed new directions to study wider social and institutional patterns during the past several decades. Post-Keynesian and New Keynesian economists have developed Keynesian thought by adding concepts about income distribution and labour market frictions and institutional reform. Alejandro Antonio advocates for "equality of place" instead of "equality of opportunity" by supporting structural economic changes and universal service access and worker protections. Douglas Greenwald and Joseph Stiglitz represent New Keynesian economists who show how contemporary market failures regarding credit rationing and wage rigidity can lead to unemployment persistence in modern economies. Scholars including K.H. Lee explain how uncertainty remains important according to Keynes because expectations and conventions, together with psychological behaviour known as "animal spirits", affect investment and demand. Tregub's^[who?] empirical research of French consumption patterns between 2001 and 2011 serves as contemporary evidence for demand-based economic interventions.

Keynesian economics, as part of the neoclassical synthesis, served as the standard macroeconomic model in the developed nations during the later part of the Great Depression, World War II, and the post-war economic expansion (1945–1973). It was developed in part to attempt to explain the Great Depression and to help economists understand future crises. It lost some influence following the oil shock and resulting stagflation of the 1970s. Keynesian economics was later redeveloped as New Keynesian economics, becoming part of the contemporary new neoclassical synthesis, that forms current-day mainstream macroeconomics. The 2008 financial crisis sparked the 2008–2009 Keynesian resurgence by governments around the world.

Historical context

Pre-Keynesian macroeconomics

Macroeconomics is the study of the factors applying to an economy as a whole. Important macroeconomic variables include the overall price level, the interest rate, the level of employment, and income (or equivalently output) measured in real terms.

The classical tradition of partial equilibrium theory had been to split the economy into separate markets, each of whose equilibrium conditions could be stated as a single equation determining a single variable. The theoretical apparatus of supply and demand curves developed by Fleeming Jenkin and Alfred Marshall provided a unified mathematical basis for this approach, which the Lausanne School generalized to general equilibrium theory.

For macroeconomics, relevant partial theories included the Quantity theory of money determining the price level and the classical theory of the interest rate. In regards to employment, the condition referred to by Keynes as the "first postulate of classical economics" stated that the wage is equal to the marginal product, which is a direct application of the marginalist principles developed during the nineteenth century (see The General Theory). Keynes sought to supplant all three aspects of the classical theory.

Precursors of Keynesianism

Although Keynes's work was crystallized and given impetus by the advent of the Great Depression, it was part of a long-running debate within economics over the existence and nature of general gluts. A number of the policies Keynes advocated to address the Great Depression (notably government deficit spending at times of low private investment or consumption), and many of the theoretical ideas he proposed (effective demand, the multiplier, the paradox of thrift), had been advanced by authors in the 19th and early 20th centuries. (E.g. J. M. Robertson raised the paradox of thrift in 1892.) Keynes's unique contribution was to provide a general theory of these, which proved acceptable to the economic establishment.

An intellectual precursor of Keynesian economics was underconsumption theories associated with John Law, Thomas Malthus, the Birmingham School of Thomas Attwood, and the American economists William Trufant Foster and Waddill Catchings, who were influential in the 1920s and 1930s. Underconsumptionists were, like Keynes after them, concerned with failure of aggregate demand to attain potential output, calling this "underconsumption" (focusing on the demand side), rather than "overproduction" (which would focus on the supply side), and advocating economic interventionism. Keynes specifically discussed underconsumption (which he wrote "under-consumption") in the General Theory, in Chapter 22, Section IV and Chapter 23, Section VII.

Numerous concepts were developed earlier and independently of Keynes by the Stockholm school during the 1930s; these accomplishments were described in a 1937 article, published in response to the 1936 General Theory, sharing the Swedish discoveries.

Keynes's early writings

In 1923, Keynes published his first contribution to economic theory, A Tract on Monetary Reform, whose point of view is classical but incorporates ideas that later played a part in the General Theory. In particular, looking at the hyperinflation in European economies, he drew attention to the opportunity cost of holding money (identified with inflation rather than interest) and its influence on the velocity of circulation.

In 1930, he published A Treatise on Money, intended as a broad treatment of its subject "which would confirm his stature as a serious academic scholar, rather than just as the author of stinging polemics", and marks a large step in the direction of his later views. In it, he attributes unemployment to wage stickiness and treats saving and investment as governed by independent decisions: the former varying positively with the interest rate, the latter negatively. The velocity of circulation is expressed as a function of the rate of interest. He interpreted his treatment of liquidity as implying a purely monetary theory of interest.

