Supply, demand, and why your grocery bill tells a story

In 2023, the price of Italian extra-virgin olive oil roughly doubled. A 1-litre bottle at the Esselunga that used to cost €7 went to €13 in the space of a year (source: ISTAT, Prezzi al consumo — oli e grassi, retrieved 2025-01). Nobody decided this. No government set it. No CEO signed off on it. The price did that on its own.

How? Because of the single most important idea in economics: supply and demand. Today’s lesson is about that idea and what it tells you about your grocery bill, your rent, your salary, and later, the price of stocks and bonds.

The scenario

Sofia got her year-end bonus — €800 — and wants to buy a decent bottle of olive oil for her father’s birthday. At the grocery store she sees the same brand her family has used for fifteen years is now €15 per litre. Her immediate reaction: “the market is broken.” Her father, Giorgio, when told this on Sunday, shakes his head and says: “no, the market is working. That’s how prices are supposed to behave.”

He’s right. Let’s unpack why.

Demand: how much people want at a price

Demand is the relationship between the price of a good and how much people are willing to buy at that price, all else equal.

General rule: as price rises, quantity demanded falls. People buy less olive oil at €15 than at €7, because:

Some switch to cheaper alternatives (sunflower oil, rapeseed oil).
Some buy less total oil (use smaller quantities, skip recipes that use it).
Some buy lower-quality brands or smaller bottles.

Drawn on a chart with price on the y-axis and quantity on the x-axis, the demand curve slopes down. Higher price, lower quantity demanded.

Supply: how much producers want to sell at a price

Supply is the relationship between the price of a good and how much producers are willing and able to sell at that price.

General rule: as price rises, quantity supplied rises. More olive-oil producers find it profitable at €15/litre than at €7. Farmers squeeze more oil from the same olives, marginal plantations that were unprofitable become profitable, producers from other countries (Spain, Greece, Tunisia) export more to Italy.

Drawn on the same chart, the supply curve slopes up. Higher price, higher quantity supplied.

Where they meet: the equilibrium price

Price
  |            /             Supply
  |           /
  |          /
  |         /  ← equilibrium here
  |        *
  |       / \
  |      /   \
  |     /     \  Demand
  |____/_______\________ Quantity

Where supply crosses demand, you get the equilibrium price and equilibrium quantity — the price at which the amount producers want to sell equals the amount consumers want to buy.

In a free market with many buyers and sellers and good information, prices gravitate toward equilibrium automatically. If the price is too high, producers have unsold inventory and lower it. If too low, consumers clean out the shelves and producers raise it. Nobody coordinates this; it emerges.

What happened to olive oil in 2023

Two things shifted the curves:

Supply shock. A severe drought in southern Spain (the world’s biggest olive-oil producer) cut the Spanish olive harvest by roughly 50% (source: USDA Foreign Agricultural Service, 2023 report on Spanish olive oil). That reduced total European supply sharply. At every possible price, less oil was available. The supply curve shifted left.
Inelastic demand. Olive oil is culturally central in Mediterranean cooking — Italians don’t easily substitute it with seed oils, at least in the short term. When supply fell, demand didn’t fall proportionally. So price had to rise a lot to clear the market.

Result: at the new equilibrium, quantity sold dropped maybe 10-15% and price nearly doubled. That’s the olive-oil story told in supply-and-demand language.

Elasticity: how “sticky” demand and supply are

Price elasticity of demand measures how much quantity demanded changes when price changes.

Elastic demand: quantity changes a lot when price changes. Luxuries and easily-substituted goods (different brands of beer, a specific restaurant, a specific clothing brand).
Inelastic demand: quantity changes little when price changes. Necessities without close substitutes (insulin, petrol for a daily commute, olive oil for an Italian kitchen).

Rough examples:

Good	Demand elasticity	Explanation
Insulin	~0 (fully inelastic)	Diabetics need it regardless of price
Petrol, short-term	~−0.2	Commuters still drive when prices rise; switch to EV only over years
Petrol, long-term	~−0.7	Given time, people buy smaller cars, move closer to work
Restaurant meals	~−1.5	Easily substituted with home-cooked
Specific brand of pasta	~−3 or more	Substitute easy (buy Barilla instead of De Cecco)

Why it matters for personal finance: income spent on inelastic goods is hard to cut. When you’re budgeting (lesson 8), the low-elasticity categories (rent, electricity, food basics) don’t give much. The elastic ones (restaurants, entertainment, subscriptions) do.

Price elasticity of supply works the same way on the producer side — how much production increases when price rises. Housing, for instance, has very low short-term elasticity of supply: building a new apartment block takes years. Ride-sharing has high elasticity: turn on the app, drivers appear.

