Economics, oriented

Every economic question starts from scarcity: wants outrun the time, labor and materials available, so using a resource one way means not using it another. The opportunity cost of a choice is the value of the single best option you passed up, not the sum of every option, because you could only have taken one. It includes costs with no invoice, such as your own unpaid hours. Spending you cannot recover, a sunk cost, has no place in a forward-looking decision, since it is gone whichever way you choose. Decisions happen at the margin: the question is whether one more unit is worth what it costs, not whether the whole activity pays.

A production possibilities frontier maps the combinations of output an economy can reach with its current resources and technology. Points on it use everything, points inside leave something idle, and points outside are out of reach. Its slope is the opportunity cost of one good in units of the other. It bows outward because resources are not equally suited to both goods, so each extra unit of one good draws on resources that were better at the other.

Specialization rests on comparative advantage: making a good at a lower opportunity cost than your partner. Suppose Ana can bake 10 loaves or knit 5 scarves in a day, and Ben can bake 4 loaves or knit 4 scarves. Ana is faster at both. Yet a scarf costs Ana 2 loaves and Ben only 1, so Ben should knit and Ana should bake. Any exchange rate strictly between 1 and 2 loaves per scarf leaves both better off.

The trap is reading absolute advantage as a reason not to trade. Being faster at everything cannot make you cheaper at everything, because opportunity costs are ratios, and one partner's lower ratio is the other's higher one.

Rule: price every choice in the best alternative given up, ignore what is already spent, and specialize where your opportunity cost is lowest.

A demand curve lists how much buyers want at every price, holding everything else fixed; a supply curve does the same for sellers. Because each curve already answers what happens at a different price, a change in the good's own price moves you along a curve and never shifts it. Everything held fixed is a shifter: income, tastes, the prices of substitutes and complements, expectations, and the number of buyers for demand; input costs, technology, taxes and the number of sellers for supply.

Equilibrium is the price at which quantity demanded equals quantity supplied. Above it, unsold stock pushes the price down; below it, buyers who cannot get the good bid it up. When a drought shrinks a wheat crop, supply shifts left and the price rises, while buyers move up their unchanged demand curve to a smaller quantity.

A price ceiling matters only if it sits below equilibrium, and a floor only if it sits above. Suppose apartments would rent for $1,500 a month. A cap of $1,800 changes nothing. A cap of $1,200 leaves more renters looking than units offered, and since price may not rise, something else does the rationing: waiting lists, payments under the table, or slipping upkeep.

Consumer surplus is what buyers would have paid minus what they paid; producer surplus is what sellers received minus the least they would have accepted. A competitive equilibrium makes their sum as large as possible. Pushing quantity away from it creates deadweight loss: trades that would have helped both sides and now never happen, which is different from money simply moving from one side to the other.

The trap is the circular story: "the price rose, so demand fell, so the price came back down." That treats a price change as a shift. Demand did not fall; quantity demanded did, along the same curve.

Rule: before drawing anything, name which curve the event touches and in which direction, then read price and quantity off the new crossing.

Price elasticity of demand is the percentage change in quantity divided by the percentage change in price, usually reported as a positive number. Because it compares percentages, it has no units, so gasoline can be compared with airline seats. Above 1, demand is elastic and quantity is the more responsive side; below 1, it is inelastic.

Use the midpoint method so the answer does not depend on direction: divide each change by the average of its two values. Suppose a ticket rises from $4 to $6 and sales fall from 100 to 80. Quantity falls 20 on an average of 90, about 22 percent; price rises 2 on an average of 5, or 40 percent. The elasticity is about 0.56, inelastic, and revenue rises from $400 to $480. Had demand been elastic, the same price rise would have cut revenue.

Demand is more elastic when close substitutes exist, when the good takes a large share of the budget, and when buyers have time to adjust. Federal energy analysts describe gasoline as relatively inelastic and vacation air travel as highly elastic, and they note that lasting price changes have bigger effects than brief ones.

For the other elasticities, the sign carries the meaning. Cross-price elasticity is positive for substitutes and negative for complements; income elasticity is positive for normal goods and negative for inferior ones.

A tax drives a wedge between what buyers pay and what sellers keep, and the less elastic side bears more of it because it has fewer ways out. Who legally writes the check does not change that split. A tax collected from fuel producers on their emissions, for instance, would push up the prices of goods made with that fuel.

The trap is reading the slope as the elasticity. A straight demand line has one slope, yet it is elastic near the top and inelastic near the bottom, because the base values change as you move along it.

