Why would the number of consumers in a market shift the demand curve left or right?

As stated earlier, the quantity of an item that either an individual consumer or a market of consumers demands is determined by a number of different factors, but the demand curve represents the relationship between price and quantity demanded with all other factors affecting demand held constant. So what happens when a determinant of demand other than price changes?

The answer is that when a non-price determinant of demand changes, the overall relationship between price and quantity demanded is affected. This is represented by a shift of the demand curve, so let's think about how to shift the demand curve.

An increase in demand is represented by the diagram above. An increase in demand can either be thought of as a shift to the right of the demand curve or an upward shift of the demand curve. The shift to the right interpretation shows that, when demand increases, consumers demand a larger quantity at each price. The upward shift interpretation represents the observation that, when demand increases, consumers are willing and able to pay more for a given quantity of the product than they were before. (Note that the horizontal and vertical shifts of a demand curve are generally not of the same magnitude.)

Shifts of the demand curve need not be parallel, but it's helpful (and accurate enough for most purposes) to generally think of them that way for the sake of simplicity.

In contrast, a decrease in demand is represented by the diagram above. A decrease in demand can either be thought of as a shift to the left of the demand curve or a downward shift of the demand curve. The shift to the left interpretation shows that, when demand decreases, consumers demand a smaller quantity at each price. The downward shift interpretation represents the observation that, when demand decreases, consumers are not willing and able to pay as much as before for a given quantity of the product. (Again, note that the horizontal and vertical shifts of a demand curve are generally not of the same magnitude.)

Again, shifts of the demand curve need not be parallel, but it's helpful (and accurate enough for most purposes) to generally think of them that way for the sake of simplicity.

In general, it's helpful to think about decreases in demand as shifts to the left of the demand curve (i.e. a decrease along the quantity axis) and increases in demand as shifts to the right of the demand curve (i.e. an increase along the quantity axis), since this will be the case regardless of whether you're looking at a demand curve or a supply curve.

Since we identified a number of factors other than price that affect the demand for an item, it's helpful to think about how they relate to our shifts of the demand curve:

• Income: An increase in income will shift demand to the right for a normal good and to the left for an inferior good. Conversely, a decrease in income will shift demand to the left for a normal good and to the right for an inferior good.
• Prices of Related Goods: An increase in the price of a substitute will shift demand to the right, as will a decrease in the price of a complement. Conversely, a decrease in the price of a substitute will shift demand to the left, as will an increase in the price of a complement.
• Tastes: An increase in tastes for a product will shift demand to the right, and a decrease in tastes for a product will shift demand to the left.
• Expectations: A change in expectations that increases current demand will shift the demand curve to the right, and a change in expectations that decreases current demand will shift the demand curve to the left.
• Number of Buyers: An increase in the number of buyers in a market will shift market demand to the right, and a decrease in the number of buyers in a market will shift market demand to the left.

This categorization is shown in the diagrams above, which can be used as a handy reference guide.

Aggregate demand (AD) is the total amount of goods and services consumers are willing to purchase in a given economy and during a certain period. Sometimes aggregate demand changes in a way that alters its relationship with aggregate supply (AS), and this is called a "shift."

Since modern economists calculate aggregate demand using a specific formula, shifts result from changes in the value of the formula's input variables: consumer spending, investment spending, government spending, exports, and imports.

• Aggregate demand (AD) is the total amount of goods and services in an economy that consumers are willing to purchase during a specific time frame.
• When aggregate demand changes in its relationship with aggregate supply, this is known as a shift in aggregate demand.
• Aggregate demand consists of the sum of consumer spending, investment spending, government spending, and the difference between exports and imports.
• When any of these aggregate demand inputs change, then there is a shift in aggregate demand.

A D = C + I + G + ( X − M ) where: C = Consumer spending on goods and services I = Investment spending on business capital goods G = Government spending on public goods and services X = Exports M = Imports \begin{aligned} &AD=C+I+G+(X-M)\\ &\textbf{where:}\\ &C = \text{Consumer spending on goods and services}\\ &I = \text{Investment spending on business capital goods}\\ &G = \text{Government spending on public goods and services}\\ &X = \text{Exports}\\ &M = \text{Imports} \end{aligned} ​AD=C+I+G+(X−M)where:C=Consumer spending on goods and servicesI=Investment spending on business capital goodsG=Government spending on public goods and servicesX=ExportsM=Imports​

Any aggregate economic phenomena that cause changes in the value of any of these variables will change aggregate demand. If aggregate supply remains unchanged or is held constant, a change in aggregate demand shifts the AD curve to the left or to the right.

In macroeconomic models, right shifts in aggregate demand are typically viewed as a sign that aggregate demand increased or is growing—typically viewed as positive. Shifts to the left, a decrease in aggregate demand, mean that the economy is declining or shrinking—typically viewed as negative.

However, this is not always the case. For example, a reduction in aggregate demand might be engineered by the government to reduce inflation, which is not necessarily something negative.

The aggregate demand curve tends to shift to the left when total consumer spending declines. Consumers might spend less because the cost of living is rising or because government taxes have increased.

Consumers may decide to spend less and save more if they expect prices to rise in the future. It might be that consumer time preferences change and future consumption is valued more highly than present consumption.

Contractionary fiscal policy can also shift aggregate demand to the left. The government might decide to raise taxes or decrease spending to fix a budget deficit. Monetary policy has less immediate effects. If monetary policy raises the interest rate, individuals and businesses tend to borrow less and save more. This could shift AD to the left.

The last major variable, net exports (exports minus imports), is less direct and more controversial. A country’s current account surplus is always balanced by the change in the capital account (that is, a trade surplus or positive net exports). This would imply a net influx of foreign currency or dollars held abroad to pay for the fact that foreigners are buying more U.S. goods than they are selling to the U.S. This situation would lead to an increase in U.S. foreign currency holdings or an influx of U.S. dollars held abroad and would generally positively shift aggregate demand.

According to macroeconomic theory, a demand shock is an important change somewhere in the economy that affects many spending decisions and causes a sudden and unexpected shift in the aggregate demand curve.

Some shocks are caused by changes in technology. Technological advances can make labor more productive and increase business returns on capital. This is normally caused by declining costs in one or more sectors, leaving more room for consumers to buy additional goods, save, or invest. In this case, the demand for total goods and services increases at the same time prices are falling.

Diseases and natural disasters can cause negative demand shocks if they limit earnings and cause consumers to buy fewer goods. For example, Hurricane Katrina caused negative supply and demand shocks in New Orleans and the surrounding areas. And post-WWII, it's commonly held that the United States experienced a positive demand shock, particularly with real commodities.

Aggregate demand is the total amount of goods and services in an economy that consumers are willing to pay for within a certain time period. Aggregate demand is calculated as the sum of consumer spending, investment spending, government spending, and the difference between exports and imports.

Whenever one of these factors changes and when aggregate supply remains constant, then there is a shift in aggregate demand. Utilizing the aggregate demand curve, a shift to the left, a reduction in aggregate demand, is perceived negatively, while a shift to the right, an increase in aggregate demand, is perceived positively.