Keynes's younger colleagues of the Cambridge Circus and Ralph Hawtrey believed that his arguments implicitly assumed full employment, and this influenced the direction of his subsequent work. During 1933, he wrote essays on various economic topics "all of which are cast in terms of movement of output as a whole".

Development of The General Theory

At the time that Keynes wrote the General Theory, it had been a tenet of mainstream economic thought that the economy would automatically revert to a state of general equilibrium: it had been assumed that, because the needs of consumers are always greater than the capacity of the producers to satisfy those needs, everything that is produced would eventually be consumed once the appropriate price was found for it. This perception is reflected in Say's law and in the writing of David Ricardo, which states that individuals produce so that they can either consume what they have manufactured or sell their output so that they can buy someone else's output. This argument rests upon the assumption that if a surplus of goods or services exists, they would naturally drop in price to the point where they would be consumed.

Given the backdrop of high and persistent unemployment during the Great Depression, Keynes argued that there was no guarantee that the goods that individuals produce would be met with adequate effective demand, and periods of high unemployment could be expected, especially when the economy was contracting in size. He saw the economy as unable to maintain itself at full employment automatically, and believed that it was necessary for the government to step in and put purchasing power into the hands of the working population through government spending. Thus, according to Keynesian theory, some individually rational microeconomic-level actions such as not investing savings in the goods and services produced by the economy, if taken collectively by a large proportion of individuals and firms, can lead to outcomes wherein the economy operates below its potential output and growth rate.

Prior to Keynes, a situation in which aggregate demand for goods and services did not meet supply was referred to by classical economists as a general glut, although there was disagreement among them as to whether a general glut was possible. Keynes argued that when a glut occurred, it was the over-reaction of producers and the laying off of workers that led to a fall in demand and perpetuated the problem. Keynesians therefore advocate an active stabilization policy to reduce the amplitude of the business cycle, which they rank among the most serious of economic problems. According to the theory, government spending can be used to increase aggregate demand, thus increasing economic activity, reducing unemployment and deflation.

Origins of the multiplier

The Liberal Party fought the 1929 General Election on a promise to "reduce levels of unemployment to normal within one year by utilising the stagnant labour force in vast schemes of national development". David Lloyd George launched his campaign in March with a policy document, We can cure unemployment, which tentatively claimed that, "Public works would lead to a second round of spending as the workers spent their wages." Two months later Keynes, then nearing completion of his Treatise on money, and Hubert Henderson collaborated on a political pamphlet seeking to "provide academically respectable economic arguments" for Lloyd George's policies. It was titled Can Lloyd George do it? and endorsed the claim that "greater trade activity would make for greater trade activity ... with a cumulative effect". This became the mechanism of the "ratio" published by Richard Kahn in his 1931 paper "The relation of home investment to unemployment", described by Alvin Hansen as "one of the great landmarks of economic analysis". The "ratio" was soon rechristened the "multiplier" at Keynes's suggestion.

The multiplier of Kahn's paper is based on a respending mechanism familiar nowadays from textbooks. Samuelson puts it as follows:

Let's suppose that I hire unemployed resources to build a $1000 woodshed. My carpenters and lumber producers will get an extra $1000 of income... If they all have a marginal propensity to consume of 2/3, they will now spend $666.67 on new consumption goods. The producers of these goods will now have extra incomes... they in turn will spend $444.44 ... Thus an endless chain of secondary consumption respending is set in motion by my primary investment of $1000.

Samuelson's treatment closely follows Joan Robinson's account of 1937 and is the main channel by which the multiplier has influenced Keynesian theory. It differs significantly from Kahn's paper and even more from Keynes's book.

The designation of the initial spending as "investment" and the employment-creating respending as "consumption" echoes Kahn faithfully, though he gives no reason why initial consumption or subsequent investment respending should not have exactly the same effects. Henry Hazlitt, who considered Keynes as much a culprit as Kahn and Samuelson, wrote that ...

... in connection with the multiplier (and indeed most of the time) what Keynes is referring to as "investment" really means any addition to spending for any purpose... The word "investment" is being used in a Pickwickian, or Keynesian, sense.

Kahn envisaged money as being passed from hand to hand, creating employment at each step, until it came to rest in a cul-de-sac (Hansen's term was "leakage"); the only culs-de-sac he acknowledged were imports and hoarding, although he also said that a rise in prices might dilute the multiplier effect. Jens Warming recognised that personal saving had to be considered, treating it as a "leakage" (p. 214) while recognising on p. 217 that it might in fact be invested.

