Aggregate Demand vs Marginal Propensity to Consume (MPC)

MPC is the slope of a line on a diagram that has no price level on it

The two objects live in different coordinate spaces, and that single fact resolves most of the confusion. Plot a consumption function and disposable income runs along the horizontal axis while consumption runs up the vertical one. Suppose an illustrative household sector follows C equals 60 plus 0.75 times disposable income, both measured in billions of dollars. At disposable income of 200, consumption is 60 plus 150, or 210. At disposable income of 280, consumption is 60 plus 210, or 270. Consumption rose by 60 while income rose by 80, and 60 divided by 80 is 0.75, which is the MPC and also the slope of that line. Nothing in the calculation mentions the price level, because the price level is not on either axis. Aggregate demand is plotted somewhere else entirely, with the price level on the vertical axis and real GDP on the horizontal one, so every point on it already assumes a stated price level. A prompt asking what happens to consumption when the price level rises is therefore never an MPC question. The prompt is asking about the wealth effect and a movement along aggregate demand. Practice separating the two fractions at /calculate/mpc-and-mps.

A change in the MPC moves the curve and changes how far every later shift travels

A rise in the MPC does two separate jobs, and exam answers usually name only the second. Suppose households grow more confident and the MPC rises from 0.6 to 0.75 with disposable income unchanged. The first job is immediate. At every level of income households now consume a larger share, so consumption is higher at every price level and aggregate demand shifts right on its own, with no policy involved. The second job is about size. The spending multiplier is 1 divided by 1 minus the MPC, so it climbs from 1 divided by 0.4, which is 2.5, to 1 divided by 0.25, which is 4. A government purchase of 40 billion dollars that would have shifted aggregate demand right by 100 billion dollars now shifts it right by 160 billion. Same policy, same budget, larger shift, purely because a behavioral fraction moved. Keep the jobs separate when you write. A question about where the curve sits wants the first. A question about the size of a fiscal package wants the second. Neither touches aggregate supply, since the MPC says nothing about production costs or productive capacity. The determinant list the first job belongs to is at /macro/aggregate-demand.

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