Rule: compute with percentages from midpoints, and before predicting who pays a tax, ask which side has fewer alternatives.

Economic cost includes explicit costs, the bills you pay, and implicit costs, the return your own time and money could have earned elsewhere. Economic profit subtracts both, so it is smaller than accounting profit. Zero economic profit is a normal return: the owner earns exactly what the resources would earn in their next-best use, so nobody has a reason to enter or leave.

In the short run at least one input is fixed; in the long run every input can change. Fixed costs are owed whether or not you produce, so they never belong in the decision to produce today. Every firm, whatever its market, chooses output where marginal revenue equals marginal cost; short of that point, another unit adds more than it costs.

Consider a food truck selling meals at $8 when its average variable cost is $6 and its average total cost is $9. It loses $1 a meal, yet each meal also covers $2 of rent it owes anyway, so it stays open this month. It shuts down only if price falls below average variable cost, and it leaves the industry only if price stays below average total cost once the lease can be dropped.

Structure decides how far price sits above marginal cost. A perfectly competitive firm is too small to move the price, so marginal revenue equals price, and free entry competes long-run profit away. A monopolist must cut the price on every unit to sell one more, so its marginal revenue falls below price and it sells less, at a higher price, than a competitive market would. Oligopolists watch each other, and a cartel is unstable because each member gains by quietly selling more. In the United States, agreements among competitors to fix prices or restrict output are illegal, while winning a dominant position with a better product is not.

The trap is treating zero economic profit as failure. It means the business pays every input, the owner included, its going rate.

Rule: decide output on marginal revenue and marginal cost, judge staying open against variable cost and staying in against total cost, and leave fixed costs out of today's choice.

Market failure means a condition behind an efficient outcome has broken, not that the market has vanished. Each kind breaks a different condition, so each needs its own remedy.

An externality is a cost or benefit that lands on someone outside the trade. Because nobody pays the external cost, a polluting good is overproduced; because nobody is paid for the external benefit, something like vaccination is underproduced. A corrective (Pigouvian) tax equal to the external cost per unit puts that cost back into the price. A 2018 Congressional Budget Office option for a $25-per-ton tax on greenhouse gas emissions spelled out the effect: firms would make the cuts costing less than $25 a ton and skip the rest. Cap-and-trade flips the choice, fixing the quantity of emissions and leaving the price uncertain.

A public good is both non-rival, so one person's use leaves no less for others, and non-excludable, so non-payers cannot be kept out. Each person does best letting others pay, so voluntary funding falls short: the free-rider problem. A common resource such as a fishery is rival but non-excludable, so it gets overused.

Asymmetric information blocks trades that would help both sides. Adverse selection happens before a deal: if healthier people drop out of an insurance pool, those left cost more to cover, and premiums rise. Moral hazard happens after: being covered changes how carefully people act. Showing that a market failed does not settle the case for stepping in, since government decisions face their own problems of information and incentives.

Taxes need a second distinction. The marginal rate applies to the next dollar; the average rate is total tax over total income. Suppose a schedule taxes the first $20,000 at 10 percent and anything above at 20 percent. A raise from $20,000 to $21,000 lifts the tax from $2,000 to $2,200, so take-home pay still rises by $800.

The trap is believing a raise into a higher bracket can lower your pay. The higher rate applies only to the part of income inside the new bracket.

Rule: name which condition failed before choosing a remedy, and use the marginal rate for decisions and the average rate for burden.

Gross domestic product (GDP) is the market value of the final goods and services produced within a country in a period. "Final" avoids double counting. For example, if a mill sells lumber for $200 to a carpenter who sells the finished table for $500, GDP rises by $500, not $700. "Produced" rules out reselling used goods, since nothing new was made. The Bureau of Economic Analysis (BEA) also leaves out unpaid care of one's own children, volunteer work and illegal activity for lack of data, and states plainly that GDP is not a measure of well-being.

By spending, GDP is consumption plus investment plus government purchases plus exports minus imports. Investment means new structures, equipment, software, homes and inventories, not buying shares. Imports are subtracted because they already sit inside the other spending totals but were made abroad. Adding up incomes reaches the same total in principle, since every dollar spent is someone's income; in practice a statistical discrepancy separates the two estimates.

Nominal GDP uses current prices; real GDP removes inflation so periods can be compared. Two straight quarters of falling GDP is a popular rule of thumb, but BEA notes it is not an official definition; a committee at a private research organization dates recessions using monthly data such as employment alongside quarterly GDP.