Milan rent: a textbook example

Milan rents have risen roughly 45% between 2015 and 2024 (source: OMI — Osservatorio del Mercato Immobiliare, Rapporto Immobiliare 2024). Why?

Demand shifted right:

Milan added tech and finance jobs at a rapid pace — consulting, banking, fashion tech.
Universities (Bocconi, Politecnico, Cattolica) attract more international students.
Remote workers from other Italian cities moved to Milan post-2020.

Supply shifted very little:

Building permits in central Milan are hard to obtain.
Historical constraints (UNESCO-adjacent areas, protected facades) cap new construction.
Turning existing units from long-term rentals to short-term (Airbnb) reduced supply to residents.

High demand, nearly-flat supply, inelastic short-term supply. Result: prices go up a lot before quantity adjusts. This is the olive-oil story, but the “good” is square metres of Milan apartment and the shock is long-term and structural.

Sofia, who pays €950/month for a 35 m² studio in the Navigli area, experiences this directly. Moving to Bologna (her father’s city) would halve her rent. She hasn’t moved because the salary premium of working in Milan more than compensates — but that calculation is also supply-and-demand, applied to labor.

Price is a signal

The deepest idea in supply-and-demand is that price is information. When olive oil got expensive, the price told:

Consumers: “use less.”
Producers: “plant more olive trees” (takes years to bear fruit).
Importers: “source from other regions.”
Innovators: “develop drought-resistant strains.”

Nobody needed to command any of these responses. The price did the coordinating.

This is why centralized price controls almost always fail. If you cap rent in Milan at €500/month, the price-signal “build more” gets muted. Developers build elsewhere. Existing landlords either exit the market (leaving units empty) or convert to short-term. Quantity supplied drops. Even with rent cap, equilibrium — the actual quantity of renters served — gets worse.

Exceptions exist. Some goods have externalities (healthcare, education, public goods) where market pricing doesn’t capture social value. Those are the arguments for policy interventions we’ll come back to in later lessons. But for most ordinary goods, prices are doing useful work.

Why your grocery bill tells a story

Every month, ISTAT publishes the CPI (Consumer Price Index — lesson 4 on inflation). Under the hood, it’s tracking the prices of thousands of specific goods and services. When you see “food and non-alcoholic beverages inflation 3.2% year-over-year”, that’s the sum of thousands of little supply-and-demand stories: olive oil up, cereals up (Ukraine), eggs up (avian flu), coffee roasted beans up (Brazil drought), wine slightly down.

Your grocery bill is a mirror of global supply chains. If you pay attention to which items rose and which fell, you get a surprisingly accurate read of what’s going on in agricultural markets, shipping, energy, and labor.

The labor market is also supply and demand

The same framework explains your salary.

Labor supply: workers willing to work at a given wage. Higher wage, more workers willing.
Labor demand: employers willing to hire at a given wage. Higher wage, fewer willing.
Equilibrium wage: where they meet.

When a skill is in high demand and short supply (software engineering in Milan 2020-2023), wages rise. When demand falls or supply rises (journalism in the 2010s), wages fall.

This is why “invest in yourself” works. If you can shift the supply curve of your specific skill leftward (become rarer) while being in a growing demand area (shift demand rightward), your wages rise. Lesson 15 — “skills that compound” — is supply-and-demand for your personal labor.

What to do with this lesson

Three habits:

Before complaining about a price, ask: supply or demand? “Milan rent is insane” → demand shifted, supply can’t catch up. “My petrol bill doubled” → supply shock (geopolitics, refinery outage). Understanding which side moved tells you whether it’s temporary or structural.
Recognize inelastic categories in your own budget. Rent, utilities, minimum food, essential transport. These are your “can’t cut” expenses. Elastic ones (restaurants, entertainment, clothing) are where budget pressure actually has room to work.
Apply the same lens to your career. Your wages are a market price. To raise them: increase demand for your specific skills (make yourself harder to replace) or reduce your alternatives (don’t be the only option you give yourself — always have an outside offer ready to walk to).

Sources

ISTAT — Prezzi al consumo. https://www.istat.it/it/dati-analisi-e-prodotti/prezzi (retrieved 2025-01).
OMI (Agenzia delle Entrate) — Rapporto Immobiliare 2024. https://www.agenziaentrate.gov.it/portale/web/guest/schede/fabbricatiterreni/omi/pubblicazioni/rapporti-immobiliari-residenziali (retrieved 2025-02).
USDA FAS — Spain: Olive Oil Annual 2023. https://www.fas.usda.gov/data/spain-olive-oil-annual-7 (retrieved 2025-01).
Eurostat — HICP — all items. https://ec.europa.eu/eurostat/web/hicp (retrieved 2025-01).

Next lesson: central banks — the people who set the price of money. What the ECB actually does, why their meetings move your mutuo rate, and what “quantitative easing” means for your grocery bill five years later.