Aggregate Demand (AD) Curve

Monday, 31 October 2011

In macroeconomics, aggregate demand (AD) is the total demand for final goods and services in the economy (Y) at a given time and price level. It is the amount of goods and services in the economy that will be purchased at all possible price levels. This is the demand for the gross domestic product of a country when inventory levels are static. It is often called effective demand, though at other times this term is distinguished.

It is often cited that the aggregate demand curve is downward sloping because at lower price levels a greater quantity is demanded. While this is correct at the microeconomics, single good level, at the aggregate level this is incorrect. The aggregate demand curve is in fact downward sloping as a result of three distinct effects; Pigou's wealth effect, the Keynes' interest rate effect and the Mundell-Fleming exchange-rate effect.


An aggregate demand curve is the sum of individual demand curves for different sectors of the economy. The aggregate demand is usually described as a linear sum of four separable demand sources.[3]

    AD = C + I + G + (X-M) \


    C \ is consumption (may also be known as consumer spending) = ac + bc(Y − T),
    I \ is Investment,
    G \ is Government spending,
    NX = X - M \ is Net export,
        X \ is total exports, and
        M \ is total imports = am + bm(Y − T).

These four major parts, which can be stated in either 'nominal' or 'real' terms, are:

    personal consumption expenditures (C) or "consumption," demand by households and unattached individuals; its determination is described by the consumption function. The consumption function is C= a + (mpc)(Y-T)
        a is autonomous consumption, mpc is the marginal propensity to consume, (Y-T) is the disposable income.

    gross private domestic investment (I), such as spending by business firms on factory construction. This includes all private sector spending aimed at the production of some future consumable.
        In Keynesian economics, not all of gross private domestic investment counts as part of aggregate demand. Much or most of the investment in inventories can be due to a short-fall in demand (unplanned inventory accumulation or "general over-production"). The Keynesian model forecasts a decrease in national output and income when there is unplanned investment. (Inventory accumulation would correspond to an excess supply of products; in the National Income and Product Accounts, it is treated as a purchase by its producer.) Thus, only the planned or intended or desired part of investment (Ip) is counted as part of aggregate demand. (So, I does not include the 'investment' in running up or depleting inventory levels.)
        Investment is affected by the output and the interest rate (i). Consequently, we can write it as I(Y,i). Investment has positive relationship with the output and negative relationship with the interest rate. For example, an increase in the interest rate will cause aggregate demand to decline. Interest costs are part of the cost of borrowing and as they rise, both firms and households will cut back on spending. This shifts the aggregate demand curve to the left. This lowers equilibrium GDP below potential GDP. As production falls for many firms, they begin to lay off workers, and unemployment rises. The declining demand also lowers the price level. The economy is in recession.

    gross government investment and consumption expenditures (G).
    net exports (NX and sometimes (X-M)), i.e., net demand by the rest of the world for the country's output.

In sum, for a single country at a given time, aggregate demand (D or AD) = C + Ip + G + (X-M).

These macrovariables are constructed from varying types of microvariables from the price of each, so these variables are denominated in (real or nominal) currency terms.

Aggregate demand curves

Understanding of the aggregate demand curve depends on whether it is examined based on changes in demand as income changes, or as price change.

Keynesian cross

Aggregate demand-aggregate supply model

Sometimes, especially in textbooks, "aggregate demand" refers to an entire demand curve that looks like that in a typical Marshallian supply and demand diagram.

Aggregate supply/demand graph
Thus, that we could refer to an "aggregate quantity demanded" (Yd = C + Ip + G + NX in real or inflation-corrected terms) at any given aggregate average price level (such as the GDP defoliator), P.
In these diagrams, typically the Yd rises as the average price level (P) falls, as with the AD line in the diagram. The main theoretical reason for this is that if the nominal money supply (Ms) is constant, a falling P implies that the real money supply (Ms/P)rises, encouraging lower interest rates and higher spending. This is often called the "Keynes effect."
Carefully using ideas from the theory of supply and demand, aggregate supply can help determine the extent to which increases in aggregate demand lead to increases in real output or instead to increases in prices (inflation). In the diagram, an increase in any of the components of AD (at any given P) shifts the AD curve to the right. This increases both the level of real production (Y) and the average price level (P).
But different levels of economic activity imply different mixtures of output and price increases. As shown, with very low levels of real gross domestic product and thus large amounts of unemployed resources, most economists of the Keynesian school suggest that most of the change would be in the form of output and employment increases. As the economy gets close to potential output (Y*), we would see more and more price increases rather than output increases as AD increases.
Beyond Y*, this gets more intense, so that price increases dominate. Worse, output levels greater than Y* cannot be sustained for long. The AS is a short-term relationship here. If the economy persists in operating above potential, the AS curve will shift to the left, making the increases in real output transitory.
At low levels of Y, the world is more complicated. First, most modern industrial economies experience few if any falls in prices. So the AS curve is unlikely to shift down or to the right. Second, when they do suffer price cuts (as in Japan), it can lead to disastrous deflation.


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