Marginal Propensity to Consume
The Marginal Propensity to Consume (MPC) is the fraction of each additional dollar of disposable income that a household spends on goods and services rather than saves, serving as the key parameter determining the size of the Keynesian fiscal multiplier.
John Maynard Keynes introduced the concept of the Marginal Propensity to Consume in his landmark 1936 work 'The General Theory of Employment, Interest and Money.' The MPC captures a simple but powerful idea: when a household receives an extra dollar of income, some fraction gets spent and the remainder gets saved. The spending fraction is the MPC, and the saving fraction (1 minus MPC) is the Marginal Propensity to Save.
Empirical estimates of the MPC vary considerably depending on income level, the form the income takes (permanent vs. transitory), and economic conditions at the time. Lower-income households typically exhibit higher MPCs because a larger share of their income is consumed meeting basic needs — food, housing, transportation — leaving little margin for saving. Upper-income households tend to have lower MPCs, particularly for transitory windfalls, because their immediate consumption needs are already met. This differential is central to debates about the distributional efficiency of fiscal stimulus.
The MPC is the engine behind the Keynesian fiscal multiplier. If the government increases spending by $1 billion and the MPC is 0.8, the initial recipients spend 80 cents of every dollar received, those second-round recipients spend 80% of what they receive, and so on in an infinite geometric series. The resulting total increase in GDP equals $1 billion divided by (1 minus 0.8), producing a $5 billion total impact. A lower MPC produces a smaller multiplier.
The Permanent Income Hypothesis, developed by Milton Friedman, challenged the simple Keynesian view by arguing that households base their consumption on expected lifetime income rather than current income. Under this view, a temporary tax cut or rebate check — perceived as transitory income — would be largely saved rather than spent, implying a low effective MPC for such measures. The empirical literature has found mixed support for both views, with the truth likely depending on liquidity constraints facing different income groups.
For equity investors, the MPC matters because it shapes the economic multiplier of fiscal policy changes. A large stimulus package directed at lower-income households (high MPC) generates more consumer spending, which benefits retailers, consumer discretionary companies, and consumer staples businesses. Rising MPC estimates also support revenue forecasts for companies dependent on broad-based consumer demand.