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# Aggregate Demand Curve

Imagine if, all of a sudden, the Central Bank of England decided to increase the interest rates. What would the economic impact of that decision be? What would happen to the price level? Would the unemployment increase? What would happen to the GDP?

You can address all the questions using the aggregate demand curve. The aggregate demand curve illustrates the relationship between the price level and the output in an economy. It is also a helpful tool in analysing what happens to the price level and GDP when external factors act upon the aggregate demand. This explanation will help you learn everything you need to know about the aggregate demand curve to analyse any important economic event.

## The aggregate demand graph

The aggregate demand curve is a curve that illustrates the total amount of goods and services produced in the economy over a period of time. The aggregate demand curve shows the relationship between the total and the general price level in the economy.

In figure 1, you can see a standard aggregate demand curve that demonstrates a movement along the curve. On the x-axis, we have the real GDP, which represents the amount of output in an economy. On the y-axis, we have the general price level (£) at which the output in the economy is produced.

Fig 1. - Movement Along Aggregate Demand Curve

Remember, aggregate demand is a measure of the total expenditure on a country’s goods and services. We are measuring the total amount of spending in an economy from households, firms, the government, and exports minus imports.

Table 1. Aggregate Demand Curve explanation
 Contraction of AD Expansion of AD We can take a given level of output Q1 at a general price level of P1. Let’s just assume that the general price level has increased from P1 to P2. Then, the real GDP, the output, would decrease from Q1 to Q2. This movement along the aggregate demand curve is called a contraction of aggregate demand. This is shown in Figure 1 above. We can take a given level of output Q1 at a general price level of P1. Let’s just assume that the general price level has decreased from P1 to P3. Then, the real GDP, the output, would increase from Q1 to Q3. This movement along the aggregate demand curve is called an expansion or extension of aggregate demand. This is shown in Figure 1 above.

## Derivation of the aggregate demand curve

There are three reasons why the AD curve is downward sloping. Aggregate demand can only ever change if households’ consumption, firms’ investments, government spending, or net export spending increase or decrease. If the AD is sloping downward, the aggregate demand changes purely due to price level changes.

### The wealth effect

The first reason for a downward-sloping curve is the so-called ‘Wealth Effect’, which states that as the price level decreases, the purchasing power of households increases. This means that people have more disposable income and are therefore more likely to spend on goods and services in the economy. In this case, consumption increases solely due to a price level decrease and there is an increase in aggregate demand otherwise known as an extension of AD.

The second reason is the ‘Trade Effect’, which states that if price level decreases, causing a depreciation in the domestic currency, exports become more internationally price competitive and there will be greater demand for exports. The exports will generate more revenue, which will increase the value of X in the AD equation.

Imports, on the other hand, will become more expensive because the domestic currency will depreciate. If the import volumes are to remain the same, there will be more spending on imports, causing an increase in the value of ‘M’ in the AD equation.

The overall effect on the aggregate demand due to a decrease in the price level via the trade effect is therefore ambiguous. It will depend on the relative proportion of export and import volumes. If the export volumes are larger than the import volumes, there will be an increase in the AD. If the import volumes are larger than the export volumes, there will be a fall in the AD.

To understand the effects on the aggregate demand always refer to the aggregate demand equation.

### Interest effect

The third reason is the ‘Interest Effect’, which states that if price levels were to decrease due to the rise of supply commodities relative to the demand of the commodities, the banks would also lower the interest rates for them to match the inflation target. Lower interest rates mean that the cost of borrowing money is lower and that there is a lower incentive to save money as borrowing has become easier for households. This would increase income levels and the consumption of households in the economy. It would also encourage firms to borrow more and invest more in capital goods such as machinery promoting economic activity contributing to an expansion of aggregate demand.

## Aggregate demand curve shift

What affects the aggregate demand curve? The main determinants of AD are consumption from households (C), investments of firms (I), government (G) spending on the public (health care, infrastructure, etc.) as well as the spending on net exports (X - M).

If any of these determinants of aggregate demand, excluding the general price levels, change due to external reasons, the AD curve shifts either to the left (inward) or to the right (outward) depending on whether there has been an increase or a decrease in those components.

Keep in mind this formula.

For further information about the aggregate demand components and their effects, check out our explanation on Aggregate Demand.

To summarise, if the determinants of consumption (C), investment (I), government spending (G), or net exports increase (X-M), independent of the price level, the AD curve will shift to the right.

