1. Aggregate Demand
The quantity of real GDP demanded, Y, is the total amount of final goods and services produced in the United States that households (C), businesses (I), governments (G), and foreigners (NX) plan to buy.
So, aggregate demand is the sum of the four component demands:
AD = CD + ID + GD + NXD
Keep it simple. You know that the AD curve is a subtle object—an equilibrium relationship derived from simultaneous equilibrium in the goods market and the money market. This description of the AD curve is not helpful to students in the principles course and is a topic for the intermediate macro course. At the same time that we want to simplify the AD story, we also want to avoid being misleading. The textbook walks that fine line, and we suggest that you stick closely to the textbook treatment and don’t try to convey the more subtle aspects of AD.
Not a strict ceteris paribus event. A major problem with the AD curve is that a change in the price level that brings a movement along the curve is not a strict ceteris paribus event. A change in the price level changes the quantity of real money, which changes the interest rate. Indeed, this chain of events is one of the reasons for the negative slope of the AD curve. In telling this story, we must be sensitive to the fact that the student doesn’t yet know about the demand for money. We must provide intuition with stories (like the Maria stories in the textbook) without referring to the demand for money.
Income equals expenditure on the AD curve. Some instructors want to emphasize a second and more subtle violation of ceteris paribus, that along the AD curve, aggregate planned expenditure equals real GDP. That is, the AD curve is not drawn for a given level of income but for the varying level of income that equals the level of planned expenditure. If you want to make this point when you first introduce the AD curve, you must cover the AE model of Chapter 25 before you cover this chapter. (The material is written in a way that permits this change of order.) If you do not want to derive the AD curve from the equilibrium of the AE model, don’t even mention what’s going on with income along the AD curve. Silence is vastly better than confusion. You can pull this rabbit out of the hat when you get to Chapter 25 if you’re covering the material in the order presented in the textbook.

2. Aggregate Demand The Aggregate Demand Curve
Aggregate demand is the relationship between the quantity of real GDP demanded and the price level.
The aggregate demand (AD) curve plots the quantity of real GDP demanded against the price level.

3. Slope of the AD Curve
We assume that the AD curve slopes downward: All else equal, as P increases the quantity demanded of the good decreases.
The rationale is slightly different than the usual rationale for downward sloping demand curves in goods markets. The usual rationale is that as the price of a good increases, buyers substitute away from that good toward other goods that are now relatively less expensive. But in our model economy there is only one good!

4. Aggregate Demand Figure 23.6 shows an AD curve.
The AD curve slopes downward for two reasons
A wealth effect
Substitution effects

5. Aggregate Demand
Wealth effect A rise in the price level, other things remaining the same, decreases the quantity of real wealth (money, bonds, stocks, etc.).
To restore their real wealth, people increase saving and decrease spending, so the quantity of real GDP demanded decreases.
Similarly, a fall in the price level, other things remaining the same, increases the quantity of real wealth.
With more real wealth, people decrease saving and increase spending, so the quantity of real GDP demanded increases.

6. Aggregate Demand
Intertemporal substitution effect A rise in the price level, other things remaining the same, decreases the real value of money and raises the interest rate.
Faced with a higher interest rate, people try to borrow and spend less so the quantity of real GDP demanded decreases.
Similarly, a fall in the price level increases the real value of money and lowers the interest rate.
Faced with a lower interest rate, people borrow and spend more so the quantity of real GDP demanded increases.

7. Aggregate Demand
International substitution effect A rise in the price level, other things remaining the same, increases the price of domestic goods relative to foreign goods, so imports increase and exports decrease, which decreases the quantity of real GDP demanded.
Similarly, a fall in the price level, other things remaining the same, decreases the price of domestic goods relative to foreign goods, so imports decrease and exports increase, which increases the quantity of real GDP demanded.

8. Aggregate Demand
The AD curve shifts when the quantity demanded changes for any reason other than a price change.
Among the things that will shift AD:
changing expectations about future prices
changes in foreign prices; changes in the exchange
rate
changing expectations about future income and future
profits
fiscal and monetary policy

9. Aggregate Demand Changes in Aggregate Demand
Expectations about future income, future inflation, and future profits change aggregate demand.
Increases in expected future income increase people’s consumption today, and increases aggregate demand.
A rise in the expected inflation rate makes buying goods cheaper today and increases aggregate demand.
An increase in expected future profits boosts firms’ investment, which increases aggregate demand.
Fiscal and monetary policy

10. Aggregate Demand Changes in Aggregate Demand
Fiscal policy is the government’s attempt to influence economic activity by changing its taxes, spending, deficit, and debt policies.
A tax cut or an increase in transfer payments increases households’ disposable income—aggregate income minus taxes plus transfer payments.
An increase in disposable income increases consumption expenditure and increases aggregate demand.

11. Aggregate Demand Changes in Aggregate Demand
Monetary policy is changes in the interest rate and quantity of money.
An increase in the quantity of money increases aggregate demand. (More on this later in the course.)
A cut in the interest rate increases expenditure and increases aggregate demand.

12. Aggregate Demand Figure 23.7 illustrates changes in aggregate demand.
When aggregate demand increases, the AD curve shifts rightward…
… and when aggregate demand decreases, the AD curve shifts leftward.