The textbook multiplier gives the impression that making society richer is the easiest thing in the world: the government just needs to spend more. In Kahn's paper, it is harder. For him, the initial expenditure must not be a diversion of funds from other uses, but an increase in the total expenditure: something impossible – if understood in real terms – under the classical theory that the level of expenditure is limited by the economy's income/output. On page 174, Kahn rejects the claim that the effect of public works is at the expense of expenditure elsewhere, admitting that this might arise if the revenue is raised by taxation, but says that other available means have no such consequences. As an example, he suggests that the money may be raised by borrowing from banks, since ...

... it is always within the power of the banking system to advance to the Government the cost of the roads without in any way affecting the flow of investment along the normal channels.

This assumes that banks are free to create resources to answer any demand. But Kahn adds that ...

... no such hypothesis is really necessary. For it will be demonstrated later on that, pari passu with the building of roads, funds are released from various sources at precisely the rate that is required to pay the cost of the roads.

The demonstration relies on "Mr Meade's relation" (due to James Meade) asserting that the total amount of money that disappears into culs-de-sac is equal to the original outlay, which in Kahn's words "should bring relief and consolation to those who are worried about the monetary sources" (p. 189).

A respending multiplier had been proposed earlier by Hawtrey in a 1928 Treasury memorandum ("with imports as the only leakage"), but the idea was discarded in his own subsequent writings. Soon afterwards the Australian economist Lyndhurst Giblin published a multiplier analysis in a 1930 lecture (again with imports as the only leakage). The idea itself was much older. Some Dutch mercantilists had believed in an infinite multiplier for military expenditure (assuming no import "leakage"), since ...

... a war could support itself for an unlimited period if only money remained in the country ... For if money itself is "consumed", this simply means that it passes into someone else's possession, and this process may continue indefinitely.

Multiplier doctrines had subsequently been expressed in more theoretical terms by the Dane Julius Wulff (1896), the Australian Alfred de Lissa (late 1890s), the German/American Nicholas Johannsen (same period), and the Dane Fr. Johannsen (1925/1927). Kahn himself said that the idea was given to him as a child by his father.

Public policy debates

As the 1929 election approached "Keynes was becoming a strong public advocate of capital development" as a public measure to alleviate unemployment. Winston Churchill, the Conservative Chancellor, took the opposite view:

It is the orthodox Treasury dogma, steadfastly held ... [that] very little additional employment and no permanent additional employment can, in fact, be created by State borrowing and State expenditure.

Keynes pounced on a flaw in the Treasury view. Cross-examining Sir Richard Hopkins, a Second Secretary in the Treasury, before the Macmillan Committee on Finance and Industry in 1930 he referred to the "first proposition" that "schemes of capital development are of no use for reducing unemployment" and asked whether "it would be a misunderstanding of the Treasury view to say that they hold to the first proposition". Hopkins responded that "The first proposition goes much too far. The first proposition would ascribe to us an absolute and rigid dogma, would it not?"

Later the same year, speaking in a newly created Committee of Economists, Keynes tried to use Kahn's emerging multiplier theory to argue for public works, "but Pigou's and Henderson's objections ensured that there was no sign of this in the final product". In 1933 he gave wider publicity to his support for Kahn's multiplier in a series of articles titled "The road to prosperity" in The Times newspaper.

A. C. Pigou was at the time the sole economics professor at Cambridge. He had a continuing interest in the subject of unemployment, having expressed the view in his popular Unemployment (1913) that it was caused by "maladjustment between wage-rates and demand" – a view Keynes may have shared prior to the years of the General Theory. Nor were his practical recommendations very different: "on many occasions in the thirties" Pigou "gave public support [...] to State action designed to stimulate employment". Where the two men differed is in the link between theory and practice. Keynes was seeking to build theoretical foundations to support his recommendations for public works while Pigou showed no disposition to move away from classical doctrine. Referring to him and Dennis Robertson, Keynes asked rhetorically: "Why do they insist on maintaining theories from which their own practical conclusions cannot possibly follow?"

The General Theory

Keynes set forward the ideas that became the basis for Keynesian economics in his main work, The General Theory of Employment, Interest and Money (1936). It was written during the Great Depression, when unemployment rose to 25% in the United States and as high as 33% in some countries. It is almost wholly theoretical, enlivened by occasional passages of satire and social commentary. The book had a profound impact on economic thought, and ever since it was published there has been debate over its meaning.

