Principles of Economics — Complete Course

ECONOMICS is the social science that studies how individuals, firms, governments, and whole societies allocate SCARCE resources among competing uses. Every choice has an OPPORTUNITY COST — the value of the next-best alternative given up. Rational decision-makers think at the MARGIN: they compare the marginal benefit (MB) of a small additional unit of activity to its marginal cost (MC) and continue as long as MB > MC, stopping where MB = MC. The four factors of production — LAND, LABOR, CAPITAL, and ENTREPRENEURSHIP — are combined by producers to make goods and services, and each factor earns a corresponding income (rent, wages, interest, profit). MICROECONOMICS studies individual decision units (households, firms, markets), while MACROECONOMICS studies economy-wide aggregates (GDP, unemployment, inflation, growth).

The PRODUCTION POSSIBILITIES FRONTIER (PPF) is the workhorse model of scarcity and trade-offs. Points on the PPF are PRODUCTIVELY EFFICIENT (no waste); points inside are inefficient; points outside are unattainable with current resources and technology. The PPF is typically bowed outward because resources are not equally suited to all uses — moving more resources from Good A to Good B forces the use of increasingly less-suited resources, producing INCREASING OPPORTUNITY COSTS. Economic growth (better technology, more capital, more or better-trained workers) shifts the PPF outward. Comparative advantage — producing at a LOWER opportunity cost than another party — is why individuals, firms, and nations gain from SPECIALIZATION AND TRADE, even when one party has an absolute advantage in everything.

Economists build MODELS — deliberate simplifications that isolate the forces that matter. Two common assumptions are CETERIS PARIBUS ('other things equal') and rational optimization subject to constraints. POSITIVE economics describes what IS ('a $15 minimum wage will reduce teen employment by X percent'), while NORMATIVE economics prescribes what OUGHT to be ('the minimum wage should be $15'). Confusing the two — or ignoring TRADE-OFFS, INCENTIVES, UNINTENDED CONSEQUENCES, and the difference between CORRELATION and CAUSATION — is the source of most bad economic reasoning. The core PRINCIPLES to memorize: (1) people face trade-offs; (2) cost is what you give up; (3) rational people think at the margin; (4) people respond to incentives; (5) trade can make everyone better off; (6) markets are usually a good way to organize activity; (7) governments can sometimes improve market outcomes; (8) a country's standard of living depends on its productivity; (9) prices rise when the government prints too much money; (10) society faces a short-run trade-off between inflation and unemployment.

The LAW OF DEMAND says that, ceteris paribus, the quantity demanded of a good falls as its price rises — the demand curve slopes downward because of the SUBSTITUTION EFFECT (buyers switch to relatively cheaper alternatives) and the INCOME EFFECT (a higher price reduces real purchasing power). A MOVEMENT ALONG the demand curve is caused by a change in the good's own price; a SHIFT of the entire curve is caused by a change in one of the DEMAND SHIFTERS — income (normal vs inferior goods), the price of related goods (substitutes vs complements), tastes and preferences, expectations of future prices or income, and the number of buyers.

The LAW OF SUPPLY says that, ceteris paribus, the quantity supplied rises as the price rises, so the supply curve slopes upward. Shifters of supply include input prices, technology, taxes and subsidies, expectations, and the number of sellers. MARKET EQUILIBRIUM occurs where the demand and supply curves intersect: the EQUILIBRIUM PRICE is the price at which quantity demanded equals quantity supplied, and any deviation is corrected by market forces — a price above equilibrium creates a SURPLUS that pushes price down, while a price below equilibrium creates a SHORTAGE that pushes price up. COMPARATIVE STATICS asks how the equilibrium changes when demand, supply, or both shift; the three-step method is (1) identify which curve(s) shift, (2) determine the direction, (3) read off the new equilibrium.

Government PRICE CONTROLS interfere with equilibrium. A binding PRICE CEILING (maximum legal price) set below equilibrium — rent control, gasoline price caps — produces a persistent SHORTAGE, queues, black markets, and quality deterioration. A binding PRICE FLOOR (minimum legal price) set above equilibrium — agricultural price supports, minimum wages that bind — produces a persistent SURPLUS (unemployment in the labor market). TAXES drive a wedge between the price buyers pay and the price sellers receive; the TAX INCIDENCE (who bears the burden) does not depend on which side legally pays but on the relative ELASTICITIES of supply and demand — the more inelastic side of the market bears the larger share of the burden. Taxes and price controls also generate DEADWEIGHT LOSS — the loss of mutually beneficial trades that no longer occur.

