What is the difference between MPC and APC?
MPC (Marginal Propensity to Consume) measures the change in consumption from a change in income: ΔC/ΔY. APC (Average Propensity to Consume) is total consumption divided by total income: C/Y. MPC describes how households respond at the margin to a new dollar, while APC describes their overall consumption rate. In Keynesian models APC typically falls as income rises, and MPC is usually below APC for higher-income households.
How does MPC determine the fiscal multiplier?
The simple Keynesian spending multiplier equals 1 / (1 − MPC). If MPC is 0.7, the multiplier is roughly 3.33; if MPC is 0.5, it falls to 2.0. Real-world multipliers are smaller because of leakages: taxes, imports, and saving. IMF research finds short-run multipliers for transfers to liquidity-constrained households of 0.6–1.5, lower than the textbook value, but still meaningfully positive in recessions.
Why do low-income households have a higher MPC?
Low-income households are more likely to be liquidity-constrained: they lack savings buffers and limited access to credit, so a marginal dollar must be spent on near-term needs (food, rent, utilities). Carroll, Slacalek, and co-authors document MPCs of 0.7–0.9 for the bottom income quintile in the US and euro area, compared to 0.2–0.4 at the top. This heterogeneity is central to modern HANK (Heterogeneous Agent New Keynesian) models.
What does the Permanent Income Hypothesis predict for MPC?
Milton Friedman's Permanent Income Hypothesis (PIH) argues households smooth consumption against permanent income, not transitory shocks. PIH predicts a very low MPC out of temporary windfalls—maybe 0.05–0.10—because rational savers spread a one-time gain over their remaining lifetime. Empirical work since the 1990s rejects pure PIH: actual MPCs out of tax rebates and stimulus checks are 0.2–0.6, far higher than PIH predicts, especially for constrained households.
What is the typical MPC in the United States?
Aggregate US MPC out of transitory income shocks is estimated at roughly 0.5–0.7 in the first year, based on studies of 2001, 2008, and 2020–2021 stimulus payments. Disaggregated by income segment, MPCs range from about 0.3 for high-income households to 0.85–0.95 for low-income or credit-constrained households. The 2020 CARES Act and 2021 Economic Impact Payments showed average MPCs of 0.4–0.5 within three months.
Can MPC exceed 1?
Sustained MPC above 1 is impossible because a household cannot consume more than 100% of additional income indefinitely. However, in the short run, households can spend more than a one-time income shock by drawing on savings or credit, producing measured MPC ≥ 1 in narrow windows. This is occasionally observed in studies of large lump-sum payments to deeply credit-constrained consumers.
How does MPC change during recessions?
Two opposing forces operate. Liquidity constraints tighten and unemployment rises, raising MPC for affected households. But aggregate uncertainty prompts precautionary saving, lowering MPC for the unaffected. Net effect is empirically positive: aggregate MPC tends to rise in recessions, which is why fiscal stimulus has historically been more effective in downturns than booms (Auerbach and Gorodnichenko, 2012).
Why is MPC important for policy design?
Stimulus targeted at high-MPC groups (low-income households, the unemployed, families with children) produces larger consumption and GDP gains per dollar spent than broad-based tax cuts that flow primarily to high savers. The 2021 expanded Child Tax Credit and SNAP benefits had estimated multipliers of 1.2–1.5, while corporate tax cuts typically score 0.3–0.5 (CBO and JCT analyses). MPC heterogeneity is the technical reason.
