How economies work · 2 of 9
Supply, demand, and price as information
Read a price as a message about scarcity, and say what a cap does to that message.
Nobody decides the price of a lemon. There is no committee, no formula, and no file anywhere that says what a lemon should cost. Yet a lemon has a price in every market on earth, that price rises when a frost hits the groves, and it carries the news of that frost to a person making lemonade three thousand miles away who has never heard of the frost. That is the thing worth understanding here. The price is a message.
Two curves, one crossing
Demand slopes down: the higher the price, the fewer people want one. Supply slopes up: the higher the price, the more people are willing to make. They cross at one price, and at that price the amount people want equals the amount people will make.
Everything below that crossing is worth doing. Every buyer to the left of it would have paid more than they did, and every seller would have accepted less. The two shaded areas are that difference added up, and they are the reason the trades happen at all.
Move a curve and watch the crossing move. Then cap the price and watch what happens to the area that used to be there.
The figures are exact for the straight-line market drawn above and reproducible by moving the slider to $25. Real curves bend, which changes the numbers and none of the directions.
The queue is the part nobody measures
A capped price does not remove the competition for a scarce thing. It removes money as the way of settling it, and something else settles it instead: queueing, waiting lists, knowing somebody, arriving first, or a black market at a price higher than the one that was banned.
Those costs are real and mostly invisible, because a shortage shows up as an absence. A newspaper can photograph a high price. It cannot photograph the generator that was never driven in, or the apartment that was never built.
ConventionEconomists say elastic for a curve that responds a lot to price and inelastic for one that barely responds. Insulin is inelastic: the price can double and people buy the same amount, because the alternative is dying. One brand of cereal is elastic: raise the price and buyers move to the next box along. Almost everything interesting about who bears a tax, and about how badly a shortage hurts, comes down to which of the two curves bends more.
What to do with this
When a price moves, ask which curve moved, and use the quantity to tell you. Price up with quantity down is a supply story. Price up with quantity up is a demand story. That single check kills most of the bad explanations before you have to argue with them.
And when somebody proposes to fix a price, ask what happens to the amount that gets made, and who ends up in the queue. Those two questions are not an argument against the policy. They are the cost side of it, and a policy is only worth judging once both sides are on the table.
Test yourself
01A city caps rents at 20% below the market rate. Ten years later rents in uncapped buildings are higher than they would have been. Explain the mechanism.
The cap lowers the return on building and on maintaining rental housing, so less of both happens. Existing tenants in capped units do well and stay put, which takes those units off the market entirely. Everybody else competes for a smaller uncapped pool.
The demand did not go anywhere. It moved to the part of the market where the price is still allowed to move, and it arrived there concentrated. This is the general shape of a binding cap: the pressure does not disappear, it relocates to wherever the rule is not.
02During a hurricane, generators sell for four times the usual price. What is that price doing, and what is the case against letting it?
It is doing two useful things. It stops the first buyers from taking three generators each, so more households get one. And it makes it worth somebody's while to drive a truckload in from four states away, which is the only thing that actually ends the shortage.
The case against is that willingness to pay is not the same as need. At four times the price the generator goes to whoever has the most money rather than to whoever has a relative on an oxygen machine. That is a real objection and it is about who gets served rather than about how many exist, which is why the useful answers to it are things like direct payments rather than a cap.
03Bread gets more expensive. Is that supply or demand, and does it matter?
It matters because the quantity tells you which. If the price rose and less bread is being sold, supply moved: it got harder or costlier to make. If the price rose and more bread is being sold, demand moved: more people want it. Price alone is one number from a two dimensional picture, and reading it without the quantity is how people end up blaming the wrong thing.
A tutor that knows this lesson. It asks before it explains, and it will not tell you what to do with your own money.
Educational material, not investment or policy advice. Figures are cited where they come from a filing or a statistical series, and labelled as illustrative where they do not.