PRICE ELASTICITY OF DEMAND (Ed) measures the responsiveness of quantity demanded to a change in price: Ed = %ΔQd / %ΔP. Demand is ELASTIC when |Ed| > 1 (quantity is very responsive), UNIT ELASTIC when |Ed| = 1, and INELASTIC when |Ed| < 1. The MIDPOINT (arc) formula uses averages of the two prices and quantities to avoid direction bias. Determinants of elasticity: (1) availability of close substitutes — more substitutes → more elastic; (2) necessities vs luxuries — luxuries more elastic; (3) share of the budget — big-ticket items more elastic; (4) time horizon — the long run is more elastic than the short run; (5) narrowness of market definition — 'red apples' is more elastic than 'food'.

The TOTAL REVENUE (TR = P × Q) test links elasticity to firm revenue: when demand is ELASTIC, a price cut RAISES total revenue and a price hike lowers it; when demand is INELASTIC, a price cut LOWERS total revenue and a price hike raises it; when demand is UNIT ELASTIC, total revenue is unchanged. INCOME ELASTICITY of demand distinguishes NORMAL goods (positive) from INFERIOR goods (negative), and within normal goods NECESSITIES (0 < Ei < 1) from LUXURIES (Ei > 1). CROSS-PRICE ELASTICITY distinguishes SUBSTITUTES (positive) from COMPLEMENTS (negative). PRICE ELASTICITY OF SUPPLY is typically larger in the long run because firms have time to build new capacity.

CONSUMER SURPLUS is the area between the demand curve and the price — the value buyers receive above what they pay. PRODUCER SURPLUS is the area between the price and the supply curve — the payment sellers receive above their minimum acceptable price. TOTAL SURPLUS = CS + PS, and it is MAXIMIZED at the competitive equilibrium — this is the ALLOCATIVE EFFICIENCY property of well-functioning markets and a central result of the First Welfare Theorem. Taxes, subsidies, price controls, tariffs, and monopoly power all reduce total surplus by creating DEADWEIGHT LOSS (DWL) — the triangle of mutually beneficial trades that no longer occur. The size of the DWL from a tax rises with the SQUARE of the tax and with the elasticities of supply and demand, which is why economists prefer taxing goods with inelastic demand (Ramsey rule).

The neoclassical CONSUMER CHOICE model assumes a rational consumer with a UTILITY function ranking bundles of goods, facing a BUDGET CONSTRAINT (I = P₁Q₁ + P₂Q₂). MARGINAL UTILITY (MU) is the extra satisfaction from one more unit and typically DIMINISHES. The UTILITY-MAXIMIZATION RULE requires the consumer to equalize MU per dollar across goods: MU₁/P₁ = MU₂/P₂ = ... = λ. Equivalently, on an indifference-curve diagram the consumer picks the bundle where the highest INDIFFERENCE CURVE (constant-utility contour) is tangent to the budget line — the MARGINAL RATE OF SUBSTITUTION (MRS = MU₁/MU₂) equals the price ratio (P₁/P₂). This tangency is the microeconomic foundation of the downward-sloping demand curve.

A price change produces a SUBSTITUTION EFFECT (movement along the same indifference curve as the relative-price ratio changes) and an INCOME EFFECT (movement to a new indifference curve as real income changes). For a NORMAL good, both effects push quantity demanded in the same direction, reinforcing the law of demand. For an INFERIOR good, the income effect works against the substitution effect; when the income effect dominates entirely — a rare case — we get a GIFFEN good with an upward-sloping demand curve. The consumer surplus interpretation of the tangency condition provides the theoretical link between utility, willingness to pay, and the demand curve used in the previous module.

BEHAVIORAL ECONOMICS documents systematic departures from the rational model, drawing on the work of Kahneman, Tversky, and Thaler. Key findings include: PROSPECT THEORY (people evaluate outcomes as gains or losses from a reference point, with LOSS AVERSION making losses roughly twice as painful as equivalent gains); MENTAL ACCOUNTING (money is treated as non-fungible depending on its source or label); ANCHORING (arbitrary initial numbers bias subsequent judgments); PRESENT BIAS / HYPERBOLIC DISCOUNTING (people undervalue the future in a time-inconsistent way, explaining under-saving and procrastination); the ENDOWMENT EFFECT (people demand more to give up a good than they would pay to acquire it); and HERD BEHAVIOR. NUDGES (Thaler & Sunstein) are choice-architecture interventions — default enrollment in 401(k) plans, calorie labels, opt-out organ donation — that preserve freedom of choice while steering people toward welfare-improving outcomes.

