Page 215 - Krugmans Economics for AP Text Book_Neat

figure 17.1

The Aggregate Aggregate price
level (GDP deflator,
Demand Curve
2005 = 100)
The aggregate demand curve shows
the relationship between the aggregate
price level and the quantity of aggre- A movement down the
gate output demanded. The curve is AD curve leads to a lower
aggregate price level and
downward sloping due to the wealth
higher aggregate output.
effect of a change in the aggregate 1933
price level and the interest rate effect 7.9
of a change in the aggregate price
level. Corresponding to the actual 1933
data, here the total quantity of goods 5.0
and services demanded at an aggre-
gate price level of 7.9 is $716 billion in
2005 dollars. According to our hypo- Aggregate demand
thetical curve, however, if the aggre- curve, AD
gate price level had been only 5.0, the 0 $716 950 Real GDP
quantity of aggregate output demanded (billions of
would have risen to $950 billion. 2005 dollars)

goods and services demanded would have been $950 billion in 2005 dollars instead of
$716 billion.
The first key question about the aggregate demand curve involves its negative slope.

Why Is the Aggregate Demand Curve Downward
Sloping?

In Figure 17.1, the curve AD slopes downward. Why? Recall the basic equation of na-
tional income accounting:

(17-1) GDP = C + I + G + X − IM

where C is consumer spending, I is investment spending, G is government purchases of
goods and services, X is exports to other countries, and IM is imports. If we measure
these variables in constant dollars—that is, in prices of a base year—then C + I + G +
X − IM represents the quantity of domestically produced final goods and services de-
manded during a given period. G is decided by the government, but the other variables
are private -sector decisions. To understand why the aggregate demand curve slopes
downward, we need to understand why a rise in the aggregate price level reduces C, I,
and X − IM.
You might think that the downward slope of the aggregate demand curve is a natural
consequence of the law of demand. That is, since the demand curve for any one good is
downward sloping, isn’t it natural that the demand curve for aggregate output is also
downward sloping? This turns out, however, to be a misleading parallel. The demand
curve for any individual good shows how the quantity demanded depends on the price
of that good, holding the prices of other goods and services constant. The main reason the quan-
tity of a good demanded falls when the price of that good rises—that is, the quantity of a
good demanded falls as we move up the demand curve—is that people switch their con-
sumption to other goods and services that have become relatively less expensive.
But when we consider movements up or down the aggregate demand curve, we’re con-
sidering a simultaneous change in the prices of all final goods and services. Furthermore, changes

module 17 Aggregate Demand: Introduction and Determinants 173