Factors Affecting Demand and Supply
Demand and supply are the building blocks of markets. They're shaped by factors like income, preferences, and production costs. When demand or supply shifts, prices and quantities adjust to find a new equilibrium. Understanding what causes these shifts and which direction they go is the core skill in this section.
Factors shifting demand curves
A demand curve shifts when something other than the good's own price changes how much consumers want to buy. Here are the main shifters:
Changes in consumer income shift demand differently depending on the type of good:
- Normal goods (like cars or restaurant meals): When income rises, demand increases (rightward shift). When income falls, demand decreases (leftward shift).
- Inferior goods (like instant noodles or bus tickets): These work in reverse. When income rises, people switch to preferred alternatives, so demand decreases (leftward shift). When income falls, demand increases (rightward shift).
The key distinction is whether people buy more or less of a good as they get richer. That's what separates normal from inferior goods.
Shifts in consumer preferences alter demand directly. If plant-based meat becomes trendy, demand for it shifts right. If consumers turn against fur coats, demand shifts left. These preference changes can come from advertising, health research, cultural trends, or social movements.
Price changes in related goods affect demand through two channels:
- Substitutes are goods that replace each other (Pepsi and Coca-Cola). If the price of Pepsi rises, consumers switch to Coca-Cola, shifting Coca-Cola's demand curve rightward.
- Complements are goods used together (smartphones and phone cases). If the price of smartphones rises and fewer are sold, demand for phone cases decreases (leftward shift). If smartphone prices drop, demand for cases increases (rightward shift).
Other demand shifters include changes in population size, consumer expectations about future prices, and income distribution across the population.
Demand shifts and market equilibrium
When the demand curve shifts, the market moves to a new equilibrium:
- A rightward shift (increase in demand) leads to a higher equilibrium price and a larger equilibrium quantity: and
- A leftward shift (decrease in demand) leads to a lower equilibrium price and a smaller equilibrium quantity: and
Both price and quantity move in the same direction when only demand shifts. That's a useful pattern to remember.
Factors Affecting Supply
Causes of supply curve shifts
A supply curve shifts when something other than the good's own price changes how much producers are willing and able to sell.
Input cost changes are the most common supply shifter. If wages or raw material prices rise, production becomes more expensive, and supply decreases (leftward shift). If input costs fall, supply increases (rightward shift). For example, a spike in oil prices raises transportation costs for almost every industry, shifting supply curves left across many markets.
Technological progress generally increases supply. Advances like automation or improved manufacturing processes let firms produce more at every price level (rightward shift). True technological setbacks are rare, but disruptions like cyberattacks on production systems could temporarily reduce supply.
Government policies and regulations shift supply in both directions:
- Subsidies and tax breaks lower producers' costs, increasing supply (rightward shift)
- Taxes, tariffs, and stricter regulations raise costs or limit production, decreasing supply (leftward shift)
Number of sellers in the market also matters. More firms entering an industry shifts market supply rightward; firms exiting shifts it leftward.
Natural conditions and resource availability affect supply too. A drought reduces agricultural supply; discovery of a new mineral deposit increases supply of that resource.
Supply shifts and market outcomes
When the supply curve shifts, the market adjusts to a new equilibrium:
- A rightward shift (increase in supply) leads to a lower equilibrium price and a larger equilibrium quantity: and
- A leftward shift (decrease in supply) leads to a higher equilibrium price and a smaller equilibrium quantity: and
Notice that when only supply shifts, price and quantity move in opposite directions. Compare this to demand shifts, where they move together. This distinction helps you figure out whether a change originated on the demand side or the supply side.
Market Dynamics
Market forces of supply and demand interact to determine equilibrium prices and quantities. The price mechanism coordinates this process: when demand or supply shifts, the resulting surplus or shortage pushes the price toward a new equilibrium, which in turn reallocates resources across the economy.
These shifts also affect consumer surplus (the benefit buyers get from paying less than their maximum willingness to pay) and producer surplus (the benefit sellers get from receiving more than their minimum acceptable price). For instance, an increase in supply lowers the price, which expands consumer surplus but may reduce producer surplus.