A PRODUCTION FUNCTION Q = f(L, K, ...) maps inputs into output. In the SHORT RUN at least one input (usually K, capital) is fixed; in the LONG RUN all inputs are variable. TOTAL PRODUCT (TP), AVERAGE PRODUCT (AP = TP/L), and MARGINAL PRODUCT (MP = ΔTP/ΔL) describe how output responds to labor. The LAW OF DIMINISHING MARGINAL RETURNS says that, with at least one fixed input, adding more of a variable input eventually causes MP to fall. This mirror-images the shape of the cost curves: rising MP corresponds to falling MC, and falling MP corresponds to rising MC.

COSTS split into FIXED (FC — do not vary with Q) and VARIABLE (VC — vary with Q); TC = FC + VC. The per-unit costs are AFC = FC/Q, AVC = VC/Q, and ATC = TC/Q. MARGINAL COST (MC = ΔTC/ΔQ) is the cost of producing one more unit and cuts BOTH the AVC and ATC curves at THEIR MINIMA. In the LONG RUN the firm chooses the least-cost input mix at any output level, tracing out the LONG-RUN AVERAGE COST (LRAC) curve. LRAC displays ECONOMIES OF SCALE where average cost falls with size (specialization, network effects, spreading fixed costs), CONSTANT RETURNS TO SCALE over a range, and DISECONOMIES OF SCALE where average cost rises (coordination, bureaucracy).

The firm maximizes ECONOMIC PROFIT = Total Revenue − Total ECONOMIC Cost, where economic cost includes IMPLICIT (opportunity) costs — the return the owner's time and capital could earn elsewhere. This differs from ACCOUNTING PROFIT (revenue minus explicit costs only). The universal profit-maximization rule is MR = MC: produce every unit whose marginal revenue exceeds its marginal cost, and stop at the last unit where MR ≥ MC. In the SHORT RUN, keep operating as long as price covers AVC (shut-down price = min AVC); in the LONG RUN, exit if price is below ATC. These rules apply in every market structure — perfect competition, monopoly, monopolistic competition, and oligopoly — with different MR curves reflecting each firm's market power.

PERFECT COMPETITION has many small firms, a homogeneous product, free entry and exit, and perfect information. Firms are PRICE TAKERS — the demand curve facing each firm is a horizontal line at the market price, so P = MR = AR. The short-run equilibrium is where MR = MC (which equals P), and firms may earn positive, zero, or negative economic profit. Free entry and exit drive long-run economic profit to ZERO — the price equals min ATC, and the industry is both ALLOCATIVELY (P = MC) and PRODUCTIVELY (min ATC) efficient. This is the benchmark against which all other structures are compared.

A MONOPOLY is a single seller of a good with no close substitutes and high barriers to entry (patents, natural monopoly from economies of scale, control of a key resource, government franchise). The monopolist faces the entire market demand curve, so MR lies BELOW price at every quantity; profit is maximized where MR = MC, but price is read off the demand curve above that quantity. Monopoly output is LOWER and price HIGHER than the competitive equilibrium, producing DEADWEIGHT LOSS. Remedies include ANTITRUST enforcement (Sherman Act §§1–2, Clayton Act, FTC Act) and regulation. PRICE DISCRIMINATION — charging different prices to different buyers for the same good — can increase output but transfers surplus from consumers to the firm.

MONOPOLISTIC COMPETITION (Chamberlin) has many firms selling DIFFERENTIATED products with free entry — restaurants, clothing, apps. Firms have a slightly downward-sloping demand curve and earn zero long-run economic profit, but produce with EXCESS CAPACITY (P > min ATC). OLIGOPOLY has a few large firms whose decisions are strategically interdependent — analyzed with GAME THEORY. The PRISONER'S DILEMMA shows why competing firms often fail to cooperate on price even though joint profits would be higher. A NASH EQUILIBRIUM is a strategy profile in which no player can improve by deviating unilaterally. Repeated games permit tacit collusion via strategies like TIT FOR TAT. The Herfindahl–Hirschman Index (HHI) is the standard concentration measure used in U.S. merger review (DOJ/FTC 2023 Merger Guidelines).

A MARKET FAILURE occurs when the free-market allocation is not socially efficient. The four canonical sources are: (1) EXTERNALITIES — costs or benefits imposed on third parties; (2) PUBLIC GOODS — non-rival and non-excludable, so private markets undersupply them; (3) COMMON RESOURCES — rival but non-excludable, subject to the TRAGEDY OF THE COMMONS (overuse of fisheries, aquifers, atmosphere); and (4) INFORMATION ASYMMETRY (adverse selection, moral hazard). A NEGATIVE EXTERNALITY (pollution, congestion) means marginal SOCIAL cost > marginal PRIVATE cost, so the market over-produces. A POSITIVE EXTERNALITY (vaccination, education, R&D) means marginal SOCIAL benefit > marginal PRIVATE benefit, so the market under-produces.

