Principles of Economics

Economics is the study of how people and societies use limited resources to satisfy unlimited wants. The starting point is scarcity: there is never enough time, money, labor, or land to do everything, so every choice means giving something up. Economists split the field into microeconomics, the study of individual households and firms, and macroeconomics, the study of the economy as a whole.

Opportunity cost and the margin

The true cost of anything is its opportunity cost: the value of the next-best alternative you gave up. If you spend a Saturday working a shift that pays $80 instead of studying, the opportunity cost of studying is that $80. Suppose you have $12 and a movie ticket costs $12 while a paperback costs $12 - buying the movie means the book is your opportunity cost, and vice versa.

Rational actors also think at the margin, weighing the extra benefit of one more unit against its extra cost. You do not ask "is pizza worth it?" but "is one more slice worth its price?" When marginal benefit exceeds marginal cost, do it; when it does not, stop.

Finally, economists separate positive statements (what is, and testable - "a higher minimum wage raises some teens' pay") from normative statements (what ought to be, a value judgment - "the minimum wage should be raised"). Good analysis keeps the two apart.

The production possibilities frontier (PPF) shows the maximum combinations of two goods an economy can make with its resources fully and efficiently employed. Points on the curve are efficient, points inside are wasteful, and points outside are (for now) unattainable. Because resources are scarce, the PPF slopes downward: making more of one good means making less of the other. Its bowed-out shape reflects increasing opportunity cost - resources are not equally good at everything.

Absolute vs. comparative advantage

A producer has an absolute advantage if it can make more of a good with the same resources. But trade is governed by comparative advantage: the ability to make a good at a lower opportunity cost. Suppose in one hour Ana can bake 4 loaves or grow 8 kg of tomatoes, while Ben can bake 1 loaf or grow 4 kg. Ana's opportunity cost of a loaf is 2 kg of tomatoes; Ben's is 4 kg. Ana gives up less, so she should specialize in bread; Ben should grow tomatoes.

When each specializes where its opportunity cost is lowest and they trade, total output rises and both can consume beyond their own PPF. This is why specialization and exchange - between people, firms, and nations - expand the economic pie even when one side is better at everything.

In a competitive market, price is set by the tug-of-war between buyers and sellers. The law of demand says that, all else equal, a higher price reduces the quantity demanded; the demand curve slopes downward. The law of supply says a higher price raises the quantity supplied; the supply curve slopes upward. Where the two curves cross is the equilibrium - the price at which quantity demanded equals quantity supplied.

A worked example

Suppose at $4 buyers want 60 units and sellers offer 60 units: equilibrium is $4 and 60 units. At $6 sellers offer 90 but buyers want only 40, a surplus of 50 that pushes price down. At $2 buyers want 100 but sellers offer 20, a shortage of 80 that pushes price up. The market self-corrects toward $4.

Be careful to separate a movement along a curve (caused only by that good's own price) from a shift of the whole curve. Demand shifts when income, tastes, prices of related goods, expectations, or the number of buyers change. Supply shifts when input costs, technology, or the number of sellers change. A rise in demand raises both equilibrium price and quantity; a rise in supply lowers price but raises quantity. Learning to shift the right curve in the right direction is the workhorse skill of microeconomics.

Elasticity measures how strongly quantity responds to a change in price or income. The most important is the price elasticity of demand: the percentage change in quantity demanded divided by the percentage change in price. Because the two move in opposite directions the raw number is negative, so economists usually read its absolute value.

Reading the number

If a 10% price rise cuts quantity demanded by 20%, elasticity is 20% / 10% = 2. Because that exceeds 1, demand is elastic - buyers are very responsive. If the same 10% rise cut quantity by only 4%, elasticity is 0.4, so demand is inelastic. An elasticity of exactly 1 is unit elastic. Necessities like insulin tend to be inelastic; luxuries and goods with close substitutes tend to be elastic. To avoid getting different answers depending on direction, economists use the midpoint method, dividing each change by the average of the start and end values.

Elasticity has real payoffs. When demand is inelastic, raising price raises total revenue (price up beats quantity down); when demand is elastic, raising price lowers revenue. Elasticity also decides tax incidence: the more inelastic side of the market - the side less able to walk away - ends up paying most of a tax.

