Give someone an extra pound, and only part of it ever gets spent

John Maynard Keynes defined the marginal propensity to consume as the fraction of each extra pound of income a household actually spends rather than saves, a number he expected to sit between zero and one for most people. That figure drives the whole Keynesian multiplier: a small jump in government spending can ripple through the economy and add up to several times its own value, because each recipient spends part of it, and the next recipient spends part of that, and so on.

— John Maynard Keynes, The General Theory of Employment, Interest and Money, Chapter 10 — 1936
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