1.1 What is economics?

A good that is scarce, so its production or use has an opportunity cost, whether or not consumers pay a price for it.

Economics as a social science

Why economics is a social science

A social science studies human behaviour and the relationships between people in society. Psychology, sociology, political science and anthropology are social sciences, and so is economics. Its subject is people: how they decide what to buy, where to work, how much to save, and how those decisions combine to shape markets, countries and the world economy.

Economics differs from a natural science such as physics because people are not atoms. They learn, change their minds, react to what others do and respond to new laws or prices in ways that are hard to predict. A rise in the price of bread affects millions of buyers and bakers, each with different incomes, tastes and expectations. Economists therefore build simplified models of behaviour and test them against evidence, knowing that real behaviour is more varied than any model.

Economics is social in a second sense: decisions are interconnected. A worker’s wage is a firm’s cost; a household’s spending is a firm’s revenue; one country’s imports are another country’s exports. This web of connections is called interdependence, and it means that a decision taken in one place often has effects, intended or not, somewhere else.

Microeconomics and macroeconomics

Economics is divided into two main branches. Microeconomics looks at the individual parts of the economy: single consumers, firms, workers and markets. It asks, for example, why the price of coffee rose, how a tax on sugary drinks changes what people buy, or why a few large firms dominate the market for smartphones.

Macroeconomics looks at the economy as a whole. It studies total output, the general level of prices, the number of people out of work, the rate of economic growth, and how government and central bank policies affect them. A third area, the global economy, extends these ideas to trade, exchange rates and development between countries.

The two branches are closely linked. Total spending in the economy is the sum of millions of individual spending decisions, and a recession in the whole economy changes the conditions every individual firm faces. Many issues, such as unemployment or climate change, need both a microeconomic and a macroeconomic view.

The nine key concepts

Nine key concepts run through every topic in economics. They are ideas that help to organise thinking about any economic issue, from the market for coffee to the growth of a whole country. Each is defined fully where it is first needed; the table gives a first introduction.

The problem of choice

Needs and wants

People have needs and wants. Needs are the things required for survival: food, clean water, shelter, clothing and basic healthcare. Wants are everything people desire beyond these needs, such as a faster phone, a holiday abroad or a meal in a restaurant. The line between them is not always sharp, and it shifts over time: internet access was a luxury in 1995 but is close to a need today for work and education in many countries.

Economists assume that wants are unlimited. Once one want is satisfied, another appears: a household that buys its first car may soon want a second one, a bigger home or better schooling for its children. Society as a whole also has endless wants, such as better hospitals, cleaner energy, more police and lower taxes, all at the same time.

The factors of production

Goods and services are produced with resources, which economists call factors of production. They are grouped into four categories, and each one earns a payment, called a factor income, when it is used in production.

The quantity and the quality of each factor both matter. Labour depends not only on the number of workers but also on their education, skills and health; land includes both its area and its fertility. Economists call the skills and knowledge of the workforce human capital, which grows through education and training.

Scarcity

Every factor of production is limited. There is a fixed amount of land, a limited number of workers with limited hours, and only so many machines at any moment. Because unlimited wants meet limited resources, not every want can be satisfied. This is the basic economic problem, scarcity.

Scarcity affects everyone, rich and poor. A billionaire still has only 24 hours a day and must choose how to spend them. A rich country still has a limited number of doctors and must decide which patients are treated first. Scarcity is a relationship between wants and resources, so it never disappears: as incomes rise, wants rise too.

Because of scarcity, every individual, firm and government faces a choice about how to use the resources available. A household chooses how to divide its income between rent, food and saving. A firm chooses which products to make and which machines to buy. A government chooses how to divide its budget between health, education, defence and other uses.

Goods that are scarce are economic goods: producing them uses resources that could have produced something else. A small number of goods are not scarce. These free goods, such as the air people breathe or seawater, exist in such quantities that using more of them does not mean anyone has to go without.

Whether a good is free depends on circumstances, not on its price. Clean air in a large city with heavy traffic is scarce, because keeping it clean requires giving up some car journeys or paying for cleaner engines. Seawater is free, but the desalinated drinking water made from it is an economic good, because desalination plants use land, labour, capital and energy. A good can also have a zero price and still be an economic good: a state school charges no fees, but the teachers and buildings it uses have other uses.

