Law of Demand
Introduces the law of demand: as the price of a good rises, the quantity demanded of it falls, all other factors held constant (ceteris paribus), producing an inverse relationship between price and quantity demanded. Explains why the ceteris paribus assumption is essential to isolating the price effect from other influences on demand. Contains: text explanation, a key-concept callout defining ceteris paribus, a worked example distinguishing a price-driven change from a non-price-driven change, and a common-mistake callout on confusing 'demand' with 'quantity demanded'.
The law of demand is one of the foundational principles of microeconomics. It states that, ceteris paribus, as the price of a good rises, the quantity demanded of that good falls; conversely, as the price falls, the quantity demanded rises. This produces an inverse relationship between price and quantity demanded: the two variables move in opposite directions.
Ceteris paribus is Latin for "all other things being equal" (or unchanged). Economists use this assumption to isolate the relationship between two variables — here, price and quantity demanded — by holding every other influence on demand (income, tastes, prices of related goods, expectations, number of consumers) constant. Without this assumption, we could never be sure whether a change in quantity demanded was caused by the price change itself or by something else happening at the same time.
Quantity demanded refers to the specific amount of a good or service that consumers are willing and able to purchase at a given price, within a given time period. It is important to recognise that the law of demand describes how this quantity responds only to changes in the good's own price, with every other determinant of demand held fixed. This distinction between a change in the price of the good itself and a change in any other factor is central to understanding demand throughout this subtopic.
Applying the law of demand: isolating a price effect
- Scenario: the price of coffee rises from 4 per cup, and nothing else in the market changes (consumer incomes, tastes, the price of tea, expectations, and the number of coffee drinkers all stay the same).
- Under the law of demand, ceteris paribus, this price rise alone should cause the quantity demanded of coffee to fall — consumers buy fewer cups at 3.
- Now compare a different scenario: the price of coffee stays at $3, but a health report claims coffee reduces long-term risk of a certain illness, so consumer tastes shift in coffee's favour and quantity demanded rises at every price.
- In the second scenario, quantity demanded changed even though price did not — this is not an application of the law of demand, because ceteris paribus was violated (tastes changed). This second case is a change in demand itself, not a movement described by the law of demand.
Common mistake: Students often say "demand fell" when the price of a good rises, when in fact the law of demand describes a fall in quantity demanded, not in demand itself. "Demand" refers to the whole relationship between price and quantity demanded (the entire schedule or curve); "quantity demanded" refers to one specific amount at one specific price. A price change causes quantity demanded to change — it does not cause demand itself to change.
- Law of demand: as price rises, quantity demanded falls, ceteris paribus — an inverse relationship.
- Ceteris paribus means "all other things being equal"; it holds every non-price determinant of demand constant.
- A change in price alone changes quantity demanded, never "demand" itself.
- The law of demand applies only to the good's own price — changes caused by income, tastes, related-good prices, expectations, or number of consumers are separate from this law.
Income Effect on Demand
Explains the income effect as one of the two reasons behind the law of demand: when the price of a good falls, a consumer's real income (purchasing power) rises even though their money income is unchanged, allowing them to buy more of that good and other goods. The key insight is distinguishing real income (purchasing power) from money/nominal income, and separating the income effect from the substitution effect and from a non-price shift in demand caused by a change in actual income. Contains: text explanation, worked example tracing a price fall through to increased real income and quantity demanded, and a common-mistake callout distinguishing the income effect from income as a determinant of demand.
The law of demand states that, ceteris paribus, as the price of a good falls, the quantity demanded of that good rises. Two effects explain why consumers behave this way: the substitution effect and the income effect. This block focuses on the income effect.
Real income refers to a consumer's purchasing power -- what their money can actually buy -- rather than the number of currency units they receive (their money income or nominal income). When the price of a good falls and money income stays constant, the consumer's real income rises: their fixed budget now stretches further and can purchase a greater quantity of goods and services than before.
The income effect describes how this rise in real income, triggered by a fall in price, leads a consumer to buy more of the good whose price fell (and potentially more of other goods too). Crucially, the consumer's actual money income has not changed at all -- only their purchasing power has increased because the good is now cheaper relative to their unchanged budget.
Key concept: The income effect operates through a price change. A fall in the price of a good increases the real purchasing power of a consumer's existing income, so they can afford to buy more -- of that good and/or other goods -- without their money income changing at all.
Tracing the income effect after a price fall
- A consumer has a fixed weekly budget of 5 per cup, allowing 10 cups to be purchased if all income were spent on coffee.
- The price of coffee falls to 50 -- nothing has changed in their pay packet.
- However, their real income (purchasing power) has risen: the same $50 can now buy up to 12.5 cups instead of 10, so the consumer is effectively 'richer' in terms of what they can consume.
- This increase in purchasing power is the income effect. It leads the consumer to buy more coffee (and possibly more of other goods) than before, reinforcing the substitution effect and producing the overall downward-sloping demand curve for coffee.
The income effect works alongside the substitution effect (where a good becomes relatively cheaper compared to substitutes, encouraging consumers to switch towards it) to produce the overall negative relationship between price and quantity demanded described by the law of demand. Together, these two effects -- combined with the law of diminishing marginal utility -- underpin why demand curves slope downward.
Common mistake: Do not confuse the income effect (a change in real purchasing power caused by a price change, which explains a movement along the demand curve) with income as a non-price determinant of demand (a change in a consumer's actual money income, which causes the entire demand curve to shift). If a consumer's pay rises and they buy more of a good at the same price, that is a shift in demand due to changed income -- not the income effect.
- Real income = purchasing power; money (nominal) income = the actual currency amount received -- they are not the same thing.
- The income effect: a fall in price raises real income, so the consumer can buy more of the good without any change in money income.
- The income effect and substitution effect together explain the downward slope of the demand curve (a movement along the curve, not a shift).
- Do not confuse the income effect with 'income' as a non-price determinant of demand -- the latter is a genuine change in money income that shifts the whole demand curve.