Demand & Supply

Unit 1 was the warm-up. We learned that scarcity forces choices, that opportunity cost measures what we give up, and that smart decisions compare marginal benefit against marginal cost. In Section 1.5 you applied those ideas to a single consumer trying to spread a fixed budget across goods. But every one of those decisions takes place inside something we haven't formally introduced yet: a market.

A market is just a place — physical, online, abstract, whatever — where buyers and sellers meet to trade. Buyers want to pay as little as possible. Sellers want to charge as much as possible. The price ends up somewhere between those two wishes, set by the back-and-forth of all the people on both sides. To understand how that price gets settled, we first need to understand the two sides separately — and that's exactly what 2.1 does.

Heads up: this is the most important section in the entire course. Every diagram you draw in Units 2 through 6 — price controls, taxes, tariffs, monopolies, factor markets, externalities — starts with a demand and supply curve. Slow down here. If 2.1 clicks, the rest of the year clicks with it. If 2.1 stays fuzzy, you'll be confused for months. We're going to take our time.

Let's start with a vocabulary correction that trips up almost every student. In everyday English, when you say you "demand" something, you mean you really, really want it. In economics, demand means something much more specific.

Notice those two words underlined in italic: willing AND able. You might be willing to buy a $200,000 sports car — but if you can't afford it, you have no demand for it in the economic sense. Likewise, a billionaire might be perfectly able to buy a million dollars worth of cabbage but has zero willingness to do so. Demand requires both. A consumer's wishes only count when backed by the dollars to act on them.

From Demand Schedule to Demand Curve

How do we actually represent demand? Two ways: a table or a graph. The table is called a demand schedule — it lists how many units a buyer would purchase at each price. Let's look at one for Maya buying iced coffees:

Plot those price-quantity pairs on a graph (price on the vertical axis, quantity on the horizontal), connect the dots, and you get the demand curve:

That downward slope is no accident. It's the visual signature of the most important principle in this section: the Law of Demand.

In plain English: when something gets more expensive, people buy less of it. When it gets cheaper, they buy more. This isn't a hypothesis — it's an empirical regularity that holds for nearly every good in nearly every market. It's why the demand curve slopes downward.

But "people just buy less when prices go up" isn't a complete answer on the AP exam. The College Board expects you to know three specific reasons why the law of demand holds. Each one explains a different part of the same phenomenon.

Reason 1: The Substitution Effect

When the price of a good rises, that good becomes relatively expensive compared to its alternatives. Rational consumers respond by switching toward the now-cheaper substitutes.

Imagine you usually buy Coke at $1.50 a can. The price suddenly jumps to $3.00. Pepsi is still $1.50. What do you do? You probably buy less Coke and more Pepsi — not because you hate Coke now, but because relatively, Pepsi looks like a much better deal. That switching behavior, summed across millions of consumers, is the substitution effect. Higher Coke price → less quantity of Coke demanded.

Reason 2: The Income Effect

When a price falls, the same dollar in your wallet stretches further. You effectively have more purchasing power — without anyone giving you a raise. So you can afford to buy more of the good (and more of other things, too).

Suppose movie tickets drop from $15 to $10. You haven't earned a cent more, but with the leftover $5 you could see an extra movie, buy popcorn, or save it. The price drop has effectively made you richer in real terms. Lower price → more quantity demanded, partly because consumers feel they can afford more.

The income effect runs in the other direction too: when a price rises, your dollars don't go as far, you feel poorer in real terms, and you cut back. Same logic, opposite direction.

Reason 3: Diminishing Marginal Utility

This one comes straight from Section 1.5. Each additional unit of a good gives you less additional satisfaction than the one before. The first slice of pizza is amazing; the fifth is just OK.

Connecting that to demand: if extra pizza slices give you less and less marginal utility, you'll only buy more of them when the price drops to match that lower MU. To get you to buy your fifth slice, the price has to come down to where MU₅/P still equals what you're getting elsewhere. Higher quantities require lower prices — which is exactly the downward slope.

Here's where students lose the most points on Unit 2. There are two completely different things that can happen to a demand curve — and the AP exam tests whether you can tell them apart on nearly every single exam.

Let's visualize the difference:

Why This Distinction Matters So Much

Imagine the question: "The price of apples rises. What happens to the demand for apples?" If you answer "demand decreases," you're wrong — even though it sounds intuitively right. The correct answer is "quantity demanded decreases." The demand curve itself didn't move; we just slid up along it to a higher price and lower quantity.

