Marginal Propensity to Consume (MPC)

Start with everyday intuition

Think about what happens when you get some extra money — say, a ₹500 bonus from your part-time job, or a cash gift on your birthday. You don't save all of it, and you don't spend all of it either. Most people spend a part and save the rest. That part you spend — the fraction of the extra income that goes into consumption — is exactly what economists call the Marginal Propensity to Consume.

The word "marginal" here means "extra" or "additional." So MPC answers one simple question: Out of every extra rupee you earn, how much do you spend?

The precise meaning

Formally, MPC is the ratio of change in consumption expenditure to the change in income that brought it about.

For example, if your income rises by ₹1,000 and your consumption rises by ₹750, your MPC is $750/1000 = 0.75$ (or 75%). This means you spend 75 paise of every extra rupee and save the remaining 25 paise.

The other side of the coin is the Marginal Propensity to Save (MPS) — the fraction of extra income that is saved. Since every extra rupee is either spent or saved:

This is not a theory; it's an accounting identity. If MPC = 0.75, then MPS must be 0.25.

Why MPC matters

MPC is not just a number — it is the engine of the multiplier effect, one of the most powerful ideas in macroeconomics.

When someone spends money, that spending becomes someone else's income. That second person, in turn, spends a fraction (their MPC) of that income, which becomes a third person's income, and so on. A single initial injection of spending — say, government investment in a road — ripples through the economy, generating total income many times larger than the original spending.

The size of this ripple depends directly on MPC. The higher the MPC, the larger the multiplier.

If MPC = 0.8, the multiplier is $1/(1-0.8) = 5$. An initial ₹100 crore investment can generate ₹500 crore of total income. If MPC = 0.5, the multiplier is only 2.

A word on the diagram …

Excess Demand: When Wants Outstrip What's Available

Think of a popular new smartphone launch. Hundreds of people queue up outside a store, but the shop only has 50 phones. Some people walk away empty-handed. That gap — between how many phones people want to buy at the current price and how many phones the shop actually has — is the core idea behind excess demand.

In economics, we apply this same logic to the entire economy. Excess demand in a market means that at the prevailing price, buyers want to purchase more of a good than sellers are willing to supply. The quantity demanded exceeds the quantity supplied.

The Precise Meaning

For a single market, excess demand is simply:

Excess Demand = Quantity Demanded − Quantity Supplied (at a given price)

When this number is positive, there is excess demand. When it is negative, there is excess supply (a surplus).

Now, in your Class 12 macroeconomics syllabus, the term "excess demand" takes on a specific national-income meaning. It refers to a situation where aggregate demand (AD) exceeds aggregate supply (AS) at the full-employment level of output. Let me unpack that.

Aggregate demand (AD) is the total planned spending in the economy: consumption, investment, government spending, and net exports. Aggregate supply (AS) is the total value of final goods and services that firms plan to produce — which, in the simplest model, equals national income (Y).

The full-employment level of output is the maximum sustainable output the economy can produce using all its resources normally. At this level, the economy is operating at its potential.

When AD is greater than what the economy can produce at full employment, you get excess demand. The NCERT textbook calls this an inflationary gap — a situation where too much money is chasing too few goods.

Why It Matters

Excess demand is not just a textbook curiosity. It signals that the economy is overheating. When demand persistently exceeds supply:

• Prices rise — general inflation sets in.
• Real purchasing power falls — your rupee buys less.
• Imports may surge — as domestic supply falls short, people buy from abroad.
• The government and central bank step in — they use fiscal policy (cut spending, raise taxes) or monetary policy (raise interest rates) to cool down demand.

In the NCERT framework, understanding excess demand helps you grasp why governments sometimes deliberately reduce spending or why the RBI raises repo rates. It's the opposite of deficient demand (recession), where AD falls short and unemployment rises. …