Why do people buy what they buy? Economists model consumers as trying to get the most satisfaction, called utility, from a limited budget. The key regularity is diminishing marginal utility: each additional unit of a good adds less extra satisfaction than the one before. The first slice of pizza is bliss; the fifth is just okay. Marginal utility is the extra utility from one more unit.

The utility-maximizing rule

A consumer spending a fixed budget constraint across goods does best when the marginal utility per dollar is equal across everything they buy: MU of A over price of A equals MU of B over price of B. Suppose a burger gives 40 utils and costs $8 (5 utils per dollar) while a smoothie gives 18 utils and costs $2 (9 utils per dollar). The smoothie delivers more satisfaction per dollar, so shifting spending toward smoothies raises total utility until the ratios even out.

This rule quietly explains the law of demand. As you buy more of a good its marginal utility falls, so it is worth paying only a lower price for additional units - which is exactly a downward-sloping demand curve. It also illuminates the old diamond-water puzzle: water is far more useful in total, but because it is abundant its marginal utility is low, so its price is low.

To understand markets we first open up the firm. A firm combines inputs to produce output, and its costs split into two kinds in the short run. Fixed costs do not change with output (rent, a lease), while variable costs rise as the firm makes more (materials, hourly labor). Their sum is total cost. Two per-unit measures matter most: average total cost (total cost divided by quantity) and marginal cost (the cost of one more unit).

Why costs eventually rise

In the short run at least one input is fixed, so adding more of a variable input runs into the law of diminishing marginal returns: past some point each extra worker adds less extra output, which makes marginal cost rise. Suppose fixed cost is $100 and producing 10 units adds $150 in variable cost: total cost is $250 and average total cost is $25. If the 11th unit adds $20 of cost, its marginal cost is $20 - below the $25 average, so it pulls average cost down.

Costs also decide what "profit" means. Accounting profit is revenue minus explicit money costs. Economic profit subtracts implicit costs too - the opportunity cost of the owner's time and capital. A firm can show an accounting profit yet a zero or negative economic profit, meaning its resources would earn more elsewhere.

A perfectly competitive market has many small firms selling an identical product, with easy entry and exit and well-informed buyers. No single firm can influence the price, so each is a price taker: it can sell all it wants at the market price but nothing above it. That makes the firm's demand curve horizontal, and its marginal revenue - the extra revenue from one more unit - equal to the price.

How much to produce

A profit-maximizing firm expands output as long as the revenue from one more unit beats its cost, and stops where marginal revenue equals marginal cost. In competition, since MR equals price, the rule becomes produce where P = MC. Suppose the price is $12 and the marginal cost of the 8th unit is $12 while the 9th would cost $14: the firm makes 8 units. If average total cost at that output is $10, profit per unit is $2 and total profit is $16.

The magic happens over the long run. Positive economic profit attracts entry, which raises supply and pushes price down; losses cause exit, which lifts price. Entry and exit continue until price is driven to minimum average total cost and economic profit is zero. Firms still earn a normal return, but the relentless competition delivers goods at the lowest sustainable cost - a key reason economists prize competitive markets.

When firms have market power they can influence price rather than merely accept it. The extreme case is monopoly: a single seller of a product with no close substitutes, protected by barriers to entry such as patents, control of a key resource, network effects, or high fixed costs that create a natural monopoly. Because the monopolist is the market, it faces the downward-sloping market demand curve and must lower its price to sell more.

Why monopoly output is lower

That price cut applies to every unit, so the monopolist's marginal revenue is below its price. It still produces where MR = MC, but because MR sits under the demand curve it chooses a lower quantity and charges a higher price than a competitive industry would. The result is a deadweight loss: some trades that buyers valued above cost never happen, so total surplus shrinks even as the monopolist earns profit.

Most real markets sit between the extremes. Under monopolistic competition many firms sell differentiated products (restaurants, clothing brands); each has a sliver of pricing power, but easy entry pushes long-run profit toward zero. Under oligopoly a few large firms dominate (airlines, phone carriers) and must anticipate each other's moves, which game theory studies. Policy responds to market power through antitrust law and regulation.

