Econ 101, Part 2: Supply, Demand & Markets
The Law of Supply
As the price of a good rises, the quantity producers are willing to sell tends to rise too - the mirror image of the law of demand.
The previous lesson looked at buyers; this one looks at sellers. Just as buyers respond predictably to price, so do producers - and their response runs in the opposite direction, which is exactly what makes markets work the way they do.
Stating the law
The law of supply says that, all else held equal, as the price of a good rises, the quantity supplied of that good rises too, and as price falls, quantity supplied falls. This creates a direct, upward-sloping relationship between price and quantity supplied, typically drawn as an upward-sloping supply curve on the same type of graph used for demand.
As with quantity demanded, “quantity supplied” refers to the specific amount producers are willing and able to sell at a specific price - not simply how much they’d like to sell in some abstract sense.
Imagine a farmers' market for tomatoes. At a low price, only the most efficient growers - those who can produce tomatoes cheaply - find it worthwhile to sell there at all. As the price rises, it becomes profitable for less efficient growers to join in too, since they can now cover their higher production costs and still turn a profit. Existing growers may also expand production, planting more tomato plants or investing in better equipment. Both effects push total quantity supplied upward as price rises.
Why the law tends to hold
The law of supply follows fairly directly from how production costs typically behave, a topic covered in depth in the next module of this curriculum. As a producer makes more of something, the cost of producing each additional unit - the marginal cost, covered in the foundations module’s lesson on marginal thinking - tends to rise, because producers eventually have to use less efficient resources, pay overtime wages, or push equipment harder. A higher price is needed to make producing those more expensive additional units worthwhile, which is exactly why a higher price calls forth a higher quantity supplied.
The law of supply describes a tendency, but how strongly and how quickly quantity supplied responds to a price change varies a great deal depending on the good and the time frame - an idea explored fully in this module's lesson on price elasticity of supply. A farmer can't instantly grow more wheat just because the price jumped yesterday; that takes an entire growing season. A factory, by contrast, might be able to add a night shift within days. Assuming supply always adjusts quickly and fully is a common oversimplification of what the law of supply actually predicts.
Supply as the other half of the market story
The law of supply is the mirror image of the law of demand, and together they set up the next lesson in this module: how the upward-sloping supply curve and the downward-sloping demand curve interact to determine the actual price and quantity that emerge in a real market. Just as buyers respond to incentives created by price, covered in the foundations module, so do sellers - a higher price is an incentive to produce and sell more, and a lower price is a disincentive that pushes some producers out of the market entirely.
- The law of supply states that quantity supplied rises as price rises, and falls as price falls, all else equal.
- This produces the typical upward-sloping supply curve.
- Rising marginal costs as production increases help explain why producers need a higher price to supply more.
- How quickly and strongly supply responds to price varies by good and by time frame.
- Supply and demand together set up how markets determine actual prices and quantities.
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