Why is the marginal propensity to consume and the marginal propensity to save always equal to one?

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The reason for this is that there are only really two things that you can do with your money.  You can spend it or you can save it, but there is no third option.

The marginal propensity to consume is a measure of how much of your next dollar (the next dollar you get) you will spend.  The marginal propensity to save measures how much of your next dollar you will save.

So the two of these measures added together have to equal one because there is no other option for what you can do with your money.

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