This paper examines the concepts of marginal propensity to consume (MPC) and marginal propensity to save (MPS) in macroeconomics, explaining how these complementary measures determine what share of an aggregate income increase is spent versus saved. Using the GDP accounting identity (GDP = C + I + G + X − M), the paper explores how variations in MPC and MPS influence overall economic output. It discusses scenarios ranging from full consumption to full saving, considers the role of banking and business investment, and addresses how prevailing economic conditions — particularly consumer confidence during downturns — shape these propensities and their downstream effects on gross domestic product.
The marginal propensity to consume (MPC) refers to the proportion of an aggregate raise in pay that is spent on the consumption of goods and services (Investopedia, 2011). That is, when more money enters the economy, it must either be spent or saved. The MPC represents the share that is spent. The marginal propensity to save (MPS) is the reverse — it refers to how much of a new aggregate raise in pay will be saved rather than spent. The two have a complementary relationship: the entirety of any aggregate increase in pay goes toward one or the other, and together they always sum to one.
According to the GDP accounting identity — GDP = C + I + G + X − M — an increase in aggregate pay will affect overall economic output. The degree to which such an increase affects GDP depends directly on the marginal propensity to consume. If consumers spend 100% of their aggregate income increase, then GDP will rise by the full amount of that increase, because the consumption component (C) in the identity rises by exactly that amount.
Beyond the direct effect on consumption, additional channels may amplify the impact. Because many goods purchased with this money are subject to taxation, government revenues may increase, raising the government spending component (G) as well. Businesses would also earn greater profits, potentially leading to an increase in business investment (I). However, the income increase could also stimulate demand for imports, which would widen the current account deficit and partially offset the gains, since higher imports reduce the net export component (X − M).
The opposite scenario occurs when all of an aggregate pay increase is saved. The marginal propensity to save is defined as "the ratio of change in saving to change in income" (Economic Concepts, 2011). If this ratio equals 1, then all additional income flows into savings, and the direct effect on GDP through consumer spending would be negligible — in theory, there would be no GDP increase from consumption at all. In practice, however, banks would have more deposits available to lend, which could still stimulate an increase in business investment (I) or even in exports (X), providing a secondary pathway through which savings eventually contribute to economic growth.
In practice, there is likely to be some portion of an aggregate income increase saved and some spent. The key takeaway is that the more money that is spent — that is, the higher the marginal propensity to consume — the greater the resulting increase in GDP from any given rise in aggregate pay. Conversely, the higher the marginal propensity to save, the less of that income increase will translate into growth in gross domestic product.
Consumer spending is the most important variable to consider in relation to the marginal propensity to consume and the marginal propensity to save. Both propensities are shaped by prevailing conditions in the economy. As a general rule, an increase in aggregate pay will lead to an increase in consumer spending and likely a higher marginal propensity to consume as well.
"Describes mixed real-world spending and saving patterns"
"Connects economic downturns to shifts in saving behavior"
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