components of aggregate demand

Components of Aggregate Demand Introduction Aggregate demand tells the quantity of goods and services demanded in an economy at a given price level. In effect, the aggregate demand curve is a just like any other demand curve, but for the sum total of all goods and services in an economy. It tells the total amount that all consumers, businesses, and the government are willing to spend on goods and services at different price levels. The aggregate demand curve can be thought of just like a demand curve for a firm. When the price level is high, aggregate demand is low; when the price level is low, aggregate demand is high. The aggregate demand curve lies in a plane consisting of the price level and income or output. It shows a downward slope with price level on the vertical axis and income or output on the horizontal axis. As such, the aggregate demand curve outlines the relationship between income or output and the price level. It is important to notice that aggregate demand is a schedule because as the price level changes, the income or output also changes. There are four major components of aggregate demand. The equation for aggregate demand, Y = C(Y - T) + I(r) + G + NX(e), tells much about the nature of both aggregate demand and the curve that represents this schedule. Components of aggregate demand The equation for aggregate demand proposed by the Mundell-Fleming model of a large open economy is Y = C(Y - T) + I(r) + G + NX(e). Y represents income or output. C(Y - T) represents consumption as a function of disposable income, defined as income less taxes. I(r) represents investment as a function of the interest rate, where an increase in the interest rate decreases investment. G represents government spending, which is predominately unaffected by interest rates. Finally, NX(e) represents net exports, defined as exports less imports as a function of the real exchange rate, where an increase in the real exchange rate decreases net exports. Understanding the details of each component of aggregate demand is an important first step toward understanding aggregate demand. The first piece of the aggregate demand equation is Y. This represents output or income. Because Y is the total amount of goods and services purchased by consumers, businesses, and the government, taking into account foreign trade, it is necessarily the output for the economy. This number is also the gross domestic product of an economy. Because every unit of output within an economy turns into income for members of the economy, it is reasonable to call output income. More specifically, the output of an economy is the national income for the economy. The per capita income is the national income for the economy divided by the population. This number is useful for comparing the standard of living across countries. All of this information directly results from the aggregate demand equation. The second piece of the aggregate demand equation is C(Y - T). This signifies that consumption is a function of disposable income. Disposable income is the money that consumers have left to spend after taxes. The function for consumption is aggregated across all consumers and thus is applicable for all incomes and tax brackets. Consumption captures spending by households on goods and services. Examples include purchasing food, movie tickets, and vacations. The third piece of the aggregate demand equation is I(r). This signifies that investment spending is a function of the real interest rate. That is, as the real interest rate increases, investment spending falls because the cost of borrowing money increases. The real interest rate is simply the nominal interest rate as published in the media corrected for expected inflation. When firms consider investment spending, they routinely take into account the nominal interest rate, inflation, and the real interest rate. Examples of investment spending include machinery, buildings, education, and new housing. The fourth piece of the aggregate demand equation is G. Government spending encompasses every expenditure made by the government. The total amount of money spent by the government is often surprising. In fact, it is not unusual for government spending to constitute upwards of one third of gross domestic product. The level of government spending is a hotly debated topic as political parties vie for their programs in the annual budget. Examples of government spending include salaries to government employees, defense spending, welfare and social security programs, and foreign aid. The fifth piece of the aggregate demand equation is NX(e). Net exports are defined as the difference between exports and imports. It is important to recognize that net exports are dependent upon the real exchange rate. As the real exchange rate rises, domestic currency is relatively more valuable and thus the price of domestic goods is relatively more expensive than the price of foreign goods. In this case, exports fall and imports rise, causing net exports to decline. Interestingly, a thriving domestic economy will result in a higher real exchange rate and thus lower net exports. Examples of exports include cars and electronics made in the US and sold Asian countries. Examples of imports include fruits and vegetables grown in New Zealand and sold in the US. The equation for aggregate demand of Y = C(Y - T) + I(r) + G + NX(e) has now been deciphered. This equation has many meanings such as output, national income, and GDP. It is difficult, or impossible, to think of economic activity that is not represented in the aggregate demand equation. This is the idea of aggregate demand: to capture all economic activity within an economy
The Aggregate Demand Curve Downward sloping aggregate demand curve