Scarcity and sustainability

Some resources are used up when they are used. Oil, gas, coal and metal ores are non-renewable: once extracted and used, they are gone. Others, such as forests, fish stocks, groundwater and fertile soil, are renewable, but only if they are used no faster than nature can replace them. When a renewable resource is used faster than it renews, it too becomes depleted.

This links scarcity to sustainability. Using resources to satisfy wants today can reduce the resources available to future generations. Overfishing can collapse a fish stock for decades; pumping groundwater faster than rain refills it can leave farmers without water; burning fossil fuels changes the climate on which future production depends.

Sustainability adds a time dimension to the problem of choice. A society chooses not only between different goods today, but also between consumption today and the resources it leaves for the future. Many of the hardest choices in economics, from carbon taxes to water rationing, involve this trade-off between present and future economic well-being.

Opportunity cost

The real cost of a choice

When resources are used for one purpose, they cannot be used for another. The cost of any choice is therefore what has to be given up to make it. Economists call this the opportunity cost: the value of the next best alternative that is forgone.

Opportunity cost is the next best alternative only, not every possible alternative. A student with a free Saturday might choose between working a shift at a café, revising for a test and playing football. If the student works, and revising was the second choice, the opportunity cost of working is the revision time lost, not the football as well, because the student could only have done one of them with those hours.

The same idea applies to every decision-maker. For a firm, the opportunity cost of building a new factory might be the research project it can no longer afford. For a government, the opportunity cost of a new aircraft carrier might be the hospitals that the same money, workers and materials could have built. For a farmer, planting cotton on a field means not planting wheat on it.

Opportunity cost for a whole country

Opportunity cost matters most clearly at the level of a whole economy, where a country’s factors of production are switched from one use to another. Between 1942 and 1945, the United States turned its car industry over to war production. All passenger car production in the United States had stopped by February 1942, and car plants built jeeps, trucks, tanks and aircraft engines instead. The opportunity cost of the military equipment was the cars American households could not buy until the war ended.

Governments face the same trade-off in every budget. Money, staff and equipment spent on defence cannot also be spent on schools or hospitals, so a choice to raise one is, in effect, a choice not to raise the others. A government that cuts taxes gives up the public services the tax revenue could have paid for.

The basic economic questions

Scarcity forces every society to answer three basic economic questions. They apply to every economy, whatever its size, wealth or political system.

The first question is about choice and opportunity cost: producing more of one thing means producing less of another. The second is about efficiency: producing in a way that wastes as few resources as possible, and in a way that is sustainable. The third is about equity, because it decides who gains from what is produced.

The “for whom” question is usually answered by income: people with higher incomes can buy more. Some goods, however, are allocated by rules instead. During the COVID-19 pandemic, the first vaccines in the United Kingdom went to care home residents and their carers, then to people aged 80 and over and frontline health and care workers, following a priority list of nine groups set by expert advisers. The vaccines were scarce, and the government chose to share them by medical need and risk rather than by willingness to pay.

How economies answer the basic questions: economic systems

Societies answer the basic economic questions in different ways. The main difference is who decides: individual consumers and firms acting through a market, or the government. An economic system is the way a society organises the ownership of resources and makes these decisions.

A market is any arrangement in which buyers and sellers exchange a good, service or factor of production at a price. It can be a physical place, such as a fruit market, or an online platform, or simply a network of buyers and sellers who never meet, as in the market for oil. Economic systems can be placed on a spectrum, with the free market economy at one end, the planned economy at the other, and mixed economies in between.

The free market economy

In a free market economy, the factors of production are owned privately, by households and firms. Consumers decide what to buy and firms decide what to produce, each pursuing their own interests. The government’s role is limited to protecting property rights, enforcing contracts and keeping order.

The basic questions are answered by prices, through the price mechanism. What to produce is decided by consumer demand: if many people want a product, its price rises, it becomes more profitable, and firms make more of it. How to produce is decided by firms seeking the lowest cost, which rewards efficient methods. For whom is decided by who can pay: goods go to the consumers with the income and the willingness to buy them.

Supporters of free markets point to strong incentives to work, invest and innovate, wide consumer choice, and fast responses to changes in what people want. Critics point to the problems markets do not solve on their own. Some people have very low incomes and cannot afford basic needs; some goods, such as street lighting or national defence, are not provided at all because firms cannot charge for them; and firms may ignore the costs they impose on others, such as pollution. No country in the world is a pure free market economy.

