The Marginal Propensity

The marginal propensity to spend is the slope of the planned spending line. It tells us how much planned spending increases if there is a $1 increase in income. The marginal propensity to spend is positive: Increases in income lead to increased spending by households and firms. The marginal propensity to spend is less than one, largely because of consumption smoothing by households. If household income increases by $1, households typically consume only a fraction of the increase, saving the remainder to finance future consumption. This equation, together with the condition that GDP equals planned spending, gives us the aggregate expenditure model. Toolkit: Section 16.19 “The Aggregate Expenditure Model” The aggregate expenditure model takes as its starting point the fact that GDP measures both total spending and total production. The model focuses on the relationships between output and spending, which we write as follows: planned spending = GDP and planned spending = autonomous spending + marginal propensity to spend × GDP.