Understanding Keynesian Equilibrium Income

According to Keynesian economic theory, the equilibrium level of national income is reached when there is no tendency for the income level to change. This equilibrium can be understood through two main approaches:

1. The Aggregate Demand (AD) and Aggregate Supply (AS) approach.
2. The Planned Savings (S) and Planned Investment (I) approach.

Aggregate Demand and Aggregate Supply Approach

In a simple two-sector economy (Households and Firms), aggregate demand (AD) is the sum of consumption expenditure (C) and planned investment expenditure (I). Aggregate supply (AS), which is equivalent to national income (Y), is the sum of consumption (C) and savings (S).

• Aggregate Demand \( AD = C + I \)
• Aggregate Supply \( AS = Y = C + S \)

Equilibrium income is achieved when aggregate demand equals aggregate supply:

\( AD = AS \)

\( C + I = C + S \)

Planned Savings and Planned Investment Approach

The condition \( C + I = C + S \) derived from the AD = AS equilibrium can be simplified. By subtracting consumption (C) from both sides of the equation, we get:

\( I = S \)

or, conventionally written as:

\( S = I \)

This means that according to Keynesian theory, the equilibrium level of income is achieved when planned savings by households equals planned investment by firms. This is the essence of the Savings-Investment equality condition for equilibrium.

Why Savings = Investment is the Equilibrium Condition

Let's consider situations where planned savings and planned investment are not equal:

• If Planned Savings > Planned Investment (\( S > I \)): In this case, the amount households plan to save is more than firms plan to invest. This implies that aggregate demand (\( C + I \)) is less than aggregate supply (\( C + S \)), or \( AD < AS \). When AD is less than AS, there is an accumulation of unsold goods, inventories rise unexpectedly, and firms will reduce production. Reduced production leads to a decrease in national income, moving towards the equilibrium level where \( S = I \).
• If Planned Savings < Planned Investment (\( S < I \)): In this case, the amount households plan to save is less than firms plan to invest. This implies that aggregate demand (\( C + I \)) is greater than aggregate supply (\( C + S \)), or \( AD > AS \). When AD is greater than AS, there is a depletion of inventories, and firms will increase production to meet the demand. Increased production leads to an increase in national income, moving towards the equilibrium level where \( S = I \).

Only when Planned Savings equal Planned Investment (\( S = I \)) is there no tendency for the level of income to change. Aggregate demand equals aggregate supply, production matches planned expenditure, and the economy is in equilibrium.

Analyzing the Options

• Option 1: Planned Savings > Planned Investment - This is a condition for disequilibrium where income will tend to fall. Incorrect.
• Option 2: Planned Savings = Planned Investment - This is the standard Keynesian condition for the equilibrium level of income. Correct.
• Option 3: Planned Savings < Planned Investment - This is a condition for disequilibrium where income will tend to rise. Incorrect.
• Option 4: Income = Consumption - This condition (\( Y = C \)) implies that Savings (\( S = Y - C \)) are zero. While zero savings is possible at a certain income level (specifically, the break-even income level where the consumption function intersects the 45-degree line), it is not the general condition for equilibrium income in Keynesian theory. Equilibrium income is where \( S = I \), and \( I \) is usually assumed to be positive. So, equilibrium income is typically where \( S \) equals a positive \( I \), not necessarily zero. Incorrect.

Therefore, according to Keynesian theory, the equilibrium level of income is achieved when Planned Savings equal Planned Investment.

Keynesian Equilibrium Revision Table

Additional Information on Keynesian Theory

Keynesian economics focuses on how aggregate demand influences output, employment, and inflation. John Maynard Keynes, in his work "The General Theory of Employment, Interest and Money" (1936), challenged classical economic views. Key aspects include:

• Role of Aggregate Demand: Keynes argued that insufficient aggregate demand can lead to prolonged periods of high unemployment, even if prices and wages are flexible.
• Sticky Prices/Wages: Unlike classical models where prices and wages adjust quickly to restore full employment, Keynesian theory often assumes some stickiness, especially downwards.
• Multiplier Effect: An initial change in spending (like investment) can lead to a larger final change in national income due to the multiplier effect. The size of the multiplier depends on the marginal propensity to consume.
• Government Intervention: Keynes advocated for active government intervention through fiscal and monetary policies to manage aggregate demand and stabilize the economy, especially during recessions.
• Paradox of Thrift: An attempt by everyone to save more may lead to a decrease in aggregate demand and thus a decrease in income, potentially leading to the same or even lower total savings. This highlights the difference between individual rationality and aggregate outcomes.

The \( S = I \) equilibrium condition discussed is fundamental to understanding the basic Keynesian model of income determination in a closed economy without government.