Theory of Demand in Economics
The law of demand, demand schedules, demand curves, what shifts them, types of demand, and price elasticity — everything your economics assignment is likely to ask about, with diagrams and worked examples.
The theory of demand is one of the most foundational concepts in economics. You’ll encounter it in every microeconomics course, every market analysis, and almost certainly in your exams. The good news: once you understand the logic behind it — not just the definitions — it stops being something you memorize and starts being something you actually use to think about markets.
What This Guide Covers
What Is Demand? (Not Just Wanting Something)
Demand is not the same as desire. That distinction is critical — and it’s where a lot of students lose marks early in an answer. Wanting a Ferrari doesn’t constitute demand. Demand requires two things working together: the willingness to buy and the ability to pay. Both must be present.
In economic terms, demand is defined as the quantity of a good or service that consumers are willing and able to purchase at various prices during a given time period, all other factors held constant. That last part — “all other factors held constant” — is the ceteris paribus assumption. It’s how economists isolate the price-quantity relationship without every variable changing at once.
Want: A desire for a good with no backing purchasing power. “I want a yacht.”
Need: A necessity, but still not demand unless backed by ability to pay.
Demand: Willingness + ability to purchase at a specific price. “I want a yacht and I have £500,000 to spend on one.” That’s demand. The distinction matters because economics studies actual market behavior, not wishes.
The Law of Demand
The law of demand states that, all else being equal (ceteris paribus), as the price of a good rises, the quantity demanded falls — and as the price falls, the quantity demanded rises. Price and quantity demanded move in opposite directions. That’s the inverse relationship at the core of demand theory.
Two Effects That Explain Why Higher Prices Reduce Quantity Demanded
It’s not enough to state the law — you need to be able to explain the economic reasoning behind it. Two effects drive this relationship.
Substitution Effect: When the price of a good rises, consumers look for cheaper alternatives (substitutes) that satisfy the same need. If bread gets more expensive, people buy more rice or pasta. They substitute away from the now-pricier good. Quantity demanded for that good falls.Income Effect: When the price of a good rises, a consumer’s real purchasing power effectively falls — they can buy less with the same money. Even if their nominal income hasn’t changed, they feel poorer. This reduced real income leads them to buy less of the good. Both effects point in the same direction: higher price → lower quantity demanded.
“There is an inverse relationship between the price of a good and the quantity demanded, ceteris paribus. When price rises, quantity demanded falls; when price falls, quantity demanded rises.”
Ceteris paribus — Latin for “all other things being equal.” This assumption is essential. Without it, you can’t isolate the price effect — other variables like income or consumer tastes might be changing at the same time, muddying the relationship.
Demand Schedule Explained
A demand schedule is simply a table that shows how much of a good consumers would buy at different prices, holding everything else constant. It’s the data behind the demand curve. Think of it as the “before the graph” version of demand.
| Price per Unit (£) | Quantity Demanded (units/week) | Relationship |
|---|---|---|
| £10 | 100 | High price → lowest quantity |
| £8 | 150 | ↑ |
| £6 | 220 | ↑ |
| £4 | 320 | ↑ |
| £2 | 500 | Low price → highest quantity |
Each row of the schedule gives one point on the demand curve. When you plot all those points and join them, you get the demand curve. The schedule is useful for illustrating the law of demand with concrete numbers — and in exam questions that ask you to derive a curve from data, this is the starting point.
The Demand Curve
The demand curve is a graphical representation of the relationship between price and quantity demanded. Price goes on the vertical axis (Y-axis). Quantity demanded goes on the horizontal axis (X-axis). For a normal good, the curve slopes downward from left to right — reflecting the inverse price-quantity relationship.
Figure: D₁ — original demand curve (downward sloping). D₂ — demand curve shifted right by a non-price factor (e.g. income rise). Orange arrow shows a movement along D₁ caused by a price change.
In economics, price (P) goes on the vertical axis and quantity (Q) on the horizontal axis — even though you might instinctively think of price as the independent variable on the X-axis. This is an established convention in economics going back to Alfred Marshall. Your exam diagram must follow this convention or your curve will be drawn on the wrong axes.
Determinants of Demand (What Shifts the Curve)
The demand curve assumes ceteris paribus — everything except price is held constant. But in the real world, other things do change. When they do, the entire demand curve shifts. These non-price factors are called the determinants of demand. Here’s what they are and what direction each one shifts the curve.
Shift of the Curve vs. Movement Along the Curve
This is one of the most commonly tested distinctions in demand theory. Get it right and you’ll pick up marks other students drop.
Movement Along the Curve
Caused only by a change in the good’s own price. The demand curve itself doesn’t move. You slide up or down the same curve. When price rises: move up-left (lower quantity demanded). When price falls: move down-right (higher quantity demanded). This is called a change in quantity demanded, not a change in demand.
Shift of the Curve
Caused by a change in any non-price determinant — income, tastes, substitute prices, complement prices, expectations, number of buyers. The whole curve moves. Rightward shift = demand increase (more demanded at every price). Leftward shift = demand decrease. This is called a change in demand.
Ask yourself: what caused this change? If the answer is “the price of this good changed” → movement along the curve, change in quantity demanded. If the answer is anything else → shift of the curve, change in demand. That one question resolves most of the confusion around this topic.