The planned economy

In a planned economy (also called a command or centrally planned economy), the state owns most land and capital, and central planners decide what is produced, how and for whom. Production targets are set for each factory and farm, prices are fixed by the state, and workers may be assigned to jobs.

A planned economy can direct resources quickly towards national goals, such as heavy industry or defence, and can aim to give everyone access to basic goods, jobs, housing and healthcare. The Soviet Union industrialised rapidly in the 1930s in this way. The weaknesses are serious, however. Planners need enormous amounts of information about millions of products and the preferences of millions of people, which they rarely have. Without prices to signal what is wanted, planned economies often produce too much of some goods and too little of others, leading to queues and shortages. With little reward for effort or new ideas, incentives to work hard and innovate are weak.

A natural comparison is the Korean peninsula, divided in 1945 into two states that chose very different systems. North Korea kept a centrally planned economy; South Korea developed a market-based mixed economy. In 2024, South Korea’s gross national income per person was estimated at about 29 times North Korea’s. The comparison is not a controlled experiment, since many other factors differ, but it shows how much the choice of system can matter over several decades.

Sources: Bank of Korea estimates of North and South Korean GNI per capita (2025), as reported by the Korea Economic Institute of America (2025).

The mixed economy

Every real economy is a mixed economy: it combines a market sector, where private firms and consumers make most decisions through prices, with a public sector, where the government owns some resources, provides some goods and services, and regulates markets. The difference between countries is the balance between the two.

Governments in mixed economies typically provide or fund education, healthcare, roads and defence, pay pensions and unemployment benefits, tax incomes and spending, and set rules on safety, competition and the environment. Most other goods, from food and clothes to cars and phones, are produced by private firms and sold in markets. One way to measure the size of the government’s role is government spending as a percentage of a country’s total output (GDP).

Sources: IMF, Fiscal Monitor data via IMF DataMapper, general government expenditure (accessed October 2026).

The table shows that even the United States, often described as a market economy, has government spending of over a third of its output, while France, with spending above half of GDP, still relies on private firms to produce most goods. Spending is only a partial measure: governments also influence the economy through laws and regulation, and some countries, such as China, keep large state-owned firms whose output is not counted as government spending.

Economies also move along the spectrum over time, which illustrates the key concept of change. From 1978, China began to replace central planning with market incentives: farm households were allowed to sell output above their quotas, private firms were permitted, and foreign investment was invited into special economic zones. China’s real GDP per person rose from $386 in 1978 to $13,119 in 2024 (constant 2015 US$), about 34 times higher. In the other direction, many market economies widened the government’s role during the COVID-19 pandemic, paying wages and supporting firms directly.

Sources: World Bank, World Development Indicators, GDP per capita (constant 2015 US$), China (2026).

Markets or intervention?

The debate over how far governments should intervene in markets runs through all of economics. Markets are good at coordinating the decisions of millions of people through prices and at rewarding efficiency and innovation. Governments can correct some of the things markets do badly: providing goods that markets underprovide, protecting the environment, and making the distribution of income fairer. Government intervention has costs and risks of its own, however, such as poor information, high administrative costs and decisions influenced by political pressure.

How much intervention is right depends partly on facts, such as whether a particular policy works, and partly on values, such as how much weight to give to equity relative to efficiency. These questions recur in every later topic, from price controls and carbon taxes to the role of governments in economic development.

The production possibilities curve (PPC) model

What the PPC shows

Economists use simplified models to explain how the economy works. The first is the production possibilities curve (PPC). It shows the different combinations of two goods or services that an economy can produce when it uses all its resources fully and efficiently.

The PPC is built on a set of assumptions that keep the model simple:

• Two goods only. The economy produces just two goods (or two groups of goods, such as capital goods and consumer goods), so they can be drawn on two axes.

• Fixed quantity and quality of resources. The amount of land, labour, capital and entrepreneurship does not change.

• Fixed technology. The methods of production available do not change.

• A fixed time period. The curve shows what can be produced in a given period, such as a year.

• Full and efficient use of resources at every point on the curve: no factor of production is unemployed or wasted.

Real economies produce millions of goods, and their resources and technology change all the time, so the PPC is not a description of any real economy. Its value is that, by holding everything else fixed, it shows clearly how scarcity, choice and opportunity cost work.