PIGOUVIAN policy remedies internalize the externality: a PIGOUVIAN TAX equal to the marginal external cost causes firms to produce the socially efficient quantity; a Pigouvian SUBSIDY does the same for positive externalities. TRADEABLE PERMITS — cap-and-trade systems for SO₂ (1990 CAA Amendments) and CO₂ (EU ETS, RGGI) — set the quantity and let the price emerge, and are cost-effective because permits flow to whoever values them most. The COASE THEOREM (1960) says that with well-defined property rights and zero transaction costs, private bargaining will reach an efficient outcome regardless of the initial assignment — but real transaction costs (many parties, information gaps) usually preclude bargaining, which is why regulation or Pigouvian pricing is used.

PUBLIC GOODS — national defense, basic research, lighthouses, GPS — are non-rival (my use does not diminish yours) and non-excludable (nonpayers cannot be prevented from consuming). Private markets undersupply them because of the FREE-RIDER problem, so government provision (financed by taxes) is standard. COMMON RESOURCES are rival but non-excludable (open-access fisheries, groundwater); solutions include catch quotas, ITQs (individual transferable quotas), community management (Elinor Ostrom's Nobel work), and privatization. Government intervention itself can fail — REGULATORY CAPTURE, rent-seeking, information problems, and unintended consequences — which is why cost–benefit analysis (OMB Circular A-4) and sunset review are standard practice for major rules.

GROSS DOMESTIC PRODUCT (GDP) is the market value of all FINAL goods and services produced within a country's borders in a given period. It can be measured three equivalent ways: the EXPENDITURE approach — GDP = C + I + G + (X − M) (consumption + investment + government purchases + net exports); the INCOME approach — sum of wages, rent, interest, and profit plus depreciation and indirect taxes; and the PRODUCTION (value-added) approach. NOMINAL GDP uses current-year prices; REAL GDP uses base-year prices to strip out inflation. The GDP DEFLATOR = 100 × Nominal / Real. Per-capita real GDP is the standard measure of average living standards; it excludes non-market production, leisure, environmental quality, and the distribution of income.

The CONSUMER PRICE INDEX (CPI) tracks the price of a fixed basket of goods and services bought by a typical urban household (BLS). Inflation is measured as the percentage change in CPI year-over-year. CPI can OVERSTATE true cost-of-living increases because of substitution bias, new-goods bias, and quality-change bias — the CHAINED CPI and the PCE deflator (Fed's preferred measure) partially correct for these. Distinguish DEMAND-PULL inflation (AD > potential output) from COST-PUSH inflation (adverse supply shocks like oil prices). Nominal vs REAL interest rates: FISHER equation r_real ≈ r_nominal − π_expected. Unexpected inflation redistributes wealth from creditors to debtors and from workers with sticky wages to employers.

The UNEMPLOYMENT RATE (BLS Household Survey) = unemployed / labor force, where the labor force = employed + unemployed (those without a job actively looking in the last 4 weeks). The LABOR FORCE PARTICIPATION RATE = labor force / working-age population; U-6 adds marginally attached and part-time-for-economic-reasons. Types: FRICTIONAL (short-term job search), STRUCTURAL (skill or geographic mismatch — includes long-term technological displacement), and CYCLICAL (downturn-driven). The NATURAL RATE of unemployment is frictional + structural; when actual > natural, there is a NEGATIVE OUTPUT GAP. Full-employment output equals POTENTIAL GDP, estimated by the CBO. OKUN'S LAW is the empirical rule that a 1 pp rise in the unemployment rate is associated with roughly a 2 pp fall in real GDP relative to potential.

The AGGREGATE DEMAND (AD) curve slopes downward in price-level / real-output space because of the WEALTH effect, the INTEREST-RATE effect, and the EXCHANGE-RATE effect. AD shifts with C, I, G, or NX changes. The SHORT-RUN AGGREGATE SUPPLY (SRAS) curve slopes upward because wages and other input prices are sticky; the LONG-RUN AGGREGATE SUPPLY (LRAS) is vertical at POTENTIAL OUTPUT (Y*). Long-run equilibrium is where AD, SRAS, and LRAS intersect. A NEGATIVE demand shock (2008–09) reduces both output and the price level; a NEGATIVE supply shock (1973 oil, 2022 energy) causes STAGFLATION — falling output with rising prices. SELF-CORRECTION via wage adjustment eventually returns the economy to Y*, but the process can be slow, motivating stabilization policy.

