Demand: What Determines How Much Gets Bought

Demand is not wanting something. Demand is how much buyers will actually take at each price, with the money to pay standing behind it. So demand is written as a list of prices and quantities rather than as one number. The commonest error in the whole subject is calling a price change a change in demand. A price change is not a change in demand.

Two things sit underneath that. First, a price is only a number attached to a quantity, so a price on its own says nothing about how much anybody will buy. Second, a market is where two separate lists meet, one written by buyers and one written by sellers, and neither side can read the other list before trading starts. Everything in this guide is the buyers' list, worked on an invented country called the Republic of Sankhya and its invented onion market.

What is demand, and how is it different from wanting something?

Ask a hundred households in Sankhya whether they would like onions and every one of them says yes. The answer is worth nothing. It does not say at what price, and it does not say whether the money is there. Demand only counts a want backed by the ability to pay it at a stated price. Economists name the backed want effective demandWanting something and having the money to pay for it at that price. Wanting on its own never appears in a schedule. rather than desire. A student who would love a motorcycle and has no savings contributes nothing to the motorcycle market. The market never hears from him.

Once that is accepted, demand stops being a single number and becomes a table. At each price, buyers would take a particular quantity, and the honest way to record buyer behaviour is to write down all of those pairs at once. The list of pairs is called a demand scheduleA list that pairs every price with the quantity buyers would take at that price. A schedule is a table, not a single number.. The Sankhya onion schedule below is the one the whole of this guide runs on.

Reading that table wrongly for a second leads to reading the rest of this guide wrongly too. Demand is not the row that happens to be true today; demand is all five rows held at once. Four of those five prices are not being charged by anybody, and the quantities attached to them are still real facts about buyers. Those quantities are what buyers would do if asked. The same five pairs drawn on a pair of axes give a demand curveThe same list drawn as a line, with quantity along the bottom and price up the side. The line adds a shape and no new facts., which adds a shape and not one new fact.

What Shifts a Demand Curve, and What Only Moves Along It?

The distinction the rest of the subject stands on takes thirty seconds to learn and years to stop getting wrong. When the price of onions in Sankhya rises from Rs 2,000/- to Rs 2,200/-, buyers take 95 lakh quintals instead of 100. Nothing about buyers changed. The list was asked a different question and gave back the answer already written on it. A change of that kind is a movement along the curveA change in the quantity bought caused only by a change in price, with the underlying list left exactly as it was., and the honest sentence to write about it is that the quantity bought fell, not that demand fell.

Now take the other case. The price is left exactly where it was, at Rs 2,000/-, and something else changes: wages across Sankhya fall. Buyers now take 90 lakh quintals at that same Rs 2,000/-, and they would take 10 lakh quintals fewer at every other price too. The list itself has been rewritten. Rewriting the list is a shiftA change in the list itself, so the quantity attached to every price is different, including prices nobody is charging., and here the honest sentence is that demand fell. A movement changes which row is being read; a shift changes what the rows say, and no amount of staring at the quantity alone reveals which of the two has happened.

Which six things actually rewrite the list, and why is price not one of them?

If price is disqualified as a cause of a shift, something else has to do the work, and there are six candidates worth knowing by name. A buyer who has less takes less at every price, so income comes first. Then the price of a substituteSomething a buyer would use instead. If it gets cheaper, buyers move part of their spending to it.: if a rival staple in Sankhya gets cheaper, some spending walks across and the onion list falls, so at the same Rs 2,000/- buyers take 92 lakh quintals rather than 100. Then the price of a complementSomething normally bought alongside. If it gets dearer, buyers take less of both together.: dearer cooking oil means fewer dishes get cooked, and onions go into those dishes.

The last three are easier to feel. Then the number of buyers. Sankhya towns keep adding households, and each new kitchen buys at whatever price is standing. Then expectations. A household that believes next season's harvest will fail buys early. Early buying lifts this season's list and lowers the next one. And last, tastes. The word covers everything about what people want that is not income and not another price. Six things move the list, and the price of the good itself is not among them. The price is the question the list answers, not a cause that could rewrite the answers.

How Demand and Supply Shape Prices, and who actually decides the number?

