Core Theoretical Framework of Keynesian Economics

Keynesian Economics: Core Concepts and the IS-LM Framework Introduction Keynesian economics fundamentally changed how economists understand the macroeconomy. Rather than assuming the economy always adjusts to full employment, John Maynard Keynes argued that the economy can settle at equilibrium with persistent unemployment when aggregate demand falls short of potential output. Understanding Keynesian theory means grasping how aggregate demand determines income and employment, how monetary and fiscal policy affect this demand, and how the IS-LM model synthesizes these relationships. Aggregate Demand and Effective Demand Aggregate demand is the total spending on goods and services in the economy at a given price level. It consists of two main components: consumption spending and investment spending. Effective demand is Keynes's key concept: it is the level of aggregate demand that actually determines equilibrium output and employment. This is crucial because Keynes argued that the economy gravitates toward the level of output where aggregate demand equals aggregate supply—not necessarily toward full employment. In the Keynesian cross diagram, equilibrium occurs where the aggregate demand curve intersects the 45-degree line. This 45-degree line represents all points where output ($Y$) equals income (since total output generates total income). The intersection tells you the equilibrium level of output and employment. The key insight here is directional: when aggregate demand falls short of potential output, unemployment rises as firms produce less and hire fewer workers. Conversely, when aggregate demand exceeds potential output, inflation pressures build up as the economy overheats. The Multiplier Effect One of Keynes's most important discoveries is that changes in spending have amplified effects on total income—this is the multiplier effect. Suppose the government increases spending by $1 billion on new infrastructure. Workers hired for this project earn wages and spend some of that income on consumption. Their spending creates income for others, who in turn spend some of their income, and so on. The total increase in income exceeds the initial $1 billion. The size of the multiplier depends on the marginal propensity to consume (MPC), denoted $c$. This measures how much of each additional dollar of income households spend. The simple multiplier is: $$k = \frac{1}{1-c}$$ Equivalently, since the marginal propensity to save is $s = 1 - c$, we can write: $$k = \frac{1}{s}$$ Example: If $c = 0.8$ (households spend 80 cents of each dollar earned), then $k = \frac{1}{1-0.8} = 5$. A $1 billion increase in spending generates a $5 billion increase in total income. However, leakages reduce the multiplier's size. Imports, taxes, and savings all represent income that doesn't flow back into domestic consumption. When these leakages exist, the actual multiplier is smaller than the simple multiplier. This is why the multiplier is crucial for policy analysis: a small initial policy change can have large effects on income and employment throughout the economy. Liquidity Preference and Money Supply Keynes explained money demand through the concept of liquidity preference: the desire to hold wealth in the form of money rather than bonds or other assets. In its simplest form, liquidity preference depends only on the interest rate $r$, expressed as the function $L(r)$. Why does money demand depend on interest rates? The opportunity cost of holding money is the interest you could earn on a bond. When interest rates are high, holding money is expensive—you give up significant interest income. So money demand falls. When interest rates are low, the opportunity cost is small, so money demand rises. Equilibrium in the money market occurs when: $$L(r) = \hat{M}$$ where $\hat{M}$ is the exogenous (externally determined) money supply. This equation tells you the interest rate at which the quantity of money people want to hold equals the quantity available. How monetary policy works in Keynes's framework: If the central bank increases the money supply, there is an excess supply of money at the existing interest rate. People try to use excess money to buy bonds, driving bond prices up and interest rates down. The lower interest rate increases investment spending (since investment projects are more attractive at lower borrowing costs), which increases aggregate demand. Later, Keynes acknowledged that money demand also depends on income $Y$: $L(Y, r)$. Higher income increases the demand for money (you need more cash to make transactions). This addition is important for the IS-LM model. Saving, Investment, and the Marginal Efficiency of Capital Saving is the portion of income that households do not spend on consumption. It can be expressed as a function of income: $S(Y)$. Investment is spending on new capital goods (factories, equipment, buildings). The profitability of investment depends on expected returns and the cost of borrowing. Keynes represented this relationship through the schedule of the marginal efficiency of capital, denoted $I(r)$. This shows how much investment firms are willing to undertake at each interest rate: higher interest rates mean fewer investment projects are profitable, so investment falls. The capital market reaches equilibrium when: $$I(r) = S(Y)$$ This equation says that the amount firms want to invest at the current interest rate must equal the amount households want to save at the current income level. What happens out of equilibrium? If $I(r) > S(Y)$, investment exceeds saving—firms want to borrow more than households want to lend. This excess demand for loanable funds drives the interest rate up. As $r$ rises, investment demand falls and saving rises, until equilibrium is restored. The opposite occurs when $I(r) < S(Y)$. This investment-saving relationship will be central to understanding the IS curve in the IS-LM model. Wage Rigidity and Unemployment A critical difference between Keynesian and classical economics concerns wages. Classical economists believed wages are perfectly flexible: they adjust instantly to