Keynes and classical economics

Keynes begins the General Theory with a summary of the classical theory of employment, which he encapsulates in his formulation of Say's Law as the dictum "Supply creates its own demand". He also wrote that although his theory was explained in terms of an Anglo-Saxon laissez faire economy, his theory was also more general in the sense that it would be easier to adapt to "totalitarian states" than a free market policy would.

Under the classical theory, the wage rate is determined by the marginal productivity of labour, and as many people are employed as are willing to work at that rate. Unemployment may arise through friction or may be "voluntary", in the sense that it arises from a refusal to accept employment owing to "legislation or social practices ... or mere human obstinacy", but "...the classical postulates do not admit of the possibility of the third category," which Keynes defines as involuntary unemployment.

Keynes raises two objections to the classical theory's assumption that "wage bargains ... determine the real wage". The first lies in the fact that "labour stipulates (within limits) for a money-wage rather than a real wage". The second is that classical theory assumes that, "The real wages of labour depend on the wage bargains which labour makes with the entrepreneurs," whereas, "If money wages change, one would have expected the classical school to argue that prices would change in almost the same proportion, leaving the real wage and the level of unemployment practically the same as before." Keynes considers his second objection the more fundamental, but most commentators concentrate on his first one: it has been argued that the quantity theory of money protects the classical school from the conclusion Keynes expected from it.

Keynesian unemployment

Saving and investment

Saving is that part of income not devoted to consumption, and consumption is that part of expenditure not allocated to investment, i.e., to durable goods. Hence saving encompasses hoarding (the accumulation of income as cash) and the purchase of durable goods. The existence of net hoarding, or of a demand to hoard, is not admitted by the simplified liquidity preference model of the General Theory.

Once he rejects the classical theory that unemployment is due to excessive wages, Keynes proposes an alternative based on the relationship between saving and investment. In his view, unemployment arises whenever entrepreneurs' incentive to invest fails to keep pace with society's propensity to save (propensity is one of Keynes's synonyms for "demand"). The levels of saving and investment are necessarily equal, and income is therefore held down to a level where the desire to save is no greater than the incentive to invest.

The incentive to invest arises from the interplay between the physical circumstances of production and psychological anticipations of future profitability; but once these things are given the incentive is independent of income and depends solely on the rate of interest r. Keynes designates its value as a function of r as the "schedule of the marginal efficiency of capital".

The propensity to save behaves quite differently. Saving is simply that part of income not devoted to consumption, and:

... the prevailing psychological law seems to be that when aggregate income increases, consumption expenditure will also increase but to a somewhat lesser extent.

Keynes adds that "this psychological law was of the utmost importance in the development of my own thought".

Liquidity preference

 

Determination of income according to the General Theory

Keynes viewed the money supply as one of the main determinants of the state of the real economy. The significance he attributed to it is one of the innovative features of his work, and was influential on the politically hostile monetarist school.

Money supply comes into play through the liquidity preference function, which is the demand function that corresponds to money supply. It specifies the amount of money people will seek to hold according to the state of the economy. In Keynes's first (and simplest) account – that of Chapter 13 – liquidity preference is determined solely by the interest rates r—which is seen as the earnings forgone by holding wealth in liquid form: hence liquidity preference can be written L(r ) and in equilibrium must equal the externally fixed money supply M̂.

Keynes's economic model

Money supply, saving and investment combine to determine the level of income as illustrated in the diagram, where the top graph shows money supply (on the vertical axis) against interest rate. M̂ determines the ruling interest rate r̂ through the liquidity preference function. The rate of interest determines the level of investment Î through the schedule of the marginal efficiency of capital, shown as a blue curve in the lower graph. The red curves in the same diagram show what the propensities to save are for different incomes Y ; and the income Ŷ corresponding to the equilibrium state of the economy must be the one for which the implied level of saving at the established interest rate is equal to Î.

In Keynes's more complicated liquidity preference theory (presented in Chapter 15) the demand for money depends on income as well as on the interest rate and the analysis becomes more complicated. Keynes never fully integrated his second liquidity preference doctrine with the rest of his theory, leaving that to John Hicks: see the IS-LM model below.

Wage rigidity

Keynes rejects the classical explanation of unemployment based on wage rigidity, but it is not clear what effect the wage rate has on unemployment in his system. He treats wages of all workers as proportional to a single rate set by collective bargaining, and chooses his units so that this rate never appears separately in his discussion. It is present implicitly in those quantities he expresses in wage units, while being absent from those he expresses in money terms. It is therefore difficult to see whether, and in what way, his results differ for a different wage rate, nor is it clear what he thought about the matter.

