Marginal Propensity to Save (MPS) vs Paradox of Thrift
Marginal Propensity to Save (MPS) and Paradox of Thrift are two Aggregate Demand & Supply concepts in AP Economics that students often mix up. The marginal propensity to save (MPS) is the fraction of each additional dollar of disposable income that households save. The paradox of thrift is the idea that if everyone tries to save more at once, falling spending can lower total income so that aggregate saving doesn't rise and may fall. Here is how they compare side by side.
It ranges between 0 and 1 and, together with the marginal propensity to consume, always sums to 1. A higher MPS means a smaller spending multiplier. It measures how much of new income leaks out of the spending stream.
A Keynesian result: saving is prudent for one household, but if all households cut spending simultaneously, aggregate demand drops, the multiplier shrinks output and income, and lower income means less saving overall. It is a leading example of the fallacy of composition and supports the case for fiscal stimulus during a downturn. It assumes the economy is below full employment.
MPS vs the Paradox of Thrift: A Behavioral Fraction and What Happens When Everyone Changes It
| Marginal Propensity to Save | Paradox of Thrift | |
|---|---|---|
| What it is | A number describing how one household splits an extra dollar | A claim about what follows when every household tries to save more at once |
| Level of analysis | A single household's response to extra income | Total income and total saving for the whole economy |
| What it says | A fraction between 0 and 1, equal to 1 minus MPC | Trying to save more can leave total saving flat or lower it |
| Role in the model | An input, since the spending multiplier is 1 divided by MPS | An outcome, since it follows from a leftward shift of aggregate demand |
| Direction of the effect | A higher value shrinks the multiplier | A higher desired saving rate shrinks income, and lower income funds less saving |
| When it matters most | Any multiplier calculation | When output sits below potential and weak demand is what limits production |
| How it is used in policy | To size a spending change or a tax change | As a warning against urging households to save during a downturn |
The paradox is exact in the simple model, and the numbers show why
Work an illustrative economy where the marginal propensity to save is 0.2, so the spending multiplier is 1 divided by 0.2, which is 5. Households become nervous and decide to save 50 billion dollars more than before at their current income, which means cutting consumption by 50 billion dollars. Aggregate demand shifts left, and the multiplier does its usual job in reverse: equilibrium real output falls by 50 multiplied by 5, or 250 billion dollars. Now look at saving. Households did put aside an extra 50 billion dollars from the decision itself, but they also lost 250 billion dollars of income, and at a saving rate of 0.2 that loss removes 0.2 multiplied by 250, which is 50 billion dollars of saving. The two figures cancel. Total saving ends where it started, and everyone is poorer, which is the paradox in its sharpest form. The result is not a trick of the arithmetic. In this model, equilibrium saving has to equal investment, and nothing in the story changed investment, so saving could not move. Multiplier practice with other values is at /calculate/mpc-and-mps.
The paradox is a statement about weak demand, not a case against saving
The argument holds when spending is the binding constraint on output, which is the situation during a downturn with idle factories and unemployed workers. Change the setting and it weakens or reverses. At full employment, output is limited by resources rather than by willingness to spend, so extra saving does not shrink production. It flows into the loanable funds market instead, where a larger supply of funds pushes the real interest rate down and finances more investment. More capital raises future productive capacity, which is a rightward shift of long-run aggregate supply. That is the standard growth story, and it is the reason saving is treated as the source of investment at /macro/loanable-funds. So the honest version of the paradox is conditional. Attempting to save more is contractionary when demand is what limits output and constructive when it is not. A second condition often goes unstated: the model assumes investment does not respond to the fall in interest rates that extra saving would bring. When investment does respond strongly, part of the lost consumption returns as capital spending, and the paradox is softened rather than exact.
Frequently asked questions
What is the paradox of thrift?
The paradox of thrift is the result that if every household tries to save a larger share of its income at the same time, the fall in spending lowers total income enough that aggregate saving does not rise and may fall. Each household's plan is sensible on its own. What defeats it is that one household's spending is another household's income.
Is saving bad for the economy?
No, saving is what funds investment and long-run growth, and the paradox of thrift applies only to a specific case in which weak demand rather than scarce resources is holding output down. In a recession, a sudden jump in desired saving deepens the shortfall in spending. At full employment, the same extra saving lowers real interest rates and finances new capital.
How is the MPS related to the multiplier?
The spending multiplier equals 1 divided by the MPS in the simple model, so an MPS of 0.25 gives a multiplier of 4 and an MPS of 0.1 gives a multiplier of 10. Saving is the only leakage that model recognizes, which is why the whole multiplier rests on that one fraction. Adding taxes and imports as leakages makes the true multiplier smaller.
Live Fiscal Policy graph. Drag the curves, or open the full version.
Live AD/AS Model graph. Drag the curves, or open the full version.
Get AP Econ exam tips in your inbox
Occasional emails with study tips, new interactive graphs, and exam-season reminders. Free, no spam.
No spam. Unsubscribe anytime. Read our privacy policy.
Keep track of what you have studied
A free EconLearn account adds progress tracking, your quiz history, and achievements. Studying here is free either way, and there is nothing to pay for as a student.
Create a free accountAlready have one? Sign in
Last updated