A wage is just a price - the price of labor - set by supply and demand in a labor market. The demand for labor is a derived demand: firms hire workers not for their own sake but for what they produce. A worker's contribution is the marginal product of labor, and its dollar value is the marginal revenue product (marginal product times the price of output). A firm keeps hiring while the marginal revenue product of the next worker exceeds the wage.

What moves wages

Suppose a worker adds 20 units per day and each sells for $5: their marginal revenue product is $100 a day, the most a firm would pay. Anything that raises productivity - better tools, training, technology - raises labor demand and wages. On the supply side, wages rise when fewer workers are willing or able to do a job. This is why human capital (education and skills) and scarcity command higher pay: surgeons out-earn cashiers largely because their marginal revenue product is high and their supply is limited.

Institutions matter too. A minimum wage set above the market wage can raise pay for some while reducing hours or jobs for others, an effect economists still measure and debate. Unions bargain collectively to lift wages and conditions. Gaps in pay across groups can reflect differences in skills and hours, but also discrimination, which economics analyzes as both an equity problem and a market inefficiency.

Competitive markets are usually efficient, but not always. A market failure occurs when the price of a good does not reflect its full costs or benefits to society. The classic case is an externality - a cost or benefit that spills onto third parties. A factory that pollutes creates a negative externality: its private cost is below the true social cost, so the market overproduces. A neighbor who vaccinates or plants a garden creates a positive externality, which the market underprovides.

Correcting the price

The economist's fix is to make the price tell the truth. A tax equal to the external damage - a Pigouvian tax, such as a carbon tax - raises the polluter's cost to the social cost and cuts output to the efficient level. Suppose a ton of emissions does $40 of harm the firm ignores; a $40-per-ton tax internalizes it. Positive externalities call for the mirror image: subsidies for things like education or vaccination. Sometimes clarifying property rights lets private parties bargain to an efficient outcome on their own.

A second failure is the public good - something non-excludable and non-rival, like national defense or a lighthouse. Because no one can be shut out, everyone hopes to free-ride on others, so private markets underprovide it and government typically steps in. Recognizing where markets fail, without assuming government always does better, is the heart of public economics.

We now zoom out to the whole economy. Gross domestic product (GDP) is the market value of all final goods and services produced within a country in a period. The most common way to add it up is the expenditure approach, which sums four kinds of spending: consumption (C) by households, investment (I) by firms in capital and inventories, government purchases (G), and net exports (exports minus imports, NX). In shorthand, GDP = C + I + G + NX.

Real versus nominal

Because GDP is measured in money, rising prices can make output look bigger than it is. Nominal GDP uses current prices; real GDP corrects for inflation by using constant base-year prices, so it reflects true changes in the quantity of output. Suppose an economy makes only bread: 100 loaves at $2 gives nominal GDP of $200 this year; if next year it makes 110 loaves but the price jumps to $3, nominal GDP is $330, yet real GDP in base-year prices is 110 x $2 = $220 - a 10% real gain, not 65%.

GDP is the headline gauge of an economy's size, but it is not a measure of welfare. It counts only market activity, so it misses unpaid housework and volunteering; it ignores the distribution of income, leisure, environmental damage, and whether the goods are useful. Economists therefore pair GDP with other measures rather than treating it as a scorecard for happiness.

Two numbers dominate macro headlines. The unemployment rate is the percentage of the labor force (the employed plus those actively looking) that is unemployed. If 150 million people are employed and 6 million are seeking work, the labor force is 156 million and the unemployment rate is 6 / 156 = about 3.8%. People without a job who are not looking are outside the labor force, which is why the rate can miss discouraged workers.

Types of unemployment and inflation

Economists sort joblessness into frictional (short-term, between jobs), structural (skills or location mismatch), and cyclical (from downturns). Some frictional and structural unemployment always exists; the sum of the two is the natural rate, the level around which the economy operates when healthy.

Inflation is a sustained rise in the overall price level, most often tracked by the Consumer Price Index (CPI), the cost of a fixed basket of goods a typical household buys. If the basket cost $200 last year and $206 now, inflation is 6 / 200 = 3%. Inflation erodes the value of money and savings, distorts decisions, and hurts people on fixed incomes, while deflation and very high inflation carry their own dangers. Both indicators guide the policy we study in later weeks.