Types of Demand
Individual vs. Market Demand
Individual demand is how much one consumer buys at various prices. Market demand is the horizontal sum of all individual demand curves in the market — how much all consumers combined buy at each price. Most economic analysis focuses on market demand.
Joint Demand (Complementary)
Goods demanded together — like cars and fuel. When demand for one rises, demand for the other follows. The demands are linked by the nature of consumption. A fall in car demand will reduce fuel demand even if fuel’s own price hasn’t changed.
Composite Demand
A good that is demanded for multiple different uses is in composite demand. Milk is used for drinking, cheese-making, butter, and infant formula. A rise in demand from one use competes with the others — increased cheese production can reduce milk available for direct consumption.
Derived Demand
Demand for a good that arises from demand for another. Labour is a classic example — firms demand workers because consumers demand the goods workers produce. Steel demand is derived from demand for cars, buildings, and appliances. The demand is indirect.
Competitive Demand (Substitutes)
Two goods are in competitive demand when they’re close substitutes — demand for one comes at the expense of demand for the other. Android phones vs. iPhones. Butter vs. margarine. A rise in price for one increases demand for the other.
Effective Demand
Demand backed by actual purchasing power. This is what economics studies — not theoretical desire but real market demand supported by money. A key distinction in development economics where populations may have high needs but insufficient purchasing power.
Price Elasticity of Demand (PED)
Knowing that quantity demanded falls when price rises is one thing. Knowing by how much is something else entirely. That’s what price elasticity of demand measures.
PED = % Change in Quantity Demanded ÷ % Change in Price
The formula gives a number. The number tells you how sensitive quantity demanded is to a price change. Because of the inverse relationship, PED is always negative — a price rise produces a quantity fall. In practice, economists often use the absolute value.
|PED| > 1 — Elastic demand: Quantity demanded responds more than proportionately to a price change. A 10% price rise causes more than a 10% fall in quantity demanded. Common for luxury goods, goods with many close substitutes, goods that take up a large share of income.|PED| < 1 — Inelastic demand: Quantity demanded responds less than proportionately. A 10% price rise causes less than a 10% fall in quantity demanded. Common for necessities, goods with few substitutes, habit-forming goods (like cigarettes).
|PED| = 1 — Unit elastic: Quantity demanded changes by exactly the same proportion as price.
PED = 0 — Perfectly inelastic: Quantity demanded doesn’t change at all regardless of price. Theoretical extreme (e.g. life-saving medication with no substitute).
PED = ∞ — Perfectly elastic: Any price rise leads to zero demand. The demand curve is horizontal.
| PED Value | Description | Curve Shape | Typical Goods |
|---|---|---|---|
| |PED| > 1 | Elastic | Flatter slope | Luxury cars, holidays, branded clothing |
| |PED| < 1 | Inelastic | Steeper slope | Petrol, insulin, salt, cigarettes |
| |PED| = 1 | Unit elastic | Rectangular hyperbola | Theoretical benchmark |
| PED = 0 | Perfectly inelastic | Vertical line | Life-saving drugs (no substitute) |
| PED = ∞ | Perfectly elastic | Horizontal line | Commodity in perfectly competitive market |
Exceptions to the Law of Demand
The law of demand holds for normal goods — but there are recognized cases where it breaks down. Your exam may ask you to identify and explain these.
Price Rises — Quantity Demanded Rises Too
Named after economist Robert Giffen, these are inferior goods so dominant in a poor household’s budget that the income effect overwhelms the substitution effect. When the price of bread (a staple) rises, very poor households can no longer afford meat — so they buy even more bread. Quantity demanded rises with price. Giffen goods are rare and usually found only in extreme poverty contexts.
Higher Price Signals Higher Status — Demand Rises
Named after economist Thorstein Veblen, these are luxury or status goods where a higher price actually increases demand because exclusivity and prestige are part of what’s being purchased. Designer handbags, rare watches, and certain premium spirits can behave this way. The high price is part of the appeal — lowering the price could reduce demand among status-conscious buyers.
Rising Prices Attract More Buyers
In asset markets — property, stocks, cryptocurrencies — rising prices can signal future gains. Buyers rush in because they expect the price to rise further, not because the current price seems cheap. This produces upward-sloping demand behavior in speculative markets, which is one reason asset bubbles form. This is a price expectations effect, not a true violation of the law of demand in the conventional sense.
Frequently Asked Questions
Need Help With Your Economics Assignment?
Demand and supply analysis, market structures, elasticity calculations, essays, and research papers — our economics writing team works across microeconomics, macroeconomics, and applied economics topics.
Economics Homework Help Get StartedApproaching Your Demand Theory Question
Most demand theory exam questions follow one of three patterns. The first asks you to define and explain — state the law, explain the two effects behind it, give a brief example. The second asks you to draw and annotate — the demand curve, label axes correctly (P on vertical, Q on horizontal), show a shift or a movement, explain what caused it. The third asks you to apply — here’s a scenario, what happens to demand and why?
The shift vs. movement distinction catches students out most often. Read the question carefully: if a non-price factor changes, you’re drawing a new curve. If only the good’s own price changes, you’re moving along the existing one. Get that right and you’ll handle most scenario questions cleanly.
On elasticity: always connect PED to the formula, then to whether the good is a necessity or luxury, and whether substitutes exist. Those two characteristics — necessity-ness and availability of substitutes — are the main drivers of whether demand is elastic or inelastic. A good answer links the PED value to a real-world implication, whether that’s firm revenue or government tax policy.