Complementary Goods vs Determinants of Supply
Complementary Goods and Determinants of Supply are two Supply & Demand concepts in AP Economics that students often mix up. Complementary goods are goods that are typically used or consumed together. Determinants of supply are factors that shift the supply curve, changing the quantity supplied at each price. Here is how they compare side by side.
When the price of one good increases, the demand for its complement decreases, as consumers buy less of both goods. Examples include cars and gasoline, or printers and ink cartridges.
The main determinants of supply are technology, input costs, government policies, and expectations. When these factors change, the supply curve shifts to the right or left. For example, if a new technology makes production more efficient, supply will shift to the right, indicating an increase in supply at each price level.
Complementary Goods vs Determinants of Supply: Which Curve Moves
| Complementary Goods | Determinants of Supply | |
|---|---|---|
| Side of the market | A demand-side link between two goods | The list of things that move a supply curve |
| Curve that moves | Demand for the partner good | Supply of the good in front of you |
| Trigger | The partner good's price changing | Input prices, technology, number of sellers, expectations, taxes and subsidies, prices of related production |
| Direction rule | Partner price up, demand for this good shifts left | Cost up, supply shifts left; cost down, supply shifts right |
| Where the word complement reappears | Complements in consumption, bought together | Complements in production, made together |
| Effect on this good's price | Falls when the partner good gets pricier | Falls when supply shifts right |
| Measured by | Cross-price elasticity of demand | No single elasticity, each determinant is judged on its own |
Complements in production shift supply, and they push the partner's price the opposite way
Hides come off the same animal as beef, so leather and beef are joint products, or complements in production. When the beef price rises, ranchers process more cattle, more hides reach tanneries at every leather price, and the supply of leather shifts right. Compare hot dogs and buns, complements in consumption: a higher hot dog price shifts the demand for buns left. Same word, different curve, and opposite pressure on the partner good's price. Numbers make the stakes obvious. Suppose leather starts with Qd equal to 180 minus 4P and Qs equal to 20 plus 6P, which clear at a price of 16 with 116 units traded. The beef price rise brings 30 extra hides to market at every price, so supply becomes 50 plus 6P. Setting 180 minus 4P equal to 50 plus 6P gives a price of 13 and a quantity of 128. Price falls by 3 and quantity rises by 12. Had you filed leather under consumption complements and shifted demand left instead, you would have predicted a lower price with a lower quantity, so half the answer would be wrong.
Ask whether the goods are bought together or made together, then draw
That single question decides the curve. Bought together points at demand for the partner good, and the shift always runs opposite to the partner's price. Made together points at supply, and the direction depends on how production is linked. Joint products such as beef and hides move together, so a higher price for one raises supply of the other. Goods that compete for the same land, machines, or hours are substitutes in production and move against each other: a farmer with fixed acreage who sees corn prices rise plants more corn and less soy, so soybean supply shifts left. Consumption relationships therefore land on demand every time, while production relationships can send supply either way, which is why a rubric that asks you to name the determinant will not accept the bare word complement. Say prices of goods related in production, or say price of a complement in consumption, and the grader knows which curve you meant. Neither list ever contains the good's own price. The full shifter lists sit at /glossary/determinants-of-supply and /glossary/determinants-of-demand, and you can push either curve yourself at /sandbox/supply-demand.
Frequently asked questions
Are complementary goods a determinant of demand or a determinant of supply?
The price of a complement is a determinant of demand for the partner good, not a determinant of supply. A rise in the price of one complement shifts the other good's demand curve left, leaving its supply curve exactly where it was. The one exception is language rather than economics: goods that are complements in production belong on the supply list.
What is the difference between complements in consumption and complements in production?
Complements in consumption are bought and used together, such as printers and cartridges, so a price rise for one shifts demand for the other left. Complements in production come out of the same process, such as beef and hides, so a price rise for one raises output of both and shifts the other's supply right. One label moves demand, the other moves supply, and they move partner prices in opposite directions.
If the price of beef rises, what happens to the supply of leather?
Leather supply increases, shifting right. Higher beef prices lead ranchers to process more cattle, and every animal yields a hide, so more leather reaches the market at every price. Working the example above, supply moving from 20 plus 6P to 50 plus 6P against demand of 180 minus 4P drops the leather price from 16 to 13 while quantity climbs from 116 to 128.
Live Supply and Demand graph. Drag the curves, or open the full version.
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