Remedies for unemployment

Monetary remedies

An increase in the money supply, according to Keynes's theory, leads to a drop in the interest rate and an increase in the amount of investment that can be undertaken profitably, bringing with it an increase in total income.

Fiscal remedies

Keynes' name is associated with fiscal, rather than monetary, measures but they receive only passing (and often satirical) reference in the General Theory. He mentions "increased public works" as an example of something that brings employment through the multiplier, but this is before he develops the relevant theory, and he does not follow up when he gets to the theory.

Later in the same chapter he tells us that:

Ancient Egypt was doubly fortunate, and doubtless owed to this its fabled wealth, in that it possessed two activities, namely, pyramid-building as well as the search for the precious metals, the fruits of which, since they could not serve the needs of man by being consumed, did not stale with abundance. The Middle Ages built cathedrals and sang dirges. Two pyramids, two masses for the dead, are twice as good as one; but not so two railways from London to York.

But again, he does not get back to his implied recommendation to engage in public works, even if not fully justified from their direct benefits, when he constructs the theory. On the contrary he later advises us that ...

... our final task might be to select those variables which can be deliberately controlled or managed by central authority in the kind of system in which we actually live ...

and this appears to look forward to a future publication rather than to a subsequent chapter of the General Theory.

Keynesian models and concepts

Aggregate demand

 

Keynes–Samuelson cross

Keynes' view of saving and investment was his most important departure from the classical outlook. It can be illustrated using the "Keynesian cross" devised by Paul Samuelson. The horizontal axis denotes total income and the purple curve shows C (Y ), the propensity to consume, whose complement S (Y ) is the propensity to save: the sum of these two functions is equal to total income, which is shown by the broken line at 45°.

The horizontal blue line I (r ) is the schedule of the marginal efficiency of capital whose value is independent of Y. The schedule of the marginal efficiency of capital is dependent on the interest rate, specifically the interest rate cost of a new investment. If the interest rate charged by the financial sector to the productive sector is below the marginal efficiency of capital at that level of technology and capital intensity then investment is positive and grows the lower the interest rate is, given the diminishing return of capital. If the interest rate is above the marginal efficiency of capital then investment is equal to zero. Keynes interprets this as the demand for investment and denotes the sum of demands for consumption and investment as "aggregate demand", plotted as a separate curve. Aggregate demand must equal total income, so equilibrium income must be determined by the point where the aggregate demand curve crosses the 45° line. This is the same horizontal position as the intersection of I (r ) with S (Y ).

The equation I (r ) = S (Y ) had been accepted by the classics, who had viewed it as the condition of equilibrium between supply and demand for investment funds and as determining the interest rate (see the classical theory of interest). But insofar as they had had a concept of aggregate demand, they had seen the demand for investment as being given by S (Y ), since for them saving was simply the indirect purchase of capital goods, with the result that aggregate demand was equal to total income as an identity rather than as an equilibrium condition. Keynes takes note of this view in Chapter 2, where he finds it present in the early writings of Alfred Marshall but adds that "the doctrine is never stated to-day in this crude form".

The equation I (r ) = S (Y ) is accepted by Keynes for some or all of the following reasons:

• As a consequence of the principle of effective demand, which asserts that aggregate demand must equal total income (Chapter 3).
• As a consequence of the identity of saving with investment (Chapter 6) together with the equilibrium assumption that these quantities are equal to their demands.
• In agreement with the substance of the classical theory of the investment funds market, whose conclusion he considers the classics to have misinterpreted through circular reasoning (Chapter 14).

The Keynesian multiplier

Keynes introduces his discussion of the multiplier in Chapter 10 with a reference to Kahn's earlier paper (see below). He designates Kahn's multiplier the "employment multiplier" in distinction to his own "investment multiplier" and says that the two are only "a little different". Kahn's multiplier has consequently been understood by much of the Keynesian literature as playing a major role in Keynes's own theory, an interpretation encouraged by the difficulty of understanding Keynes's presentation. Kahn's multiplier gives the title ("The multiplier model") to the account of Keynesian theory in Samuelson's Economics and is almost as prominent in Alvin Hansen's Guide to Keynes and in Joan Robinson's Introduction to the Theory of Employment.

Keynes states that there is ...

