Understanding how households divide additional income between spending and saving is essential for analysing changes in overall spending. MPC, MPS, and disposable income formalise this behaviour and make it measurable for macroeconomic analysis.

Disposable Income (After-Tax Income)

Disposable income is the relevant “income” concept for consumption decisions because taxes reduce what households can use, while transfers increase it.

What happens to disposable income?

Households allocate disposable income into two broad uses:

• Consumption (C): spending on goods and services

• Saving (S): income not spent on current consumption

In this context, saving is treated as a flow (per period), not a stock of wealth. When disposable income rises, the key question becomes: how much of the extra income is consumed versus saved?

Marginal Propensity to Consume (MPC)

MPC focuses on changes “at the margin,” meaning it looks at how consumption responds to changes in disposable income, not the overall level of consumption.

A consumption function graph showing consumption on the vertical axis and income/output on the horizontal axis. The slope of the line represents the MPC, so a steeper line means a larger $\Delta C$ for a given $\Delta YD$. The intercept illustrates autonomous consumption (consumption that occurs even when income is very low). Source

Interpreting MPC

• If MPC = 0.8, households spend 80 cents of each additional dollar of disposable income.

• A higher MPC implies consumer spending is more responsive to income changes.

• MPC typically lies between 0 and 1 in standard AP macro settings.

Marginal Propensity to Save (MPS)

Because any additional disposable income is either consumed or saved in this simplified framework, MPC and MPS are tightly linked.

Interpreting MPS

• If MPS = 0.2, households save 20 cents of each additional dollar of disposable income.

• A higher MPS implies a larger share of extra income “leaks” into saving instead of current consumption.

Core Relationships and Required Equations

These relationships are commonly used to describe how changes in disposable income translate into changes in consumption and saving.

Because of this identity, once you know MPC, you can immediately find MPS, and vice versa.

Two aligned graphs: the consumption function ($C$ vs. $YD$) and the saving function ($S$ vs. $YD$). The slope of the consumption line corresponds to MPC, while the slope of the saving line corresponds to MPS, illustrating how each extra dollar of disposable income is divided between $\Delta C$ and $\Delta S$. The saving graph typically crosses zero at the break-even income level where households move from dissaving to positive saving. Source

How to Think About “Extra” Disposable Income

The syllabus emphasis is on what happens to an extra dollar of disposable income:

• The part that is spent is the MPC share

• The part that is saved is the MPS share

This framing is crucial when evaluating how income changes (for example, from changes in take-home pay) can translate into changes in household spending versus saving.

Common interpretation pitfalls to avoid

• MPC is not the share of total income consumed. It is the share of additional disposable income consumed.

• Saving is not “bad” in macro analysis; it is simply income not used for consumption in the current period.

• Disposable income matters, not GDP directly, for household consumption decisions in this simplified behavioural relationship.