Scarcity, choice and efficiency on the PPC

The diagram shows a PPC for capital goods (machines, factories, infrastructure) and consumer goods (food, clothes, phones). Each point on the curve is a combination the economy can just produce when all resources are used efficiently. Together, the points show the economy’s production possibilities.

The PPC illustrates scarcity because the economy cannot produce beyond the curve. Point X, outside the curve, would give more of both goods than A or B, but it is unattainable with current resources and technology. The curve is a boundary, set by the limited quantity of resources.

It illustrates choice because the economy must pick one point on (or inside) the curve. At A, it produces many capital goods and few consumer goods; at B, more consumer goods and fewer capital goods. Which point is chosen is the society’s answer to the “what to produce” question.

It illustrates efficiency because every point on the curve uses all resources in the best possible way. At A or B, it is impossible to produce more of one good without producing less of the other. This is productive efficiency.

Point U, inside the curve, shows unemployment of resources: some workers are out of work, machines stand idle or land is unused, or resources are used inefficiently. The economy could produce more of both goods, moving towards the curve, without giving anything up. Points inside the PPC therefore show productive inefficiency. A recession, when firms cut output and lay off workers, moves an economy to a point inside its PPC.

Being on the PPC does not mean that the economy produces the best combination for society. Every point on the curve is productively efficient, but only one combination matches what society values most. Producing the mix of goods that society wants most is called allocative efficiency, an idea developed further in the study of markets.

Opportunity cost on the PPC

Because the economy is fully using its resources on the curve, producing more of one good requires moving resources away from the other. The downward slope of the PPC therefore shows opportunity cost.

The diagram shows an economy choosing between military goods and civilian goods, the trade-off the United States faced in 1942. At B, it produces C₁ civilian goods and M₁ military goods. To move to C, with more military goods (M₂), it must transfer workers, factories and materials out of civilian production, so civilian output falls to C₂. The opportunity cost of the extra M₁M₂ military goods is the C₁C₂ civilian goods given up.

The opportunity cost of one good is measured in units of the other good. If producing 100 more tanks means producing 2,000 fewer cars, then the opportunity cost of each tank is 20 cars, and the opportunity cost of each car is 1/20 of a tank.

Increasing and constant opportunity cost

The shape of the PPC depends on how suitable the factors of production are for producing each good. Two cases are important: a curve bowed outwards from the origin (concave) and a straight line.

Increasing opportunity cost: a concave PPC

In most economies, resources are not equally good at producing everything. Some land suits wheat but not vineyards; some workers are skilled engineers, others skilled nurses. When an economy starts producing more of good X, it first moves the resources that are best at producing X and worst at producing Y. Little Y is lost. As it keeps expanding X, it has to use resources that are less and less suited to X and better and better at Y, so each extra unit of X costs more and more units of Y.

This is increasing opportunity cost, and it makes the PPC bowed outwards (concave to the origin). In the left-hand panel, moving from A to B (the first 3 units of X) costs only 0.5 units of Y, from B to C costs 1.8, and from C to D costs 6.7. The curve becomes steeper as X increases.

Constant opportunity cost: a straight-line PPC

If resources were equally good at producing both goods, every unit of X would cost the same amount of Y, however much X was already produced. This is constant opportunity cost, shown by a straight-line PPC with a constant slope. In the right-hand panel, every extra 3 units of X cost exactly 3 units of Y.

Constant opportunity cost is realistic only for goods that use very similar resources, such as two types of shirt made by the same workers on the same machines. Increasing opportunity cost is the more common case for a whole economy, which is why the PPC is usually drawn bowed outwards.

Actual growth and growth in production possibilities

The PPC can show two different kinds of economic growth, which are easily confused. The difference lies in whether the economy moves towards its existing curve or the curve itself moves.

Actual growth: a movement towards the PPC

Actual growth happens when an economy produces more by using resources that were unemployed or used inefficiently. In the left-hand panel, the economy starts at A, inside its PPC, perhaps because of a recession. As unemployed workers find jobs and idle factories restart, output of both goods rises and the economy moves towards point B on the curve. The PPC itself does not move, because the economy’s resources and technology have not changed.

Growth in production possibilities: an outward shift of the PPC

Growth in production possibilities (often called potential growth) happens when the economy’s capacity to produce increases. In the right-hand panel, the PPC shifts outwards from PPC₁ to PPC₂, so combinations such as D that were unattainable before become possible. The economy needs an increase in the quantity or quality of its factors of production, or better technology.