Over the long run, nothing matters more for living standards than economic growth - a sustained rise in real GDP, and especially in real GDP per capita. Small differences compound enormously. The rule of 70 gives a quick estimate of doubling time: divide 70 by the annual growth rate. An economy growing 2% a year doubles its income in about 35 years; one growing 7% doubles in just 10. That gap explains why some nations pulled far ahead over a century.

Where growth comes from

Growth ultimately comes from rising labor productivity - output per hour worked. Its main sources are more and better physical capital (tools, machines, infrastructure), more human capital (education and skills), and above all technological progress, which lets us get more output from the same inputs. Because capital faces diminishing returns, simply piling on machines eventually slows; sustained growth in rich countries leans heavily on innovation.

Institutions set the stage. Secure property rights, the rule of law, openness to trade, and stable policy encourage the investment and risk-taking that growth requires. This framework also raises the hopeful possibility of convergence: poorer countries can grow faster by adopting existing technology, narrowing the gap with rich ones - though only where institutions let them.

Money is anything widely accepted in exchange, and economists define it by three functions: a medium of exchange (you trade it for goods), a unit of account (prices are quoted in it), and a store of value (it holds purchasing power over time). Modern money is fiat money - valuable by government decree and trust, not because it is backed by gold. Economists track it with measures like M1 and M2.

How banks create money

Most money is created by ordinary banks through fractional-reserve banking. A bank keeps only a fraction of deposits on hand as reserves and lends the rest, and those loans become new deposits elsewhere, which are lent again. The money multiplier is roughly 1 divided by the reserve ratio: with a 10% reserve ratio, an initial $1,000 deposit can support up to $1,000 / 0.10 = $10,000 in deposits across the banking system.

Overseeing all this is the Federal Reserve, the central bank of the United States. The Fed is a lender of last resort in a crisis and steers the economy with tools that change the supply of money and the level of interest rates - chiefly open-market operations (buying and selling government bonds), the interest rate it pays on bank reserves, and the discount rate. How it uses those tools to fight recession or inflation is the subject of next week.

Governments try to smooth the business cycle - the ups and downs of real GDP - with two main levers. Fiscal policy is the government's use of spending and taxes, controlled by the legislature and executive. Monetary policy is the central bank's control of the money supply and interest rates. The aggregate demand and aggregate supply model is the usual framework: policy mainly shifts aggregate demand.

Expansion versus contraction

In a recession, policymakers turn expansionary. Fiscal expansion means higher government spending or tax cuts to boost demand; monetary expansion means the Fed lowering interest rates to encourage borrowing and investment. When the economy overheats and inflation climbs, they turn contractionary: cutting spending or raising taxes, and raising interest rates. Fiscal changes can ripple through the economy via the multiplier: a dollar of spending can raise total output by more than a dollar as it is re-spent. A budget deficit results when the government spends more than it collects, adding to the national debt.

Policy is powerful but blunt. It faces time lags - recognizing a downturn, acting, and seeing effects all take time - so poorly timed policy can worsen a cycle. Large deficits raise debt and may crowd out private investment, and monetary policy loses traction when interest rates are already near zero. Economists broadly agree these tools help, while debating their size, timing, and mix.

We close by taking the whole course global. Nations trade for the same reason people do: comparative advantage lets each specialize and consume more than it could alone. Yet governments often limit trade with tariffs (taxes on imports) and quotas (limits on quantity). A tariff raises the price of imports, helping protected domestic producers and the treasury but costing consumers more, and typically shrinking total surplus. The balance of trade - exports minus imports - measures a country's net trade position.

Development and today's debates

For economic development, the tools from Week 13 return at global scale: growth in poor countries depends on human capital, investment, technology, and above all sound institutions. Debates continue over how much globalization, foreign aid, trade policy, and governance each contribute, and how to share the gains from trade so that the workers displaced by it are not left behind.

Everything you have learned - opportunity cost, supply and demand, elasticity, market failure, GDP, inflation, and policy - is a lens on the issues in today's headlines: inflation and interest-rate decisions, tariffs and trade wars, climate policy and carbon pricing, AI and the future of jobs, and government debt. Open the sidebar Latest news links, pick one current story, and read it as an economist. That habit - turning a headline into a testable question about costs, benefits, and incentives - is the real takeaway of Principles of Economics.