Figure 2.1: Graph of the aggregate demand curve. The most noticeable feature of the aggregate demand curve is that it is downward sloping, as seen in figure 2.1. There are a number of reasons for this relationship. Recall that a downward sloping aggregate demand curve means that as the price level drops, the quantity of output demanded increases. Similarly, as the price level drops, the national income increases. There are three basic reasons for the downward sloping aggregate demand curve. These are Pigou's wealth effect, Keynes's interest-rate effect, and Mundell-Fleming's exchange-rate effect. These three reasons for the downward sloping aggregate demand curve are distinct, yet they work together. The first reason for the downward slope of the aggregate demand curve is Pigou's wealth effect. Recall that the nominal value of money is fixed, but the real value is dependent upon the price level. This is because for a given amount of money, a lower price level provides more purchasing power per unit of currency. When the price level falls, consumers are wealthier, a condition which induces more consumer spending. Thus, a drop in the price level induces consumers to spend more, thereby increasing the aggregate demand. The second reason for the downward slope of the aggregate demand curve is Keynes's interest-rate effect. Recall that the quantity of money demanded is dependent upon the price level. That is, a high price level means that it takes a relatively large amount of currency to make purchases. Thus, consumers demand large quantities of currency when the price level is high. When the price level is low, consumers demand a relatively small amount of currency because it takes a relatively small amount of currency to make purchases. Thus, consumers keep larger amounts of currency in the bank. As the amount of currency in banks increases, the supply of loans increases. As the supply of loans increases, the cost of loans--that is, the interest rate--decreases. Thus, a low price level induces consumers to save, which in turn drives down the interest rate. A low interest rate increases the demand for investment as the cost of investment falls with the interest rate. Thus, a drop in the price level decreases the interest rate, which increases the demand for investment and thereby increases aggregate demand. The third reason for the downward slope of the aggregate demand curve is Mundell-Fleming's exchange-rate effect. Recall that as the price level falls the interest rate also tends to fall. When the domestic interest rate is low relative to interest rates available in foreign countries, domestic investors tend to invest in foreign countries where return on investments is higher. As domestic currency flows to foreign countries, the real exchange rate decreases because the international supply of dollars increases. A decrease in the real exchange rate has the effect of increasing net exports because domestic goods and services are relatively cheaper. Finally, an increase in net exports increases aggregate demand, as net exports is a component of aggregate demand. Thus, as the price level drops, interest rates fall, domestic investment in foreign countries increases, the real exchange rate depreciates, net exports increases, and aggregate demand increases. IS-LM model of aggregate demand There is another major model that is useful for explaining the nature of the aggregate demand curve. This model is called the IS-LM model after the two curves that are involved in the model. The IS curve describes equilibrium in the market for goods and services where Y = C(Y - T) + I(r) + G and the LM curve describes equilibrium in the money market where M/P = L(r,Y). The IS-LM model exists in a plane with r, the interest rate, on the vertical axis and Y, being both income and output, on the horizontal axis. The IS-LM model has the same horizontal axis as the aggregate demand curve, but a different vertical axis.
Figure 2.2: Graph of the IS-LM curves. The IS curve describes equilibrium in the market for goods and services in terms of r and Y. The IS curve is downward sloping because as the interest rate falls, investment increases, thus increasing output. The LM curve describes equilibrium in the market for money. The LM curve is upward sloping because higher income results in higher demand for money, thus resulting in higher interest rates. The intersection of the IS curve with the LM curve shows the equilibrium interest rate and price level. The IS curve and the LM curve shift in response to economic activities. The IS curve shifts outward as a result of increased government purchases, exogenous increases in investment, decreases in taxes, and exogenous increases in consumption. The IS curve shifts inward as a result of decreases in government purchases, exogenous decreases in investment, increases in taxes, and exogenous decreases in consumption. The LM curve shifts outward as a result of increases in the money supply and decreases in the price level. The LM curve shifts inward as a result of decreases in the money supply and increases in the price level. The aggregate demand curve can be derived using the IS-LM model. Recall that the aggregate demand curve relates price level to income and output. The simplest way to derive the downward sloping aggregate demand curve from the IS-LM model is to look at the effects of an increase in the price level on output or income. When the price level increases, the LM curve shifts inward. An inward shift in the LM curve results in an intersection of the IS-LM model at a lower level of output and income and a higher interest rate. When a line connecting the old price level and the old output and income to the new price level and the new output and income in the price level and output and income space, the downward sloping aggregate demand curve appears. In general, from the IS-LM model, it is clear that aggregate demand slopes downward because as the price level increases, output and income decrease. The IS-LM curve is a useful way to incorporate the money market into the logic driving the aggregate demand curve. By understanding the basics of the IS-LM model and the three reasons that the aggregate demand curve is downward sloping as presented under the previous heading, the nature of the aggregate demand curve is clear. The next step to work through is how shifts of and shifts along the aggregate demand curve function. In this capacity, the IS-LM model will become very useful.
