Demand and Supply

Demand and Supply

The model of demand and supply is the most powerful analytical tool in economics. It explains how prices and quantities are determined in markets, how markets respond to change, and how misalignments between buyers and sellers (shortages and surpluses) are corrected. Understanding demand and supply is essential for anyone making business decisions about pricing, production, and strategy.

The Demand Side of the Market

Quantity demanded is the amount of a good or service that a consumer is willing and able to buy at a given price. The distinction between willing and able is important: demand must be backed by both desire and purchasing power.

A demand schedule is a table showing the relationship between the price of a product and the quantity demanded. A demand curve is a graph of this relationship, with price on the vertical axis and quantity demanded on the horizontal axis.

The Law of Demand: Holding everything else constant (ceteris paribus), when the price of a product falls the quantity demanded increases, and when the price rises the quantity demanded decreases. The demand curve slopes downward.

Two effects explain the law of demand:

• Substitution effect — when the price of a good rises, it becomes more expensive relative to substitutes, causing consumers to switch to those substitutes, reducing quantity demanded of the original good
• Income effect — when the price of a good rises, the real purchasing power of consumers falls (their income goes less far), causing them to buy less of the good

Variables That Shift Market Demand

A change in the price of a good causes a movement along the existing demand curve — not a shift. The demand curve itself shifts when one of the following five variables changes:

1. Income — for a normal good, demand increases as income rises and falls as income falls. For an inferior good, demand increases as income falls and decreases as income rises (e.g. budget supermarket brands, public transport for some demographics).
2. Prices of related goods — Substitutes are goods that can be used for the same or similar purposes (e.g. coffee and tea); if the price of a substitute rises, demand for the original good increases. Complements are goods used together (e.g. printers and ink cartridges); if the price of a complement rises, demand for the original good falls.
3. Tastes and preferences — changes in consumer preferences, fashion, seasons, or trends shift demand; what consumers want changes over time and across cultures.
4. Population and demographics — as population increases, demand for most goods increases; changes in the age, gender, income, or other characteristics (demographics) of the population shift demand for specific goods (e.g. an ageing population increases demand for healthcare).
5. Expected future prices — if consumers expect prices to rise in the future, they have an incentive to buy now, increasing current demand; if they expect prices to fall, they may delay purchases, decreasing current demand.

The Supply Side of the Market

Quantity supplied is the amount of a good or service that a firm is willing and able to supply at a given price. A supply schedule and supply curve show the relationship between price and quantity supplied.

The Law of Supply: Holding everything else constant, an increase in the price of a product causes an increase in the quantity supplied, and a decrease in price causes a decrease in quantity supplied. The supply curve slopes upward because higher prices make production more profitable, inducing firms to expand output.

Variables That Shift Supply

A change in price causes a movement along the supply curve. The supply curve itself shifts when one of these variables changes:

1. Prices of inputs — if input costs (wages, raw materials, energy) rise, production becomes more expensive and supply decreases (shifts left)
2. Technological change — improvements in technology that reduce production costs increase supply (shift right)
3. Prices of substitutes in production — if the price of another good that can be produced with the same resources rises, producers may switch to producing that good, reducing supply of the original
4. Number of firms in the market — more firms increase market supply; fewer firms reduce it
5. Expected future prices — if firms expect higher prices in the future, they may reduce current supply to sell later at higher prices

Market Equilibrium

Market equilibrium is the situation in which the quantity of a product demanded by buyers equals the quantity supplied by sellers, resulting in a market-clearing price. At equilibrium, there is no tendency for price to change.

• Surplus (excess supply) — when price is above equilibrium, quantity supplied exceeds quantity demanded; unsold goods accumulate, putting downward pressure on price until equilibrium is restored
• Shortage (excess demand) — when price is below equilibrium, quantity demanded exceeds quantity supplied; consumers compete for available goods, putting upward pressure on price until equilibrium is restored

Market equilibrium adjusts automatically through the price mechanism: surpluses push prices down, shortages push prices up, until the market clears.

Predicting Price and Quantity Changes

The demand and supply model predicts the direction of price and quantity changes in response to shifts:

• Increase in demand (rightward shift) — price rises, quantity rises
• Decrease in demand (leftward shift) — price falls, quantity falls
• Increase in supply (rightward shift) — price falls, quantity rises
• Decrease in supply (leftward shift) — price rises, quantity falls

When both curves shift simultaneously, the effects on price and quantity are sometimes indeterminate without knowing the relative magnitude of the shifts — a critical point for business forecasting.

Behavioural Economics

Behavioural economics studies situations in which people act in ways that are not fully economically rational. Common errors include: ignoring non-monetary opportunity costs; failing to ignore sunk costs (costs already paid and irrecoverable, which should not influence future decisions); and being overly optimistic about future behaviour (e.g. assuming you will change your spending habits next month). Recognising these biases helps businesses design better pricing strategies and helps policymakers design more effective policies.