01 · Explore
Aggregate Demand and its Components
In macroeconomics, we distinguish between planned (ex ante) and actual (ex post) values of economic variables to understand how income is determined.
Aggregate demand (AD) represents the total planned expenditure on final goods and services in an economy. In a simple two-sector model, it consists of consumption (C) and investment (I). It is crucial to distinguish between ex ante and ex post measures. Ex ante refers to the planned or intended values of variables like consumption or investment, while ex post refers to the actual, realized values as measured in accounting.
For example, if a producer plans to add Rs 100 to her inventory (planned investment) but sells Rs 30 from existing stock due to an unforeseen upsurge in demand, her actual or ex post investment is Rs 70 (100 - 30). The difference arises from unplanned inventory investment (in this case, unplanned decumulation), which acts as a buffer between planned production and actual sales.
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What is the primary difference between ex ante and ex post investment?
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02 · Explore
The Consumption Function
The consumption function describes the relationship between household consumption and income.
The simplest consumption function is linear: C = C̄ + cY. Here, C̄ represents autonomous consumption, which is the level of spending even when income (Y) is zero. This is usually financed through past savings or borrowing. The term 'cY' represents induced consumption, which depends on the level of income.
The coefficient 'c' is the Marginal Propensity to Consume (MPC). It measures the change in consumption per unit change in income (ΔC/ΔY). Since people typically save a portion of any additional income, the MPC usually lies between 0 and 1. If MPC is 0.7, it means for every Rs 100 increase in income, consumption increases by Rs 70.
| Term | Symbol | Definition |
|---|---|---|
| Autonomous Consumption | C̄ | Consumption independent of income level |
| Marginal Propensity to Consume | MPC (c) | Rate of change in consumption as income changes |
| Average Propensity to Consume | APC | Total consumption divided by total income (C/Y) |
| Marginal Propensity to Save | MPS (s) | Rate of change in savings as income changes (1 - c) |
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If a country's consumption function is C = 150 + 0.75Y, what is the autonomous consumption and the MPC?
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03 · Explore
Investment and the Two-Sector Model
Investment is the second component of aggregate demand, representing additions to physical capital and inventories.
Investment (I) includes spending on machines, buildings, and changes in inventory. Unlike consumption, which is highly dependent on current income, investment decisions often depend on interest rates and future expectations. However, in this basic model, we assume investment is autonomous (I = Ī), meaning it remains constant regardless of the level of income.
In a two-sector model (households and firms), Aggregate Demand (AD) is the sum of planned consumption and planned investment: AD = C̄ + Ī + cY. We can combine the autonomous parts into a single term 'Ā', where Ā = C̄ + Ī. Thus, AD = Ā + cY. This equation represents the total planned spending in the economy at different income levels.
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Why is investment often treated as 'autonomous' in the basic Keynesian model?
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04 · Explore
Determination of Equilibrium Income
Equilibrium in the final goods market occurs when planned supply equals planned demand.
The economy reaches equilibrium when the ex ante aggregate supply (Y) equals the ex ante aggregate demand (AD). Graphically, the supply side is represented by a 45-degree line from the origin, where every point has equal horizontal (income) and vertical (output) values. This assumes that firms are willing to supply any amount of goods at a fixed price level due to underutilized resources.
Algebraically, equilibrium is found where Y = Ā + cY. Solving for Y, we get Y = Ā / (1 - c). This shows that the equilibrium level of national income is determined by the total autonomous expenditure and the marginal propensity to consume. If the planned demand is less than the planned output, unintended inventories accumulate, leading producers to reduce production until equilibrium is restored.
Calculating Equilibrium Income
Y = (C̄ + Ī) / (1 - c)
Given C = 50 + 0.5Y and I = 100. Here, Ā = 50 + 100 = 150. MPC (c) = 0.5. Equilibrium Y = 150 / (1 - 0.5) = 150 / 0.5 = 300. The equilibrium income is 300 units.
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What happens if ex ante aggregate demand is greater than ex ante aggregate supply?
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05 · Explore
The Multiplier Mechanism
The multiplier explains how an initial change in autonomous spending leads to a larger change in total income.
The investment multiplier (k) is the ratio of the total increment in final goods output (ΔY) to the initial increment in autonomous expenditure (ΔĀ). When autonomous investment increases, it creates income for factor owners. These individuals spend a fraction (MPC) of that new income on consumption, which in turn becomes income for others. This cycle continues in decreasing rounds.
The formula for the multiplier is k = ΔY / ΔĀ = 1 / (1 - c) or k = 1 / s, where 's' is the Marginal Propensity to Save. A higher MPC results in a larger multiplier because more money is re-spent in each round. For instance, if MPC is 0.8, the multiplier is 1 / (1 - 0.8) = 5. An initial investment of Rs 100 would eventually increase the total national income by Rs 500.
The Multiplier Process
- 1
Initial Injection
Autonomous investment (ΔĪ) increases, raising income by the same amount.
- 2
Induced Consumption
Recipients spend a portion (MPC × ΔĪ) on goods and services.
- 3
Secondary Income
This spending becomes new income for producers in the next round.
- 4
Convergent Rounds
The process repeats in smaller increments until total ΔY = ΔĪ × [1/(1-c)].
The sequence shows how an initial autonomous expenditure triggers multiple rounds of spending and income generation.
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If the MPS is 0.25, what is the value of the investment multiplier?
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06 · Explore
The Paradox of Thrift
The Paradox of Thrift suggests that individual efforts to save more can lead to lower total income for the economy.
If all people in an economy increase their propensity to save (MPS increases, MPC decreases), the aggregate demand line swings downwards. This reduction in planned spending leads to a decrease in the equilibrium level of income. Because total income falls, the total volume of savings in the economy will not increase—it will either decline or remain unchanged.
Consider an economy where MPC falls from 0.8 to 0.5. The multiplier falls from 5 to 2. Even if autonomous investment remains the same, the lower multiplier results in a much lower equilibrium income. In the textbook example, while individuals try to save more, the resulting drop in income leads to total savings remaining exactly unchanged (e.g., at 10 units) because the lower income level offsets the higher saving rate.
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According to the Paradox of Thrift, what happens to total national savings if everyone tries to save more?
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07 · Explore
Full Employment and Demand Conditions
Equilibrium income does not always guarantee that everyone who wants to work is employed.
Full employment level of income is the level where all available factors of production are utilized. However, the macroeconomic equilibrium (where Y = AD) can occur at a level below full employment. This is known as a situation of deficient demand. In this state, there is involuntary unemployment because the aggregate demand is insufficient to buy the output that would be produced at full employment.
Conversely, if planned aggregate demand exceeds the output produced at the full employment level, it is a situation of excess demand. Since the economy cannot produce more than its full capacity in the short run, this excess demand puts upward pressure on prices, leading to inflation in the long run. The Keynesian model emphasizes that the government may need to intervene to manage these demand gaps.
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What is 'deficient demand'?
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Does equilibrium always imply full employment?
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