The causes of an outward shift include:

• More resources: a larger labour force (from population growth or immigration), the discovery of new natural resources, or more capital goods from investment.

• Better resources: a more educated, skilled and healthier workforce (more human capital), or more fertile land through irrigation.

• Better technology: new methods of production, such as automation, that produce more output from the same resources.

The PPC can also shift inwards when production possibilities fall. A war that destroys factories and kills workers, a major earthquake, the exhaustion of a mineral deposit, or the loss of fertile land to desertification all reduce the economy’s capacity. Unsustainable use of renewable resources can therefore, over time, shift a country’s PPC inwards.

If an improvement affects only one good, the PPC shifts out only along that good’s axis. A new high-yield variety of wheat raises the maximum quantity of wheat but not the maximum quantity of laptops, so the curve pivots outwards along the wheat axis while its end on the laptop axis stays in place.

Capital goods, consumer goods and future growth

The choice between capital goods and consumer goods today affects production possibilities tomorrow. An economy that devotes more resources to capital goods, such as machines, factories, roads and power stations, has fewer consumer goods now, but its larger stock of capital raises its capacity to produce in the future, shifting its PPC further outwards. An economy that consumes almost everything it produces enjoys more today but grows more slowly. This is a choice between present and future economic well-being, and it is one reason why investment matters so much for long-term economic growth.

Modelling the economy: the circular flow of income

The second model shows how the main decision-makers in an economy are connected through flows of money, goods and services. The circular flow of income model shows how money moves round the economy as spending and income, and it is the starting point for measuring national income and for understanding why total output rises and falls.

The simple model: households and firms

The simplest version has two decision-makers. Households own the factors of production and supply them to firms: they work, rent out land and buildings, lend savings and start businesses. Firms hire these factors to produce goods and services, which they sell to households. Two markets connect them: factor markets, where households sell their land, labour, capital and entrepreneurship, and product markets, where firms sell goods and services.

The diagram shows two pairs of flows moving in opposite directions. The real flows (dashed) are factors of production going from households to firms, and goods and services going from firms to households. The money flows (solid) go the other way: firms pay households factor incomes (wages, rent, interest and profit), and households use that income to buy the goods and services firms produce.

In this simple model, all income is spent and all spending becomes someone’s income. Every dollar a household spends becomes revenue for a firm, which uses it to pay wages, rent, interest and profit to households, who spend it again. The flow is circular, and it stays the same size from one period to the next. This leads to an important identity: the total value of spending in an economy equals the total value of income, which equals the total value of output.

The model shows interdependence clearly. Households depend on firms for their incomes and the goods they buy; firms depend on households for their workers and their customers. If households cut their spending, firms’ revenues fall, they hire fewer workers, and household incomes fall in turn.

Leakages and injections

Real economies have more than two decision-makers. Three more are added to complete the model: the financial sector (banks and other financial institutions), the government, and the foreign sector (firms and households in other countries). Each of them takes money out of the circular flow and puts money back in.

Leakages (also called withdrawals) are income that is not passed on as spending on domestic goods and services. There are three:

• Saving (S): the part of household income that is not spent but kept, often in banks.

• Taxes (T): income tax, taxes on spending and other taxes paid to the government.

• Imports (M): spending on goods and services produced abroad, which becomes income for foreign firms rather than domestic ones.

Injections are additions to the circular flow that do not come from household consumption. There are also three:

• Investment (I): spending by firms on capital goods, such as machines and buildings, often financed by borrowing from the financial sector.

• Government spending (G): spending by the government on goods and services, such as school buildings, medical equipment and roads.

• Exports (X): spending by foreign households, firms and governments on goods and services produced in the domestic economy.

The diagram pairs each leakage with an injection through the sector that links them. Banks take in households’ savings and lend them to firms that want to invest. The government collects taxes and spends on goods, services and wages. The foreign sector receives spending on imports and buys the country’s exports. The pairs are not necessarily equal: there is no reason for this year’s saving to match this year’s investment, or for the government to spend exactly what it collects in taxes.

Saving rates can change quickly. In the United States, households saved 7.5% of their disposable income in February 2020; in April 2020, during the first COVID-19 lockdowns, the saving rate jumped to 31.8%, the highest monthly rate since the series began in 1959, as households could not spend on travel, restaurants and many shops. This sudden rise in a leakage was one reason why spending and output fell so sharply in 2020.