The consumer price index (CPI) from the Bureau of Labor Statistics (BLS) tracks the cost of a market basket for an average urban household, so your own inflation can differ. Core measures drop food and energy because those prices are relatively volatile. A fixed basket misses how people switch away from goods that got dearer, which is why the index was long seen as an upper bound on the cost of living; BLS has since changed its formula to allow some of that switching.

The unemployment rate is the unemployed divided by the labor force, and someone who stops searching leaves the labor force entirely. With 200 people in the labor force and 10 unemployed, the rate is 5 percent. If 4 of those 10 stop looking, it falls to 6 of 196, about 3.1 percent, with nobody hired.

The trap is reading a falling unemployment rate as job growth. Check the participation rate alongside it.

Rule: for any indicator, ask what its definition leaves out and what its denominator is.

Money does three jobs: it pays for things, it prices them, and it holds value until later. High inflation shows the jobs can come apart, since money keeps changing hands after it has stopped being a good store of wealth. The Federal Reserve counts money in nested measures. The narrow one is currency held by the public plus checking and other very liquid deposits; the broader one adds small time deposits and retail money market fund shares.

Banks create money when they lend. A loan to you appears as a new deposit in your account, and that deposit is the bank's liability while the loan is its asset. The textbook money-multiplier model caps this process with a reserve requirement, but the Fed cut those ratios to zero effective March 2020, so a required ratio no longer limits lending in the United States.

Congress gave the Fed two goals, maximum employment and stable prices, and the Fed reads stable prices as 2 percent inflation over the longer run, measured by the personal consumption expenditures (PCE) price index. Its main lever is the target for the federal funds rate, the rate banks pay to borrow reserves overnight. It steers that rate through the interest it pays on banks' reserve balances, because a bank is unlikely to lend for less than it can earn by leaving the money at the Fed. Open market operations, buying and selling securities, adjust the supply of reserves, and the discount window lends to banks that need cash quickly.

Bond prices and yields move in opposite directions. For example, a bond paying $50 a year yields 5 percent at a price of $1,000; if its price falls to $800, the same $50 is a 6.25 percent yield.

The trap is thinking the Fed sets the rate on your mortgage. Longer-term rates follow what lenders expect short-term rates to do over the life of the loan, which is why Fed statements about the future can move them before any decision.

Rule: trace a policy change from the overnight rate through expectations to borrowing costs, asset prices and the dollar, and expect each step to take time.

Aggregate demand is total planned spending at each price level. Short-run aggregate supply slopes upward because some wages and prices are slow to adjust, while long-run supply is vertical at potential output, which the Congressional Budget Office (CBO) describes as what the economy can produce with labor and capital employed at their maximum sustainable rates. When demand falls short, output drops below potential and unemployment rises; when it overshoots, prices climb. Wages and prices eventually pull output back; how fast is disputed, and the slower you think it is, the stronger the case for policy. An adverse supply shock is the hard case: prices rise while output falls, so demand policy aimed at one symptom worsens the other. The short-run trade-off between inflation and unemployment also fades once people come to expect the inflation.

Fiscal policy works through spending and taxes. Each dollar spent becomes someone's income, part of which is spent again. If people spend 80 cents of each extra dollar, the simple multiplier is 1 / (1 - 0.8) = 5, an upper bound, since taxes and imports also drain each round. A tax cut starts smaller, since households save part of it before anything is spent. Automatic stabilizers act without new laws: in a downturn, income tax payments fall and unemployment insurance spending rises, which supports private spending. Heavy borrowing has a cost too; CBO's assessment is that federal borrowing draws on savings that would otherwise fund private investment, an effect that is small when resources sit idle and larger near full employment.

A deficit is one year's gap between spending and revenue; the debt is the accumulated borrowing. Comparing the debt with GDP shows the economy's capacity to carry it.

In the open economy, a stronger dollar makes American exports dearer abroad and imports cheaper at home. For example, at 100 yen per dollar a $50 item costs a Japanese buyer 5,000 yen; at 120 yen, it costs 6,000. A current-account deficit has to be financed by borrowing from abroad or selling assets.

The trap is reading a shrinking deficit as a shrinking debt. Deficits of $3 trillion, then $2 trillion, then $1.5 trillion still add $6.5 trillion to the debt; only a surplus reduces it.

Rule: separate demand shocks from supply shocks, flows from stocks, and short-run trade-offs from long-run limits.