Now compare: "Income rises. What happens to the demand for apples (a normal good)?" Here the answer is "demand increases." Income isn't the price of apples — it's an "everything else" variable. So the entire curve shifts to the right.

OK, so if the price of the good doesn't move the curve, what does? Five categories of variables, and there's a handy mnemonic to remember them: MERIT.

Let's walk through each one carefully — these are the College Board's favorite multiple-choice traps.

M — Market Size

The demand curve represents the market demand — the sum of all individual demand curves added together. If more buyers enter the market, total demand at every price rises, and the curve shifts right. If buyers leave (people move away, the population ages out of a target group, etc.), demand shifts left.

Real example: A new university opens in a small town, adding 10,000 students. The local demand for pizza, off-campus housing, and used textbooks all shift right. More buyers → more demand at every price.

E — Expectations of Future Prices

This one's a bit counterintuitive. If consumers expect a price to rise in the future, they want to buy more right now before the increase hits. So current demand shifts right. Conversely, if they expect prices to fall later, they delay purchases — current demand shifts left.

Real example: Hurricane forecast on the news. Everyone races to buy bottled water and gas today, because they expect prices to spike when the storm hits. The expectation of higher future prices increases demand today.

R — Related Goods

The prices of other goods affect this good's demand. There are two types of related goods, and they work in opposite ways.

The intuition: if two goods are substitutes, they compete for your dollars. A price rise in one pushes consumers toward the other. If two goods are complements, they go together; a price rise in one makes the pair more expensive overall, so people buy less of both.

I — Income

When consumers' income changes, demand for most goods changes too. But not all goods react the same way — there are two distinct categories.

"Inferior" is an economic term, not an insult. It doesn't mean the goods are low quality in some objective sense — it just means consumers substitute toward better alternatives as their income rises. Instant ramen isn't bad food; it's just what college students eat because they can't afford steak. Give the student a raise and ramen consumption drops.

T — Tastes & Preferences

Trends, advertising, fads, fashion, health information, seasons — anything that changes consumers' subjective desire for a good shifts demand. This is the most "soft" of the shifters because it can be hard to pin down empirically, but it's real.

Real examples: A celebrity wears a particular brand → demand for that brand shifts right. New research says red meat is bad for your heart → demand for steak shifts left. It gets cold outside → demand for ski jackets shifts right.

Summary Table

Now flip the perspective. Instead of buyers wanting to consume, think about producers — bakeries, farmers, app developers, ride-share drivers — who are trying to sell. They have a completely different objective from buyers: not to maximize satisfaction from consumption, but to maximize profit from production.

Same "willing AND able" requirement as before — a baker who's willing to make pies but has no flour can't supply any. Both willingness (driven by profitability) and ability (capacity, resources, technology) matter.

From Supply Schedule to Supply Curve

Just like demand, we can represent supply with a schedule or a graph. Here's a supply schedule for a small bakery selling pies:

Notice the pattern: as the price rises, the bakery wants to bake more pies. Plot these points and you get an upward-sloping supply curve — the visual opposite of demand:

The intuition is straightforward: producers want to sell at the highest prices possible. When the market price rises, two things happen at once. First, each pie sold is now more profitable, so existing bakeries want to bake more. Second, the higher price attracts new bakeries into the market who weren't producing at the old, lower price. Both forces push quantity supplied up.

Conversely, when the price falls, existing bakeries cut production (some pies are now made at a loss), and some bakeries may exit entirely. Quantity supplied drops.

Why Does the Curve Slope Upward? Increasing Marginal Costs

There's a deeper reason behind the law of supply, and it'll appear again in Unit 3: producing more units generally costs more per unit at the margin. The first 10 pies a bakery makes use the cheapest ingredients, the most well-rested workers, and the most efficient ovens. Producing the next 10 pies requires overtime wages, lower-quality flour bought in a rush, and oven space at off-peak hours. Each additional batch costs more than the last.

For producers to be willing to incur those rising marginal costs, the price has to rise to compensate. That's why higher prices unlock higher quantities supplied. The upward slope mirrors the rising marginal cost of production.

Just like with demand, there are two distinct things that can happen to a supply curve, and the AP test loves to see if you can tell them apart.

Heads up about direction language: a rightward shift of supply is called an increase in supply (more quantity at every price). A leftward shift is a decrease in supply (less quantity at every price). The exam often phrases questions either way, so be comfortable with both wordings.

