Understanding the Components of Aggregate Demand in the Keynesian Model

When economists try to understand what drives an economy, they often start by examining aggregate demand-the total spending in an economy. In the Keynesian model, developed by British economist John Maynard Keynes during the Great Depression, aggregate demand is broken down into key components that help us understand how spending decisions shape economic outcomes. At the heart of this framework lies a simple but powerful idea: the amount people and businesses spend depends largely on their income and expectations about the future.

Think about your own spending habits. When you get a salary increase, do you spend all of it? Probably not. You might spend some of it on things you’ve been wanting, but you’ll likely save a portion too. This natural tendency is exactly what Keynes observed when studying consumer behavior across entire economies. Understanding these patterns isn’t just academic-it helps governments make better decisions about taxes, spending, and interest rates to keep economies stable and growing.

The consumption function: how households decide to spend 🔗

The consumption function is one of the most influential concepts in macroeconomics. Introduced by Keynes in 1936, it describes the relationship between household income and consumption spending. In its simplest form, the consumption function is expressed as C = a + bY, where C represents total consumption, Y represents income, ‘a’ is autonomous consumption, and ‘b’ is the marginal propensity to consume.

Let’s break this down with a real example. Imagine a household earning zero income. Even with no income, they still need to eat, pay for shelter, and cover basic necessities. They might do this by borrowing money, using savings, or relying on government support. This minimum level of spending-what happens when income is zero-is what economists call autonomous consumption. It’s the ‘a’ in our equation.

Now, when this household starts earning income, they don’t spend every additional rupee they make. If the marginal propensity to consume is 0.8, it means that for every additional ₹100 earned, the household will spend ₹80 and save ₹20. This spending that varies with income is called induced consumption-the ‘bY’ part of the equation.

Understanding the marginal propensity to consume 🔗

The marginal propensity to consume, or MPC, is a crucial concept that measures how much of an additional unit of income gets spent on consumption. According to Keynes, the MPC is always between zero and one, meaning people spend some but not all of their additional income.

Why does this matter? Because the MPC determines the slope of the consumption function. A higher MPC means a steeper consumption line-households are spending a larger fraction of any income increase. Lower-income families typically have a higher MPC because they have more immediate needs. If a family struggling to make ends meet receives an extra ₹10,000, they’re likely to spend most of it on necessities. A wealthier household receiving the same amount might save more of it.

The counterpart to the MPC is the marginal propensity to save, or MPS. Since every rupee of additional income is either spent or saved, MPC + MPS always equals 1. If your MPC is 0.75, your MPS must be 0.25.

What shifts the consumption function? 🔗

While income is the primary driver of consumption, other factors can shift the entire consumption function up or down. Think about what happened during the COVID-19 pandemic. Even people with stable incomes cut back on spending because they were worried about the future. This shift in consumer confidence moved the entire consumption function downward.

Similarly, changes in household wealth can shift consumption patterns. When housing prices rise, homeowners feel wealthier and may increase their spending even if their current income hasn’t changed. This is sometimes called the wealth effect. Tax changes also matter-if the government reduces income taxes, households have more disposable income available for consumption or saving.

Investment: the volatile component of aggregate demand 🔗

While consumption accounts for the largest share of aggregate demand, investment by businesses plays a critical but more unpredictable role. In its simplest form, investment can be treated as autonomous, meaning it doesn’t depend on current income levels. This is expressed as I = Ī, where the bar over I indicates it’s a fixed amount.

However, investment can also have an induced component. When the economy is growing and incomes are rising, businesses become more optimistic about future demand. They’re more likely to invest in new factories, equipment, and technology. This can be expressed as I = c + dY, where ‘c’ is autonomous investment, ‘d’ is the marginal propensity to invest (MPI), and Y is income.

How interest rates influence business investment decisions 🔗

One of the most important relationships in the Keynesian model is the inverse relationship between interest rates and investment. When a business considers buying new machinery or building a new factory, it must weigh the expected returns against the cost of borrowing money to finance the investment.

Keynes introduced the concept of the marginal efficiency of capital, which represents the expected rate of return on a new investment project. A business will only invest if the marginal efficiency of capital exceeds the interest rate. When interest rates rise, the cost of borrowing increases, making fewer investment projects profitable. Conversely, when central banks lower interest rates, more projects become worthwhile, stimulating investment spending.

Consider a manufacturing company deciding whether to purchase a new machine costing ₹10 lakh. If the machine is expected to generate ₹1.5 lakh in additional profits annually, that’s a 15% return. If the interest rate is 10%, the investment makes sense-the return exceeds the cost of borrowing. But if the interest rate rises to 18%, the company would be better off not making the investment.

The present value concept is key here. Higher interest rates reduce the present value of future returns from investment projects. A project that might generate ₹10 lakh in profits ten years from now is worth less today when interest rates are high, because that future money could alternatively earn high returns if simply invested in financial assets.

Putting it all together: aggregate demand in the Keynesian framework 🔗

When we combine consumption and investment (along with government spending and net exports in more complete models), we get aggregate demand. In the Keynesian view, fluctuations in aggregate demand are the primary cause of economic cycles-recessions happen when aggregate demand falls short of the economy’s productive capacity, and overheating occurs when it exceeds capacity.

Understanding these components helps explain why economies experience booms and busts. Consumption is relatively stable because it’s tied to income, which doesn’t fluctuate wildly in the short run. Investment, however, is much more volatile. It depends on business expectations about the future, which can shift quickly based on news, policy changes, or global events. A sudden loss of business confidence can cause investment to plummet, dragging down aggregate demand and causing a recession.

This framework also reveals why government policy can matter. If private consumption and investment fall during a recession, government spending can help fill the gap. Similarly, central banks can lower interest rates to stimulate investment and support aggregate demand during economic downturns.

What do you think? How might your own spending behavior change if you expected your income to increase significantly next year? And if you were a business owner, what interest rate level would make you hesitant to invest in expanding your company?

References

1. https://en.wikipedia.org/wiki/Consumption_function
2. https://courses.lumenlearning.com/wm-macroeconomics/chapter/aggregate-expenditure-consumption/
3. https://en.wikipedia.org/wiki/Marginal_propensity_to_consume
4. https://www.economicsdiscussion.net/keynesian-economics/keynes-theory/the-keynesian-theory-of-investment-with-diagram-and-example/16056