FISCAL POLICY is government spending (G) and taxation (T) used to influence AD. The SPENDING MULTIPLIER = 1/(1 − MPC), where MPC is the marginal propensity to consume; the TAX MULTIPLIER = −MPC/(1 − MPC). AUTOMATIC STABILIZERS (progressive income taxes, unemployment insurance, SNAP) dampen the cycle without new legislation. Discretionary fiscal policy faces INSIDE LAGS (recognition, decision) and OUTSIDE LAGS (implementation). CROWDING OUT — deficit spending raises interest rates and reduces private investment — is a key concern, though weaker at the zero lower bound. RICARDIAN EQUIVALENCE argues that forward-looking households save to pay future taxes, muting the effect of tax cuts. The FEDERAL DEBT-TO-GDP RATIO is the standard sustainability metric.

MONETARY POLICY is run by the FEDERAL RESERVE (12 regional Banks + Board of Governors + FOMC). Tools include OPEN MARKET OPERATIONS (buying/selling Treasuries), the DISCOUNT RATE, the RESERVE REQUIREMENT (currently 0%), INTEREST ON RESERVES (IORB) — the primary post-2008 tool — and, at the zero lower bound, QUANTITATIVE EASING and FORWARD GUIDANCE. The Fed has a DUAL MANDATE: maximum employment and stable prices (2% PCE inflation target). Expansionary monetary policy lowers rates, raises AD, and boosts output in the short run; the QUANTITY THEORY (MV = PY) says that in the long run money-supply growth translates one-for-one into inflation. The SHORT-RUN PHILLIPS CURVE shows an inflation–unemployment trade-off; the LONG-RUN Phillips curve is vertical at the natural rate, so systematic monetary expansion cannot permanently lower unemployment (Friedman–Phelps).

INTERNATIONAL TRADE is grounded in COMPARATIVE ADVANTAGE (Ricardo, 1817): countries gain by specializing in goods with the lowest opportunity cost and trading for the rest, even when one country has an absolute advantage in everything. Free trade expands consumption possibilities beyond the PPF. The HECKSCHER–OHLIN model adds that countries export goods intensive in their abundant factor. Trade produces winners (exporters, consumers) and losers (import-competing industries), which is why TRADE ADJUSTMENT ASSISTANCE and worker retraining are common complements. PROTECTIONISM — tariffs, quotas, export subsidies — reduces total surplus and produces DWL, though political-economy arguments (national security, infant industries) can support narrow exceptions. Modern trade is governed by the WTO, USMCA, and a web of bilateral FTAs.

The FOREIGN-EXCHANGE (FX) market determines EXCHANGE RATES — the price of one currency in terms of another. Under a FLOATING regime, rates are set by supply and demand for currencies driven by trade flows, interest-rate differentials (uncovered interest parity), inflation differentials (purchasing power parity in the long run), and capital flows. An APPRECIATING currency makes imports cheaper and exports more expensive; a DEPRECIATING currency does the opposite. The BALANCE OF PAYMENTS records all transactions: the CURRENT ACCOUNT (trade in goods, services, income, transfers) plus the CAPITAL / FINANCIAL ACCOUNT sum to zero (net of statistical discrepancy). A current-account deficit is financed by a capital-account surplus — foreigners buying U.S. assets. Fixed and pegged regimes (Bretton Woods, currency boards, the euro) trade monetary-policy independence for exchange-rate stability (the 'trilemma').

LONG-RUN ECONOMIC GROWTH — the sustained rise in real GDP per capita — is the single most important source of higher living standards. The SOLOW GROWTH MODEL (Nobel, 1987) attributes growth to CAPITAL DEEPENING and, in the long run, to TECHNOLOGICAL PROGRESS (total factor productivity, TFP). Ideas-based ENDOGENOUS growth models (Romer, Nobel 2018) emphasize R&D, human capital, and non-rivalry of ideas. Empirical determinants of growth include INSTITUTIONS (secure property rights, rule of law, contract enforcement — Acemoglu, Johnson, Robinson, Nobel 2024), OPENNESS to trade and ideas, INVESTMENT in physical and human capital, macroeconomic stability, and geography. Small differences in growth rates compound dramatically over decades — the RULE OF 70 says a variable growing at g% doubles in about 70/g years.