3.2 Shifts in Demand and Supply for Goods and Services

Factors Affecting Demand and Supply

Factors Shifting Demand Curves

Five main factors can shift the entire demand curve to the left or to the right. A shift means that at every price, consumers want to buy a different quantity than before. Don't confuse this with a movement along the curve, which is caused only by a change in the good's own price.

• Changes in consumer preferences — Tastes and trends evolve over time. If a popular fitness influencer promotes a certain protein bar, demand for that bar shifts right. If a food safety scandal hits a brand, demand shifts left.
• Changes in the number of buyers — More buyers in a market means more demand. Population growth, immigration, or a new demographic entering the market (think baby boomers reaching retirement age and demanding more healthcare) all shift demand right. A shrinking population shifts it left.
• Changes in consumer income — This one has a twist. For normal goods (organic produce, new cars), higher income shifts demand right. For inferior goods (instant ramen, bus tickets), higher income actually shifts demand left because people switch to preferred alternatives.
• Changes in the prices of related goods — There are two types to know:
  • Substitutes are goods you'd switch between. If the price of Coke rises, demand for Pepsi shifts right.
  • Complements are goods used together. If the price of smartphones rises, demand for phone cases shifts left.
• Changes in consumer expectations — If consumers expect prices to rise next month, they buy more now (demand shifts right today). If they expect a recession and worry about future income, they cut back on spending now (demand shifts left today).

Graphical Changes in Market Demand

When one of those five factors changes favorably for buyers, the demand curve shifts to the right. At each price, quantity demanded is higher than before. When a factor changes unfavorably, the curve shifts to the left, and quantity demanded is lower at every price.

A movement along the demand curve is different. It happens when the good's own price changes while everything else stays constant. A higher price leads to a lower quantity demanded (movement up and to the left along the curve), and a lower price leads to a higher quantity demanded (movement down and to the right). This inverse relationship is the law of demand.

Factors Shifting Supply Curves

Five main factors shift the supply curve. The logic here centers on production costs and profitability: anything that makes it cheaper or easier to produce shifts supply right, and anything that raises costs or barriers shifts supply left.

• Changes in input prices — Inputs are the resources used to make a good. If oil prices spike, it becomes more expensive to manufacture plastics, so supply of plastic goods shifts left. If lumber prices fall, supply of new homes shifts right.
• Changes in technology — Better technology lowers production costs per unit. Advances in automation or more efficient manufacturing processes shift supply right. Technology rarely declines, but equipment breakdowns or loss of skilled workers could shift supply left.
• Changes in the number of sellers — More firms entering a market (new startups, foreign competitors through trade) shift supply right. Firms exiting through mergers or bankruptcies shift supply left.
• Changes in government policies — Taxes and stricter regulations raise production costs, shifting supply left (think carbon taxes or new safety requirements). Subsidies lower production costs, shifting supply right (agricultural subsidies, renewable energy incentives).
• Changes in producer expectations — If farmers expect corn prices to be higher next season, they may plant more corn now, shifting current supply. If automakers expect stricter emissions standards soon, they may adjust production decisions today.

Graphical Changes in Market Supply

When a factor makes production cheaper or more accessible, the supply curve shifts to the right: at each price, firms are willing to supply a greater quantity. When a factor raises costs or reduces the number of sellers, the curve shifts to the left.

A movement along the supply curve happens when the good's own price changes, holding everything else constant. A higher price leads to a higher quantity supplied (movement up and to the right along the curve), and a lower price leads to a lower quantity supplied (movement down and to the left). This direct relationship is the law of supply.

Market Equilibrium and Price Controls

Market equilibrium is the price and quantity where the supply and demand curves intersect. At this point, quantity supplied equals quantity demanded, and there's no pressure for the price to change.

When demand or supply shifts, the equilibrium changes. For example, if demand shifts right (more buyers enter the market), the new equilibrium has a higher price and higher quantity. If supply shifts left (input costs rise), the new equilibrium has a higher price and lower quantity.

Price controls are government interventions that prevent the market from reaching equilibrium:

• A price ceiling is a legal maximum price, set below equilibrium to keep prices affordable (e.g., rent control). Because the price can't rise to equilibrium, quantity demanded exceeds quantity supplied, creating a shortage.
• A price floor is a legal minimum price, set above equilibrium to protect sellers (e.g., minimum wage). Because the price can't fall to equilibrium, quantity supplied exceeds quantity demanded, creating a surplus.