What moves the supply curve? Six categories of variables, all of which boil down to one underlying idea: anything that changes the cost of production or the producer's willingness/ability to produce.

1. Input Prices (Resource Costs) — The Big One

Producers use inputs (also called factors of production) to make their goods — raw materials, labor, energy, capital equipment. If the price of any input rises, producing each unit becomes more expensive, so at every price the firm is willing to supply less. Supply shifts LEFT.

Real example: Coffee bean prices double due to a drought in Brazil. Every coffee shop's cost per latte rises sharply. At the old market price, fewer lattes are profitable, so quantity supplied at every price falls. Supply curve shifts left.

The reverse is also true: if inputs get cheaper, the supply curve shifts right. Cheaper flour → bakeries can produce more pies profitably at every price.

2. Technology & Productivity

Improvements in technology lower the cost of producing each unit. Better machines, more efficient processes, smarter software — all increase how much output a firm can squeeze out of the same inputs. Supply shifts RIGHT.

Real example: A new pizza oven cooks twice as fast at the same cost. The pizzeria can make more pizzas per hour with the same labor. Supply of pizzas increases.

Technology shocks almost always shift supply right — it's rare to see "technology decreases" outside science fiction. But if equipment breaks or knowledge is lost, you could theoretically see a leftward shift.

3. Number of Sellers

Market supply is the sum of all individual firms' supply. More firms in the industry → more total quantity supplied at every price → supply shifts RIGHT. Firms exiting → leftward shift.

Real example: Twenty new ride-share drivers sign up with Uber in a city. The market supply of rides shifts right. Conversely, when many drivers quit during the pandemic, supply shifted left.

4. Government Policies (Taxes & Subsidies)

Government actions directly change production costs and therefore shift supply.

We'll go much deeper on taxes and subsidies in Section 2.4 (government intervention). For now, just internalize the direction.

5. Expectations of Future Prices

Just like with demand, producer expectations matter — but they work in a slightly tricky way. If producers expect prices to rise in the future, they may withhold goods now to sell them later at the higher price. Current supply shifts LEFT.

Real example: Oil producers expect OPEC to announce a production cut next month that will spike prices. They reduce sales today to stockpile for the higher prices tomorrow. Current supply decreases.

(This shifter is less commonly tested than the others — input prices and technology dominate AP questions — but know it exists.)

6. Prices of Related Goods in Production

Producers often can make multiple different goods with the same resources. If the price of one of those alternatives rises, producers shift their resources toward making that one, leaving less capacity for the original good. Supply of the original good shifts LEFT.

Real example: A farmer can plant corn or soybeans on the same land. If the price of soybeans suddenly rises, the farmer plants more soybeans next season — supplying less corn. Supply of corn shifts left.

Summary Table

• "When the price rises, demand decreases." Wrong wording. The price of the good itself never shifts the demand curve — it just causes movement along the curve. The correct statement is "when price rises, quantity demanded decreases." Mixing these two up is the single most common error in Unit 2.
• "The demand curve slopes down because people just don't want to pay more." Incomplete. On the AP, you need to give one of the three formal reasons: substitution effect, income effect, or diminishing marginal utility. "People don't want to pay more" is the intuition, not the answer.
• "Substitutes and complements work the same way." Opposite ways, actually. When a substitute's price rises, demand for this good rises (consumers switch toward it). When a complement's price rises, demand for this good falls (the pair becomes more expensive, so consumers buy less of both).
• "Inferior goods are low-quality goods." Not necessarily. "Inferior" is just a technical term for goods that consumers buy less of when their income rises. There's no quality judgment — used cars and instant ramen aren't objectively bad; they're just substituted away from as income rises.
• "Higher input prices make the supply curve slope up steeper." No — higher input prices shift the entire supply curve to the LEFT. The slope doesn't change in this case; the curve's position does.
• "Taxes are paid by sellers, so the demand curve shifts." Wrong logic. A tax on producers raises their costs, so it shifts the supply curve (left). Demand is the buyer's side; it doesn't move when producers' costs change. (We'll deal with tax incidence — who actually bears the burden — in Section 2.4.)
• "Expectations always work in the same direction as actual future events." Backwards. If consumers expect prices to rise next month, they buy more now, which means current demand rises (not falls). Expectations move the curve in the direction that anticipates and offsets the expected event.
• "Quantity demanded and demand are interchangeable terms." They are NOT. Quantity demanded is a specific number at a specific price (a point on the curve). Demand is the entire relationship/curve. Confusing them costs points on FRQs because graders look for the precise terminology.