Building the AD Curve: Slope versus Shift

You’ll be able to

Explain the three effects that make aggregate demand slope downward
Distinguish a movement along AD from a shift of AD
Predict the direction of an AD shift from a change in C, I, G or Xn

Why AD slopes down is NOT why microeconomic demand slopes down

A single good's demand slopes down partly because buyers substitute toward other goods. That reasoning cannot work for AD, because AD is everything — there is nothing outside it to substitute toward. The slope comes from three different channels. The wealth effect: a lower price level raises the real value of money holdings, so people feel richer and buy more. The interest-rate effect: a lower price level reduces the demand for money, lowering interest rates and encouraging investment. The net-export effect: a lower domestic price level makes domestic goods cheaper relative to foreign ones, raising Xn.

Slope versus shift

PRICE LEVEL changes → movement ALONG AD · C, I, G or Xn changes for any other reason → SHIFT of AD

The single most useful discriminator in the unit. If the cause is the price level, you move along; if it is anything else, you shift.

What shifts each component

C shifts with consumer confidence, wealth (including house and stock prices), taxes on households, and household debt. I shifts with business confidence, real interest rates, technology, and business taxes or investment credits. G shifts with any deliberate change in government purchases. Xn shifts with foreign incomes, exchange rates, and trade policy. Notice that the real interest rate shifting I is a shift, not a movement — the interest-rate effect that produces AD's slope comes from the price level, and a rate change from any other source moves the whole curve.

Worked example

For each, state whether AD shifts and in which direction: (a) the price level falls; (b) the Fed lowers the federal funds rate; (c) foreign incomes fall; (d) households become pessimistic about job security.

1.(a) The price level is the vertical axis — this is a movement along AD, not a shift.
2.(b) A lower rate raises investment for a reason other than the price level: AD shifts right.
3.(c) Poorer foreign customers buy fewer exports, so Xn falls: AD shifts left.
4.(d) Pessimism raises precautionary saving and cuts C: AD shifts left.

Answer: Only (a) is a movement along the curve. (b) shifts AD right; (c) and (d) shift it left. The test is always whether the price level is the cause.

Watch out

The wealth effect behind AD's slope concerns the real value of money holdings when the price level changes — not a change in how wealthy people are. A stock market crash also affects wealth, but that is a shift of AD, because the price level did not cause it.

Checkpoint

Which causes a movement along the aggregate demand curve rather than a shift?

Checkpoint

The interest-rate effect explains part of AD's downward slope because a lower price level:

Checkpoint

A major trading partner enters a deep recession. The effect on domestic AD is that it:

On the exam

Label your axes "Price Level" and "Real GDP" every single time. Rubrics award axis labels as a separate point, and it costs three seconds.

Answer the 3 checkpoints as you read.