Consumption Function

What Is a Consumption Function?

A consumption function is a model of how household spending on goods and services responds to changes in income. Its core idea is that people spend more when they earn more, but not by the full amount of the extra income. The concept was popularized by the economist John Maynard Keynes in his 1936 book The General Theory of Employment, Interest and Money, and it remains a building block of macroeconomic analysis.

How the Relationship Works

Keynes's version is often written as a straight-line equation:

C = a + bY

In this equation, C is consumption, Y is disposable income, a is autonomous consumption (spending that occurs even when income is zero, financed through savings or borrowing), and b is the marginal propensity to consume, the share of each additional unit of income that is spent rather than saved. Because b is usually assumed to be between zero and one, consumption rises with income but more slowly.

For example, suppose a hypothetical function is C = 100 + 0.8Y, with amounts in dollars. At an income of 1,000, consumption is 900. If income rises to 1,500, consumption rises to 1,300. The 500 increase in income produces a 400 increase in spending, and the remaining 100 is saved.

Later Refinements and Why Investors Watch It

Keynes's simple version has limits. Economists have noted that households seem to base spending less on current income and more on expected long-term income. The permanent income hypothesis, associated with Milton Friedman, and the life-cycle hypothesis, associated with Franco Modigliani, build on this idea and suggest that temporary income changes affect spending less than lasting ones.

Investors pay attention to consumption because household spending makes up a large share of gross domestic product in many economies. It also affects revenue for retailers, automakers, and other consumer-facing businesses. Analysts use consumption data, along with income and savings figures, to gauge the outlook for aggregate demand and economic growth. Because the function is a simplified model, actual spending also depends on interest rates, wealth, credit conditions, and confidence, so it is best read as a rough guide rather than a precise forecast.