If there is a decrease in any of these determinants, independent of the price level, then there will be a decrease in aggregate demand and a shift to the left (inward).

Let’s look at some examples:

An increase in consumer confidence, where households are willing and able to spend more money on goods and services due to high optimism, will increase aggregate demand and shift the aggregate demand curve outward.

Increased investments from firms in their capital goods such as machinery or factories due to potentially lower interest rates, would increase aggregate demand and shift the aggregate demand curve outward (to the right).

Increased government spending due to an expansionary fiscal policy as well as central banks setting expansionary monetary policies to promote firms’ investments and households’ borrowing are also contributing factors on why aggregate demand could shift outward.

Increases in net exports where a country is exporting more of its goods and services than it is importing will see a growth in aggregate demand as well as generating increased levels of revenue.

Conversely, a fall in consumer confidence due to lower optimism; a fall in investments from firms due to higher interest rates with the banks setting a contractionary monetary policy; decreased government spending due to a contractionary fiscal policy; and increased imports are factors that will cause the aggregate demand curve to shift inward.

## Aggregate demand diagrams

Let’s look at graphical examples for both cases of an increase in aggregate demand and a decrease in aggregate demand.

### Increase in aggregate demand

Let’s say Country X enacts an expansionary fiscal policy to boost economic growth. In this scenario, the government of Country X would reduce taxes and increase spending on the public. Let’s see how this would affect the aggregate demand curve.

Fig 2. - Outward Shift

Since Country X has implemented the expansionary fiscal policy of reducing the taxation rates on households and businesses, and increasing overall government spending on the public sector in infrastructure and health care, we can deduce how that would affect the aggregate demand curve.

The government reducing tax rates for households would lead to consumers having a higher disposable income, and thus more money to spend on goods and services. This would make the aggregate demand curve (AD1) shift to the right and overall real GDP would subsequently increase from Q1 to Q2.

The businesses would also have to pay lower taxes and would be able to spend their money on capital goods in the form of investments in machinery or building new factories. This would encourage further economic activity as the firms would need to hire more labour to work in these factories and earn a salary.

Finally, the government would also increase spending on the public sector such as building new roads and investing in public health care services. This would encourage further economic activities in the country as more jobs were being created through these various projects. The price in this structure remains constant at P1, as a shift of the AD curve only occurs in events independent of price level changes.

### Decrease in aggregate demand

Conversely, let’s say that the government of Country X enacts a contractionary fiscal policy. This policy involves raising taxes and reducing government spending to tackle the issue of inflation, for example. In this case, we would see a decrease in overall aggregate demand. Take a look at the graph below to see how that would work.

Fig 3. - Inward Shift

Based on the contractionary fiscal policy the government has enacted we will see increased taxation as well as decreased spending on the public sector. We know that government spending is one of the main components of aggregate demand, and a decrease in one of the components will cause the AD curve to shift inward.

Since taxation rates are higher, households will be less inclined to spend their money as most of it is being taxed by the government. Hence, we will see fewer households spend their money on goods and services, thus decreasing overall consumption.

Additionally, a business paying higher rates of taxes won’t be inclined to keep investing in more of their capital goods such as machinery and new factories, thus reducing their overall economic activity.

With overall investments from firms, consumption of households and spending from the government going down, the AD curve will shift inward from AD1 to AD2. Subsequently, the real GDP will decrease from Q1 to Q2. Price remains constant at P as the determining factor of the shift was the contractionary fiscal policy and not a price change.

## Aggregate demand and the national income multiplier

The national income multiplier measures the change between a component of aggregate demand (could be consumption, government spending, or investments from firms) and the resulting larger change in national income.

Let’s take a scenario where the US government increases government spending by 8 billion dollars, but their tax revenue generated in that year remains the same (constant). The increase in government spending will result in a budget deficit and it will be injected into the circular flow of income. However, the increased government spending will lead to an increase in the income of households in the US.

Now, let’s assume that the households decide to save a fraction of the amount of their income that has increased and spend the rest of the money on goods and services.

### The accelerator theory of investment

The accelerator effect is the relationship between the rate of change in national income and planned capital investment.

The assumption here is that firms desire to keep a fixed ratio, also known as the capital-output ratio, between the output of goods and services they are currently producing and the existing stock of fixed capital assets. For example, if they need 3 units of capital to produce 1 unit of output, the capital-output ratio is 3 to 1. The capital ratio is also known as the accelerator coefficient.

