A demand curve shows how much of a particular good or service buyers would purchase at different prices, assuming other relevant conditions stay the same. A change in the good’s own price moves buyers along the curve; a change in another factor, such as income or the price of a substitute, can shift the whole curve.
What is a demand curve?
A demand curve is a graph of the relationship between a good’s price and the quantity buyers demand at each price, with other economically relevant conditions held constant. In OpenStax’s explanation of demand, price is plotted on the vertical axis and quantity on the horizontal axis. A table listing quantities demanded at different prices is called a demand schedule.
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The law of demand says that, other things equal, a higher price typically corresponds to a lower quantity demanded, while a lower price corresponds to a higher quantity demanded. That inverse relationship is why an ordinary demand curve slopes downward. It is a simplified model: different goods and buyers can respond differently, and changing circumstances can alter the relationship.
The phrase ceteris paribus means “other things being equal.” The curve isolates the effect of price on quantity demanded by holding other relevant factors constant. If both price and income change, for example, they can affect purchases through separate channels; the observed change may reflect both.
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A simple demand schedule
The following hypothetical schedule illustrates the usual downward relationship. It is not market data.
| Price per item | Quantity demanded per week |
|---|---|
| $10 | 2 |
| $8 | 4 |
| $6 | 6 |
| $4 | 8 |
Plot each price-and-quantity pair with quantity on the horizontal axis and price on the vertical axis, then connect the points. The resulting line or curve makes the schedule’s relationship visible. Real demand curves need not be straight lines.
What is the difference between a change in demand and a change in quantity demanded?
A change in quantity demanded is a movement from one point to another on the same demand curve, caused by a change in the good’s own price. A change in demand is a shift of the entire curve, caused by a non-price determinant: buyers demand more or less at each possible price.
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| What changed? | What happens on the graph? | How to describe it |
|---|---|---|
| The good’s own price | Movement along the existing curve | Quantity demanded changes |
| A non-price determinant | The whole curve shifts right or left | Demand increases or decreases |
OpenStax puts the distinction this way: “A change in the price of a good or service causes a movement along a specific demand curve, and it typically leads to some change in the quantity demanded, but it does not shift the demand curve.” The statement appears in section 3.2 of Principles of Economics 3e.
A rightward shift means buyers want more at every given price; a leftward shift means they want less at every given price. A shift describes a pattern across buyers in a market, not a claim that every individual changes purchases by the same amount.
OpenStax illustrates income-driven shifts with cars: at a fixed price of $20,000, its textbook example goes from 18 million cars on the original curve to 20 million after an income increase, and to 14.4 million after an income decrease. These are textbook illustration values, not current market sales figures.
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What causes a demand curve to shift?
A shift occurs when a condition other than the good’s own price changes and affects how much buyers are willing to purchase at each price. The main determinants include:
- Income: For a normal good, demand usually rises as income rises. For an inferior good, demand falls as income rises. “Inferior” is an economic classification, not a judgment about quality: generic groceries, used cars, or renting an apartment may be choices some consumers reduce as their income increases.
- Tastes and preferences: If a product becomes more popular, demand can increase at each price; if it loses popularity, demand can decrease.
- Population size and composition: A larger potential group of buyers can raise market demand. Changes in age composition can also change demand for goods and services relevant to different age groups.
- Prices of related goods: A substitute can be used in place of the focal good. If the substitute’s price rises, demand for the focal good tends to rise. Complements are often used together; if a complement’s price rises, demand for the focal good tends to fall.
- Expectations: Buyers’ expectations about future prices or other conditions can affect purchases now. For instance, some buyers may purchase earlier if they expect a future price increase.
- Other market-specific conditions: Relevant non-price circumstances depend on the good and the period being studied. A demand curve abstracts from factors that are not changing in the comparison.
These are general relationships, not automatic outcomes in every market. To identify a shift, specify which non-price condition changed and why it would affect buyers’ willingness to purchase.
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What are the types of demand curve?
“Types” can refer to several ways of classifying demand. A useful introductory classification is by price elasticity: elastic, inelastic, or unit elastic. Elasticity measures the percentage responsiveness of quantity demanded to a percentage change in price.
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Price elasticity of demand is calculated as the percentage change in quantity demanded divided by the percentage change in price. Because price and quantity demanded usually move in opposite directions, the raw ratio is negative; textbooks conventionally use its absolute value when classifying elasticity.
| Classification | Absolute price elasticity | Meaning |
|---|---|---|
| Elastic | Greater than 1 | Quantity demanded changes by a larger percentage than price. |
| Inelastic | Less than 1 | Quantity demanded changes by a smaller percentage than price. |
| Unit elastic | Equal to 1 | Quantity demanded changes by the same percentage as price. |
OpenStax’s introduction to price elasticity notes that demand is likely to be more elastic when buyers have many substitutes or when the good takes a large share of their budget. Necessities without close substitutes may be highly inelastic. These are tendencies, not guarantees; elasticity depends on the market and the time period.
The two theoretical extremes
- Perfectly elastic demand: The curve is horizontal in the model. A deviation from the specified price produces an extreme quantity response.
- Perfectly inelastic demand: The curve is vertical in the model. Quantity does not respond to price.
These are polar cases, not assumptions to apply casually to ordinary products. OpenStax discusses them as theoretical limits in its sections on price elasticity and polar cases of elasticity.
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Slope describes the change in price relative to the change in quantity in the units shown on the graph. Elasticity instead compares percentage changes in quantity and price. Because one is based on plotted units and the other on percentage responsiveness, a curve’s visual steepness alone does not tell you whether demand is elastic or inelastic.
Elasticity can also vary at different points along a demand curve. A straight-looking curve therefore should not be labeled elastic or inelastic everywhere just by eye. Use percentage changes to assess responsiveness.
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How to read a demand-curve change
- Identify what changed. If it is the good’s own price, look for movement along the curve. If it is another determinant, consider a shift.
- Check the direction. A right shift represents greater quantity demanded at every given price; a left shift represents less.
- For an income change, classify the good. State whether the example is normal or inferior, then identify the direction of demand change.
- For a related good, identify the relationship. Decide whether it is a substitute or complement and trace how its price change affects demand for the focal good.
- For responsiveness, compare percentage changes. Use elasticity rather than visual slope to distinguish elastic, inelastic, and unit-elastic demand.
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