Sources: US Bureau of Economic Analysis, personal saving rate (PSAVERT), via FRED, Federal Reserve Bank of St Louis (2026).

Why leakages and injections change the size of the flow

The size of the circular flow measures the level of economic activity: total spending, income and output. It changes whenever total leakages and total injections are not equal.

• If injections are greater than leakages (I + G + X > S + T + M), more money enters the flow than leaves it. Total spending rises, firms sell more and produce more, and incomes and output grow.

• If leakages are greater than injections (S + T + M > I + G + X), more money leaves the flow than enters it. Spending falls, firms cut output and jobs, and incomes and output shrink.

• If leakages equal injections (S + T + M = I + G + X), the flow stays the same size, and the economy is in equilibrium.

The condition for equilibrium is about the totals. Saving need not equal investment, nor taxes equal government spending, nor imports equal exports; what matters is whether all leakages together equal all injections together. A government that spends more than it collects in taxes, for example, can be balanced by households that save more than firms invest.

Interdependence in the circular flow

The full model shows how closely the five decision-makers depend on each other. Households depend on firms for jobs and on the government for public services; firms depend on households, the government and foreign buyers for sales, and on banks for finance; the government depends on taxes paid by households and firms; the foreign sector buys exports and supplies imports.

A change anywhere in the model spreads through the rest of it. A recession in a major trading partner cuts that country’s demand for imports, which reduces the home country’s exports, firms’ revenues and households’ incomes. A rise in interest rates by the central bank discourages borrowing, reducing investment. The key concept of interdependence describes these links, and the circular flow is the clearest picture of them.

Strengths and limits of the circular flow model

Like the PPC, the circular flow is a simplification. It shows clearly why spending, income and output are linked, why changes in saving, taxes or trade affect the whole economy, and why national income can be measured by adding up spending, incomes or output. These ideas are the foundation of macroeconomics.

The model leaves a lot out, however. It does not show prices or inflation, it treats all households and firms as if they were the same, and it ignores the distribution of income between rich and poor. It also leaves out the natural environment: production uses resources from nature and returns waste and pollution to it, which matters for sustainability. Firms also spend on each other’s products, and governments make transfer payments, such as pensions, which redistribute income without buying any output; neither appears in the basic diagram.

Case study: Cape Town’s “Day Zero” water crisis, 2017–18

Cape Town, South Africa’s second-largest city, depends on six major dams that collect winter rainfall. South Africa is a dry country: in 2022 its renewable internal freshwater resources were about 718 cubic metres per person, compared with a world average of about 5,390. After good rains in 2014, three dry winters followed, and 2017 was the driest year in Cape Town since 1933. At the same time, the city’s population had grown from about 2.4 million in 1995 to an estimated 4.1 million in 2015, an increase of 71%, while dam storage capacity had grown by only 17%.

The chart shows how quickly the stored water ran down. By mid-May in both 2017 and 2018, the dams held only about a fifth of their capacity, and the water nearest the bottom of a dam is difficult to use. In January 2018 the city forecast that “Day Zero”, the day the dams would reach 13.5% of capacity, would arrive on 22 April 2018, and then brought the forecast forward to 12 April. On Day Zero, the taps in most homes would be turned off and residents would have to collect a ration of 25 litres per person per day from about 149 collection points.

How the city answered the basic economic questions

Water became extremely scarce, and the city and national government had to decide who would get it. They used a mix of rules and prices, the tools of a mixed economy. Household restrictions were tightened step by step: to 100 litres per person per day from June 2017, to 87 litres from September 2017, and to 50 litres per person per day from 1 February 2018. Water tariffs were raised steeply, with the highest rates for households that continued to use large amounts, and fines and flow-limiting devices were used against heavy users.

The burden was not shared equally across uses. Urban users take roughly 70% of the water from the supply system and farmers roughly 30%, and in 2018 farmers’ allocations were cut by around 60%, compared with a target cut of about 45% for urban users. The opportunity cost of keeping water in city taps was lost farm output: estimates put agricultural losses at about R5.9 billion (around $400 million) and some 30,000 jobs in the Western Cape. This was a direct answer to the “for whom” question, and it involved a choice between the economic well-being of city residents and of rural workers.