clear the labor market, ensuring full employment. Keynes rejected this assumption. In Keynes's view, wages are sticky (rigid downward). They are set by collective bargaining between unions and employers and adjust slowly. As a result, real wages do not adjust downward quickly enough when aggregate demand falls. This creates involuntary unemployment—workers willing to work at the prevailing wage cannot find jobs because firms lack the aggregate demand to hire them. Keynes made wages central to his analysis by expressing many variables in wage units (values measured relative to the wage rate). This made the wage rate implicit in his analysis rather than explicitly adjusting. The key point is that imperfect wage adjustment means the economy can settle at an equilibrium with unemployment, contradicting the classical full-employment assumption. The Liquidity Trap The liquidity trap is a critical special case in Keynesian theory. It occurs when the interest rate falls to a lower bound (typically zero), and the central bank cannot reduce it further through monetary policy. When the economy is in a liquidity trap, further increases in the money supply have no effect on the interest rate. Intuitively, when interest rates are already zero, holding extra money yields the same return as holding bonds (both zero), so there's no incentive to adjust your portfolio. New money is simply added to money holdings without changing interest rates. Why does this matter for policy? In a liquidity trap, monetary policy loses its effectiveness. The central bank cannot lower interest rates to stimulate investment and aggregate demand. Fiscal policy (government spending) becomes the primary policy tool to boost demand. John Hicks clarified the mechanism: when the liquidity preference curve becomes nearly vertical (extremely steep) at low interest rates, small changes in the money supply create large changes in interest rates on the steep part of the curve, but this effect vanishes as rates approach zero. The economy becomes stuck at low interest rates and low aggregate demand. <extrainfo> The liquidity trap was largely theoretical until the 2008 financial crisis and the COVID-19 pandemic, when major central banks around the world pushed interest rates to near-zero levels and encountered this constraint in practice. </extrainfo> The IS-LM Model The IS-LM model is the standard framework for presenting Keynesian macroeconomics. It translates Keynes's system into two equations and their graphical representation, showing how income and the interest rate are jointly determined. The Investment-Saving Curve (IS Curve) The IS curve represents all combinations of income $Y$ and interest rate $r$ at which the investment-saving equilibrium holds: $I(Y, r) = S(Y, r)$. The IS curve slopes downward because: When the interest rate falls, investment becomes more attractive (more projects are profitable), so $I(Y, r)$ increases To maintain equilibrium with higher investment, saving must increase Since saving rises with income, $Y$ must rise In other words: lower interest rates → higher investment → higher income required to maintain saving-investment balance. The Liquidity-Money Curve (LM Curve) The LM curve represents all combinations of income $Y$ and interest rate $r$ at which money-market equilibrium holds: $L(Y, r) = \hat{M}$. The LM curve slopes upward because: When income rises, the demand for money increases (more transactions) To maintain equilibrium with higher money demand, the interest rate must rise Higher interest rates reduce money demand back to the level of the fixed supply In other words: higher income → higher money demand → higher interest rate required to clear the money market. Equilibrium and Policy Effects The equilibrium level of income $\hat{Y}$ and interest rate $\hat{r}$ occur at the intersection of the IS and LM curves. This is where both the investment-saving and money markets clear simultaneously. Fiscal policy shifts the IS curve. When the government increases spending or cuts taxes, the IS curve shifts right. At the original interest rate, the higher spending now implies higher equilibrium income, shifting the entire curve. The new equilibrium has higher income (and likely a higher interest rate due to the movement along the LM curve). Monetary policy shifts the LM curve. When the central bank increases the money supply, the LM curve shifts right. At the original income level, the higher money supply requires a lower interest rate to clear the money market. The new equilibrium has lower interest rates and higher income (as the lower rates stimulate investment along the IS curve). Special Case: Interest-Only Liquidity Preference If liquidity preference depends only on the interest rate and not on income—that is, $L = L(r)$ with no income dependence—then the LM curve becomes horizontal. In this case, the money market determines the interest rate independently, and fiscal policy cannot affect the interest rate (though it still affects income by shifting the IS curve). This special case illustrates an important principle: the slopes and positions of the IS and LM curves determine which policy tool is more effective in any given situation. Summary: How It All Fits Together Keynesian theory presents a vision of the macroeconomy driven by aggregate demand: Aggregate demand depends on consumption and investment Investment depends on the interest rate and expected profitability The interest rate is determined by equilibrium in the money market (liquidity preference = money supply) Income and employment adjust to clear the goods market—they do not automatically adjust to full employment Wages are sticky, so involuntary unemployment can persist The IS-LM model provides a compact graphical representation of this system, making it easy to see how monetary and fiscal policies affect income and interest rates, and how special cases (like the liquidity trap) limit policy effectiveness. This framework fundamentally differs from classical economics in arguing that the economy can settle at less-than-full-employment equilibrium, justifying active government policy to restore demand and employment.