Shifts in the Aggregate Demand Curve Shifts to the left There are many actions that will cause the aggregate demand curve to shift. When the aggregate demand curve shifts to the left, the total quantity of goods and services demanded at any given price level falls. This can be thought of as the economy contracting. To understand what causes the economy to contract, let's start with the basic equation for the demand curve. Recall that the price level is not directly in the equation for aggregate demand. Rather, it is implicit in each of the terms in the equation. We know that aggregate demand is comprised of C(Y - T) + I(r) + G + NX(e) = Y. Thus, a decrease in any one of these terms will lead to a shift in the aggregate demand curve to the left. The first term that will lead to a shift in the aggregate demand curve is C(Y - T). This term states that consumption is a function of disposable income. If disposable income decreases, consumption will also decrease. There are many ways that consumption can decrease. An increase in taxes would have this effect. Similarly, a decrease in income--holding taxes stable--would also have this effect. Finally, a decrease in the marginal propensity to consume or an increase in the savings rate would also decrease consumption. The second term that will lead to a shift in the aggregate demand curve is I(r). This term states that investment is a function of the interest rate. If the interest rate increases, investment falls as the cost of investment rises. There are a number of ways that investment can fall. If the interest rate rises, say due to contractionary monetary or fiscal policy, investment will fall. Similarly, in the short run, expansionary fiscal policy will also cause investment to fall as crowding out occurs. Another interesting cause of a fall in investment is an exogenous decrease in investment spending. This occurs when firms simply decide to invest less without regard for the interest rate. The term variable that will lead to a shift in the aggregate demand curve is G. This term captures the whole of government spending. The only way that government spending is changed is though fiscal policy. Recall that the budgetary debate is an ongoing political battlefield. Thus, government spending tends to change regularly. When government spending decreases, regardless of tax policy, aggregate demand decrease, thus shifting to the left. The fourth term that will lead to a shift in the aggregate demand curve is NX(e). This term means that net exports, defined as exports less imports, is a function of the real exchange rate. As the real exchange rate rises, the dollar becomes stronger, causing imports to rise and exports to fall. Thus, policies that raise the real exchange rate though the interest rate will cause net exports to fall and the aggregate demand curve to shift left. Again, an exogenous decrease in the demand for exported goods or an exogenous increase in the demand for imported goods will also cause the aggregate demand curve to shift left as net exports fall. An example of this type of exogenous shift would be a change in tastes or preferences. Shifts to the right The aggregate demand curve also can shift right as the economy expands. When the aggregate demand curve shifts right, the quantity of output demanded for a given price level rises. Therefore, a shift of the aggregate demand curve to the right represents an economic expansion. A shift of the aggregate demand curve to the right is simply effected by the opposite conditions that cause it to shift to the left. Limits of aggregate demand The aggregate demand curve alone is useful. It tells how the price level and output or income are related. It shows the general effects of changes in many economic variables and the relationship between price level and output or income. But there are limits to its usefulness. It cannot show where the economy currently sits. Similarly, it cannot predict the effects of an economic policy upon the economy. In the next section, we will look at aggregate supply. This counterpart to aggregate demand completes the AS-AD model of the macroeconomy. That is, the aggregate supply and aggregate demand model of the economy is based on the total demand for goods and services and the total supply of goods and services. Once you are comfortable with the reasons for the downward sloping aggregate demand curve and with the ways and directions that the aggregate demand curve shifts, you are prepared to move on to the aggregate supply curve. The aggregate demand curve tells how the price level and output and income are related. It shows the general effects of changes in many economic variables on the relationship between price level and output and income.
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