Sellers in Sankhya have a list of their own, and it runs the other way: the more they are offered per quintal, the more they are willing to grow and bring to market. At Rs 1,600/- they offer 70 lakh quintals, at Rs 2,000/- they offer 100, and at Rs 2,400/- they offer 130. Put the two lists side by side and one price does something none of the others does.

Walk it in both directions and the mechanism does itself. Set the price at Rs 1,800/- and buyers want 105 lakh quintals against the 85 sellers bring. Twenty lakh quintals of buyers are left standing in a queue with money and nothing to spend it on. One of them offers a little more, and the price climbs. Set it at Rs 2,200/- and sellers have brought 115 while buyers take 95, so 20 lakh quintals sit unsold and start to rot, and a seller with rotting stock cuts. Nobody in this market sets the price. The price is the one number at which the two lists stop disagreeing, and on these figures the number is Rs 2,000/- a quintal at 100 lakh quintals.

Economic Recession: what happens to demand when a whole economy slows?

Everything so far has happened inside one market. A recessionA broad and sustained fall in activity across many markets at once, rather than a bad season in a single one of them. is what the same mechanism looks like when it happens in a great many markets at the same time. Incomes across Sankhya fall or become uncertain, and households respond in the only way available to them: they take less of almost everything, at prices nobody has changed. Onions, cloth, cement, two-wheelers, meals out. Each of those lists moves left, and they move together.

A recession is a shift and not a movement, and the distinction is not academic. It explains why cutting the price does not simply bring the old quantity back. Work it on Sankhya. Take the whole onion list 10 lakh quintals to the left and leave the sellers' list untouched. Read the straight line between the published rows and the two lists now meet at Rs 1,900/- and 92.5 lakh quintals, not at 100. The price did fall, by Rs 100/-, and the quantity still ended 7.5 lakh quintals below where it started. The very price cut meant to rescue the quantity also pulled sellers back. There is no price at which a market that has lost its buyers can pretend it has not.

When a slowdown has gone on long enough, or spread far enough, to earn the word at all is a separate question, worked out elsewhere in these notes. There are two competing tests for it and they disagree with each other. The disagreement is a genuine argument and not a detail. The claim here is narrower and safer: whatever test is eventually accepted, the mechanism underneath it is many demand lists moving left at once.

Why is the difference between a shift and a movement the whole subject?

Because the two can land on exactly the same number. Watch it happen on the Sankhya list. Raise the price to Rs 2,200/- and leave everything else alone: buyers take 95 lakh quintals. Now put the price back at Rs 2,000/- and instead knock 5 lakh quintals off every row: buyers take 95 lakh quintals. The quantity is identical, the cause is completely different, and anybody looking only at the quantity has no way at all of telling the two apart.

The overlap is not a puzzle without an answer. The separating question is short and comes before anything else: did the price move? If it did, the case is almost certainly a movement, and buyers have not changed their behaviour by one rupee. If it did not, then something rewrote the list, and now the six causes are worth going through one by one. With that order reversed, every price rise reads as customers losing interest. A price rise usually is nothing of the kind.

How does a household feel the difference, and what does a lender read into it?

Take a Sankhya household that normally buys 10 kg of onions a month at Rs 20/- a kg, so Rs 200/- a month, all illustrative. In the first story the onion price doubles to Rs 40/- a kg. The household grumbles, buys 6 kg, and spends Rs 240/-. In the second story the price never moves at all, but somebody in the household loses work. The household buys 6 kg, and spends Rs 120/-. The onion line on both slips reads 6 kg. The only line that differs is the price, and the price is exactly the line the mistake ignores.

A lender or an analyst reads the two stories the same way, and the ordering matters more for them than for the household. A shop that raised prices and sold less has customers behaving normally and a decision it can reverse, so a lender looking at falling volumes asks first whether the shop raised its prices. A shop whose prices never moved and whose volumes fell anyway has lost something discounting cannot get back. Its buyers are poorer rather than annoyed. An analyst has a second signal too. A movement shows up in one market. A shift shows up across a household's whole basket at once. If the cloth seller, the cement yard and the tea stall on the same street all report the same fall in the same season, nobody's pricing decision explains it.

The Republic of Sankhya and its onion market are invented.
Educational material. Not advice on any investment, tax, budget or market position.