The marginal propensity to consume plus the marginal propensity to save must always equal 1.

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This statement is a fundamental principle of Keynesian economics and is known as the consumption function. It states that out of any extra income an individual or household receives, they will spend a certain portion (the marginal propensity to consume) and save the rest (the marginal propensity to save). Therefore, the two must add up to 1, representing 100% of the extra income.

The Marginal Propensity to Consume Plus the Marginal Propensity to Save