12.1 Aggregate Demand in Keynesian Analysis

Keynesian analysis centers on aggregate demand as the primary driver of economic activity in the short run. Unlike classical economics, which assumes markets self-correct quickly, the Keynesian perspective argues that insufficient demand can leave an economy stuck below its potential for extended periods. This section covers how consumption, investment, government spending, and net exports combine to determine real GDP, and what happens when that total falls short of (or exceeds) what the economy can sustainably produce.

Aggregate Demand in Keynesian Analysis

Real GDP and Economic Gaps

Real GDP measures the total value of final goods and services produced in an economy, adjusted for inflation. It represents actual output over a given period (typically a year or quarter).

Potential GDP is the maximum sustainable output an economy can produce when labor, capital, and technology are fully employed. The relationship between actual and potential GDP defines two critical gaps:

• Recessionary gap: Actual real GDP falls below potential GDP. The economy is operating under capacity, which means higher unemployment and idle resources. This gap signals a need for expansionary policy (increased government spending, lower taxes, or lower interest rates) to boost aggregate demand.
• Inflationary gap: Actual real GDP exceeds potential GDP. The economy is producing beyond its sustainable capacity, which creates upward pressure on prices. This gap calls for contractionary policy (reduced government spending, higher taxes, or higher interest rates) to cool aggregate demand.

These gaps are central to Keynesian thinking because they justify active government intervention rather than waiting for markets to self-correct.

Structure of the Keynesian AD-AS Model

The AD-AS model has three curves that together determine the economy's price level and output.

Aggregate Demand (AD) represents total spending on goods and services at each price level. The AD curve slopes downward: as the price level falls, the quantity of goods and services demanded rises. AD is composed of four spending components:

$AD = C + I + G + NX$

where $C$ is consumption, $I$ is investment, $G$ is government spending, and $NX$ is net exports.

Short-Run Aggregate Supply (SRAS) slopes upward. In the short run, some input prices (especially wages) are "sticky," meaning they don't adjust immediately to changes in the price level. When output prices rise but wages stay fixed, firms find it profitable to produce more, so quantity supplied increases.

Long-Run Aggregate Supply (LRAS) is a vertical line at potential GDP. In the long run, all input prices fully adjust to changes in the price level. This means there's no tradeoff between inflation and output: the economy produces at potential GDP regardless of the price level.

Equilibrium occurs where AD intersects AS:

• Short-run equilibrium is where AD crosses SRAS. This can occur at, above, or below potential GDP.
• Long-run equilibrium is where AD, SRAS, and LRAS all intersect. At this point, actual GDP equals potential GDP.
• Shifts in AD or AS move the economy to new equilibrium points, changing both the price level and real GDP in the short run.

Determinants of Consumption and Investment

Consumption (C) is the largest component of AD in most economies. In the Keynesian model, it depends on disposable income and the marginal propensity to consume:

$C = a + MPC(Y - T)$

• $a$ = autonomous consumption (the baseline spending that occurs even at zero income, funded by savings or borrowing)
• $Y - T$ = disposable income (total income minus taxes)
• MPC (marginal propensity to consume) = the fraction of each additional dollar of disposable income that gets spent rather than saved

For example, if the MPC is 0.8, then for every extra dollar of disposable income, households spend 80 cents and save 20 cents. A higher MPC means consumption is more responsive to changes in income, which matters a lot for the multiplier effect (covered below).

Investment (I) is the most volatile component of AD. It depends on several factors:

• Interest rates: Higher rates raise the cost of borrowing, making fewer investment projects profitable. Lower rates do the opposite.
• Expectations: When businesses expect strong future demand and growth, they're more willing to invest in expansion. Pessimistic expectations have the reverse effect.
• Marginal efficiency of capital (MEC): This is the expected rate of return on an investment project. Businesses compare the MEC to the current interest rate. If the MEC exceeds the interest rate, the project is worth undertaking.
• Animal spirits: Keynes used this term to describe the role of confidence, intuition, and emotion in business decisions. Investment doesn't always follow cold calculation; waves of optimism or pessimism can drive spending decisions independent of fundamentals.

Government and Trade in Aggregate Demand

Government spending (G) directly adds to aggregate demand. Changes in G shift the AD curve:

• Increases in G (infrastructure projects, defense spending, education funding) shift AD to the right, raising real GDP and the price level in the short run.
• Decreases in G (austerity measures, spending cuts) shift AD to the left, reducing real GDP and the price level.

Net exports (NX) equal the difference between what a country sells abroad and what it buys from abroad:

$NX = X - M$

• Exports (X) generate income for the domestic economy.
• Imports (M) represent spending that flows out of the domestic economy to foreign producers.

Several factors shift NX and therefore AD:

• Rising foreign demand, improved competitiveness, or a depreciation of the domestic currency tend to increase NX (shifting AD right).
• Falling foreign demand, loss of competitiveness, or an appreciation of the domestic currency tend to decrease NX (shifting AD left).

Fiscal policy uses changes in G and taxes to influence AD:

1. Expansionary fiscal policy (increased G or reduced taxes) stimulates AD during recessions to close a recessionary gap.
2. Contractionary fiscal policy (decreased G or increased taxes) reduces AD during inflationary periods to close an inflationary gap.
3. Trade policies (tariffs, quotas, subsidies) can also be adjusted to influence NX and, by extension, AD.

Keynesian Theory and Effective Demand

Effective demand is one of Keynes's most important contributions. Classical economists assumed that supply creates its own demand (Say's Law), meaning the economy would always tend toward full employment. Keynes argued the opposite: the level of output and employment is determined by how much total spending actually exists in the economy. If households and businesses don't spend enough, firms cut production and lay off workers, and the economy can settle into an equilibrium below full employment.

The multiplier effect explains why changes in spending have an outsized impact on GDP. When the government spends an additional dollar, that dollar becomes income for someone else, who then spends a fraction of it (determined by the MPC), which becomes income for yet another person, and so on. The spending multiplier is:

$\text{Multiplier} = \frac{1}{1 - MPC}$

For example, with an MPC of 0.8, the multiplier is $\frac{1}{1 - 0.8} = 5$. A $100 billion increase in government spending would ultimately increase GDP by $500 billion. This is why Keynesians emphasize that even modest changes in spending can have large effects on output.

Liquidity preference theory is Keynes's explanation for how interest rates are determined. People hold money for three reasons: transactions (daily purchases), precaution (unexpected expenses), and speculation (waiting for better investment opportunities). The interest rate adjusts to balance the demand for money against the supply set by the central bank. Higher money demand pushes interest rates up, which discourages investment and reduces AD. Lower money demand has the opposite effect. This connects monetary policy to aggregate demand through the interest rate channel.