The response worked. Water use fell by more than half, to around 500 million litres a day by March 2018, and Day Zero was pushed back repeatedly. When strong winter rains returned from June 2018, the dams began to refill, and on 28 June 2018 Day Zero was postponed indefinitely. Restrictions were eased in stages as dam levels recovered.

What the case shows

The crisis illustrates the core ideas of economics in one setting. Scarcity became visible when a resource that most residents had treated as plentiful nearly ran out. Choice and opportunity cost appeared in every decision: water for gardens or for drinking, for cities or for farms, spending on emergency supply or on other public services. The city also considered building large desalination plants, but abandoned most of these plans because the cost per litre was too high, so it relied on cutting demand and on smaller projects such as drawing on groundwater.

The underlying sustainability problem was that demand had grown faster than the supply system for decades, so a drought of a severity estimated to occur about once in 300 years pushed the system to the edge. Without intervention, an unregulated market would have answered the “for whom” question by price alone.

The case has limits as evidence. It is one city, in one drought, and the rains that ended the crisis were good fortune rather than policy. Restrictions also depended on public cooperation that may be hard to repeat, and estimates of agricultural losses vary widely. Even so, it shows clearly how a society facing acute scarcity has to make choices about what, how and for whom, and how the answers combine prices, rules and shared effort.

Sources: City of Cape Town, dam level reports and water dashboard (2014–2021) and Western Cape Government water updates, as compiled in Wikipedia, “Cape Town water crisis” (accessed October 2026); World Bank, World Development Indicators, renewable internal freshwater resources per capita (2026).

Exam practice

Scarcity, opportunity cost, economic systems, the PPC and the circular flow appear across all the papers. They are rarely the subject of a whole question on their own, but their definitions, PPC diagrams and the circular flow model are often needed within questions on markets, macroeconomics and development. Paper 1 questions can ask you to use a PPC or the circular flow; Paper 2 can ask you to define terms from this topic or to draw and explain a PPC using data from the text.

Label every PPC as PPC (or PPC₁ and PPC₂ after a shift), name the two goods on the axes and refer to the marked points by their letters in your writing.

Paper 2-style short items:

1. Define the term opportunity cost. [2]

2. Draw a PPC diagram to show the effect of a recession on an economy that was previously producing on its PPC. [4]

3. Using a PPC diagram, explain why an improvement in education could increase a country’s production possibilities. [4]

4. Using the circular flow of income model, explain how a fall in exports could reduce national income. [4]

Summary

• Economics is the social science that studies how scarce resources are used to satisfy needs and wants; microeconomics studies individual markets and macroeconomics the whole economy.

• The four factors of production are land, labour, capital and entrepreneurship, earning rent, wages, interest and profit.

• Scarcity (limited resources, unlimited wants) forces choices, and every choice has an opportunity cost: the next best alternative given up. Free goods have no opportunity cost.

• Every economy must decide what, how and for whom to produce; free market, planned and mixed economies answer these questions differently, and all real economies are mixed.

• The PPC shows scarcity, choice, opportunity cost and productive efficiency; points inside it show unemployment of resources. A concave PPC shows increasing opportunity cost, a straight line constant opportunity cost.

• Actual growth is a movement from inside the PPC towards it; growth in production possibilities is an outward shift of the curve from more or better resources or technology.

• The circular flow of income links households, firms, the financial sector, the government and the foreign sector; leakages (S, T, M) and injections (I, G, X) determine whether national income rises or falls.

• Scarcity is linked to sustainability: using resources faster than they renew reduces the production possibilities of future generations.

Self-check

• To do:

  I can explain why economics is a social science and give examples of microeconomic and macroeconomic questions.

• To do:

  I can name the four factors of production and the factor income each one earns.

• To do:

  I can define scarcity and opportunity cost precisely and distinguish free goods from economic goods.

• To do:

  I can explain the link between scarcity and sustainability with an example.

• To do:

  I can explain how free market, planned and mixed economies answer the three basic economic questions.

• To do:

  I can draw a PPC and use it to show scarcity, choice, opportunity cost, efficiency and unemployment of resources.

• To do:

  I can draw and explain PPCs with increasing and constant opportunity cost, and calculate opportunity costs from a table.

• To do:

  I can distinguish actual growth from growth in production possibilities on PPC diagrams.

• To do:

  I can draw the circular flow of income with all leakages and injections and explain what happens when they are unequal.

• To do:

  I can use the nine key concepts to analyse a real example such as Cape Town’s water crisis.