If the growth of the amount of national output remains constant on a yearly basis, firms will invest the exact same amount of new capital each year to enlarge their capital stock and maintain their desired capital-output ratio. Hence, on a yearly basis, the level of investment remains constant.

If the growth of the amount of national output accelerates, investments from firms will also increase into their stock of capital assets to a sustainable level to maintain the desired capital-output ratio.

Conversely, if the growth of the amount of national output decelerates, investments from firms will also decrease into their stock of capital assets to maintain the desired capital-output ratio.

## Aggregate Demand Curve - Key takeaways

• The aggregate demand curve is a curve that illustrates the total amount of goods and services produced in the economy over a period of time. The aggregate demand curve shows the relationship between the total real output and the general price level in the economy.
• A fall in the general price level will lead to an expansion of aggregate demand. Conversely, a rise in the general price level will lead to a contraction of aggregate demand.
• An increase in the components of the aggregate demand, independent of the price level, leads to an outward shift of the AD curve.
• A decrease in the components of the aggregate demand, independent of the price level, leads to an inward shift of the AD curve.
• The national income multiplier measures the change between a component of aggregate demand (consumption, government spending, or investments from firms) and the resulting larger change in national income.
• The accelerator effect is the relationship between the rate of change in national income and planned capital investment.

The aggregate demand curve shifts if there are changes occurring in the main components of aggregate demand due to non-price factors.

At an aggregate level, when the price level decreases, we expect to see:
1. Increased household spending due to higher disposable income.

2. Increased investment by firms as the interest rates for borrowing decrease.

These are some reasons that are consistent with the aggregate demand curve sloping downwards.

The aggregate demand curve can be estimated by finding real GDP and plotting it with the price level on the vertical axis and the real output on the horizontal axis.

The components that affect aggregate demand are consumption, investment, government spending, and net exports.

## Final Aggregate Demand Curve Quiz

Question

What happens to the aggregate demand curve when there is a fall in price?

A fall in general price levels will lead to an expansion of aggregate demand.

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Question

What happens to the aggregate demand curve when there is a rise in price?

A rise in general price levels will lead to a contraction of aggregate demand.

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Question

When can aggregate demand change?

Aggregate demand can only ever change if consumption of households, investments of firms, spendings by the governments, or spendings on net exports increase or decrease.

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Question

What are the three effects called that make the AD curve downward sloping?

-The wealth effect.

-The interest effect.

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Question

When is there a shift in the aggregate demand curve?

When the components of the AD curve change for reasons independent of price.

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Question

What shifts the aggregate demand curve?

The AD curve shifts if there are changes in the main components of aggregate demand: consumption of households, investments from firms, government spending, and changes in net exports, that do not involve any price level changes.

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Question

Why does the aggregate demand curve slope down?

Aggregate demand can only ever change if consumption of households, investments of firms, spendings by the governments, or spendings on net exports increase or decrease. If the AD is downward sloping, the aggregate demand is changing purely for price level changes.

Price level changes are related to the wealth effect, trade effect, and interest effect.

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Question

What is the wealth effect?

The wealth effect states that as the price level decreases, the purchasing power of households increases. This means that people have more income and are richer, and are more likely to spend on goods and services in the economy, thus promoting further consumption and growth.

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Question

What is the national income multiplier?

The national income multiplier measures the change between a component of aggregate demand (consumption, government spending, or investments from firms) and the resulting larger change in national income.

Show question

Question

The Trade Effect states that if the price level decreases, causing a depreciation in the domestic currency, exports become more internationally price competitive and there will be greater demand for exports. More revenue will be generated from the exports, which will increase the value of X in the AD equation.

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Question

What is the interest effect?

The Interest Effect states that if price levels were to decrease due to the rise of supply commodities relative to the demand of the commodities, interest rates would also be lowered by the banks for them to match the inflation target.

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Question

How many types of multipliers are there and what are their names?

• Government spending multiplier
• Investment multiplier
• Tax multiplier
• Export and import multiplier

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Question

What is the marginal propensity to consume?

The marginal propensity to consume, otherwise known as MPC, represents the fraction of an increase in the disposable income (the increase in income after it has been taxed by the government), that people plan to spend domestically produced goods and services.

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Question

What is the accelerator theory of investment?

The accelerator theory otherwise known as the   accelerator effect   is the relationship between   planned capital investment    and the   rate of change in national income or aggregate demand.

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Question

What affects the aggregate demand curve?

The main determinants of AD are consumption from households (C), investments of firms (I), and government spending on the public (G).

Show question

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