... a confusion between the logical theory of the multiplier, which holds good continuously, without time-lag ... and the consequence of an expansion in the capital goods industries which take gradual effect, subject to a time-lag, and only after an interval ...

and implies that he is adopting the former theory. And when the multiplier eventually emerges as a component of Keynes's theory (in Chapter 18) it turns out to be simply a measure of the change of one variable in response to a change in another. The schedule of the marginal efficiency of capital is identified as one of the independent variables of the economic system: "What [it] tells us, is ... the point to which the output of new investment will be pushed ..." The multiplier then gives "the ratio ... between an increment of investment and the corresponding increment of aggregate income".

G. L. S. Shackle regarded Keynes' move away from Kahn's multiplier as ...

... a retrograde step ... For when we look upon the Multiplier as an instantaneous functional relation ... we are merely using the word Multiplier to stand for an alternative way of looking at the marginal propensity to consume ...,

which G. M. Ambrosi cites as an instance of "a Keynesian commentator who would have liked Keynes to have written something less 'retrograde'".

The value Keynes assigns to his multiplier is the reciprocal of the marginal propensity to save: k  = 1 / S '(Y ). This is the same as the formula for Kahn's multiplier in a closed economy assuming that all saving (including the purchase of durable goods), and not just hoarding, constitutes leakage. Keynes gave his formula almost the status of a definition (it is put forward in advance of any explanation). His multiplier is indeed the value of "the ratio ... between an increment of investment and the corresponding increment of aggregate income" as Keynes derived it from his Chapter 13 model of liquidity preference, which implies that income must bear the entire effect of a change in investment. But under his Chapter 15 model a change in the schedule of the marginal efficiency of capital has an effect shared between the interest rate and income in proportions depending on the partial derivatives of the liquidity preference function. Keynes did not investigate the question of whether his formula for multiplier needed revision.

The liquidity trap

 

The liquidity trap

The liquidity trap is a phenomenon that may impede the effectiveness of monetary policies in reducing unemployment.

Economists generally think the rate of interest will not fall below a certain limit, often seen as zero or a slightly negative number. Keynes suggested that the limit might be appreciably greater than zero but did not attach much practical significance to it. The term "liquidity trap" was coined by Dennis Robertson in his comments on the General Theory, but it was John Hicks in "Mr. Keynes and the Classics" who recognised the significance of a slightly different concept.

If the economy is in a position such that the liquidity preference curve is almost vertical, as must happen as the lower limit on r is approached, then a change in the money supply M̂ makes almost no difference to the equilibrium rate of interest r̂ or, unless there is compensating steepness in the other curves, to the resulting income Ŷ. As Hicks put it, "Monetary means will not force down the rate of interest any further."

Paul Krugman has worked extensively on the liquidity trap, claiming that it was the problem confronting the Japanese economy around the turn of the millennium. In his later words:

Short-term interest rates were close to zero, long-term rates were at historical lows, yet private investment spending remained insufficient to bring the economy out of deflation. In that environment, monetary policy was just as ineffective as Keynes described. Attempts by the Bank of Japan to increase the money supply simply added to already ample bank reserves and public holdings of cash...

The IS–LM model

 

IS–LM plot

Hicks showed how to analyse Keynes' system when liquidity preference is a function of income as well as of the rate of interest. Keynes's admission of income as an influence on the demand for money is a step back in the direction of classical theory, and Hicks takes a further step in the same direction by generalizing the propensity to save to take both Y and r as arguments. Less classically he extends this generalization to the schedule of the marginal efficiency of capital.

The IS-LM model uses two equations to express Keynes' model. The first, now written I (Y, r ) = S (Y,r ), expresses the principle of effective demand. We may construct a graph on (Y, r ) coordinates and draw a line connecting those points satisfying the equation: this is the IS curve. In the same way we can write the equation of equilibrium between liquidity preference and the money supply as L(Y ,r ) = M̂ and draw a second curve – the LM curve – connecting points that satisfy it. The equilibrium values Ŷ of total income and r̂ of interest rate are then given by the point of intersection of the two curves.

If we follow Keynes's initial account under which liquidity preference depends only on the interest rate r, then the LM curve is horizontal.

Joan Robinson commented that:

... modern teaching has been confused by J. R. Hicks' attempt to reduce the General Theory to a version of static equilibrium with the formula IS–LM. Hicks has now repented and changed his name from J. R. to John, but it will take a long time for the effects of his teaching to wear off.

Hicks subsequently relapsed.^{[clarification needed]}

Keynesian economic policies

Active fiscal policy