Microeconomics: Consumer, Producer and Market Theory
What it is
Microeconomics studies how individual consumers and firms make optimizing choices, and how those choices interact to set prices and quantities. A consumer maximizes satisfaction subject to a budget; a firm maximizes profit subject to its technology and factor prices; market structure then governs how price forms. Game theory extends this to strategic settings where no single agent is a price-taker, and the normative apparatus here — efficiency criteria and the welfare theorems — asks whether an equilibrium is, in a precise technical sense, good.
Core concepts
Theory of Consumer Behaviour. Cardinal utility rests on diminishing marginal utility; equilibrium requires MUx/Px = MUy/Py = ... (the equi-marginal principle). Ordinal theory uses convex, non-intersecting indifference curves with diminishing MRSxy; equilibrium is tangency with the budget line, MRSxy = Px/Py = MUx/MUy. A price fall splits into a substitution effect (always raises quantity of the cheaper good) and an income effect (positive for a normal good, negative for an inferior good); a Giffen good is where the income effect reverses the law of demand.
Theory of Production and Costs. Q = f(L, K). Short run: one factor fixed, so the law of variable proportions applies — product rises at an increasing, then decreasing (diminishing marginal returns), then negative rate as the variable factor grows. Long run: all factors vary; returns to scale asks whether output more than, exactly, or less than keeps pace when every input scales up together. Isoquants are convex with diminishing MRTS, set equal to w/r at the cost-minimizing input mix. AC and AVC are U-shaped, and MC cuts both at their minimum points; LAC envelopes every short-run AC curve.
Decision-Making under Uncertainty and Attitude towards Risk. Expected-utility theory (von Neumann–Morgenstern) ranks a lottery by the probability-weighted average utility of its outcomes, not by expected money. Concave utility of wealth marks risk-aversion (prefers a certain sum to a fair gamble, hence insurance); linear marks risk-neutrality; convex marks risk-loving. The certainty equivalent is the sure sum giving the gamble's expected utility; for a risk-averse agent it lies below the expected value, and the gap is the risk premium.
Game Theory (Non-Cooperative Games). A normal-form game specifies players, strategies, and a payoff per combination. A strategy is dominant if it beats every alternative regardless of rivals' choices. A Nash equilibrium is a combination where no player gains by unilaterally deviating, given the others' choices — not necessarily the jointly best outcome. Dominant strategies always form a Nash equilibrium, though not conversely; see the worked example, the Prisoner's Dilemma.
Market Structures. Perfect competition (many price-takers, free entry) settles at P = MR = MC; long-run entry pushes price to minimum average cost — both allocatively and productively efficient. Monopoly sets MR = MC, but since MR lies below price, output is restricted and price exceeds MC, a deadweight loss. Monopolistic competition adds product differentiation to many sellers; zero long-run profit, but excess capacity — output stops short of minimum AC. Oligopoly (few, interdependent sellers) has no single equilibrium concept; outcomes range from collusion to rivalry, price generally above MC.
Factor Pricing. Marginal productivity theory: a firm hires a factor until its marginal revenue product (MRP = marginal physical product × marginal revenue) equals the factor's price; under perfect competition, MRP = value of marginal product = factor price, the exact point where a profit-maximizing firm stops hiring more of that factor. Wages reward labour, rent is the Ricardian surplus over transfer earnings, interest rewards capital, and profit is the entrepreneur's residual — per Knight, the reward for bearing uninsurable uncertainty.
General Equilibrium Analysis. Partial equilibrium (Marshall) studies one market holding others constant; general equilibrium (Walras) studies every market at once, since a price change ripples through linked markets. The Edgeworth box depicts a two-good exchange between two people; its contract curve, where indifference curves are tangent, traces every Pareto-efficient division of the endowment.
Efficiency Criteria. An allocation is Pareto-optimal if no reallocation can help anyone without hurting someone else. Kaldor-Hicks asks a weaker, compensation-based question: could gainers hypothetically compensate losers and still be better off? Since compensation need not actually be paid, a Kaldor-Hicks improvement need not be a Pareto improvement. Wealth maximization simply ranks allocations by total value generated.
Welfare Economics. The First Fundamental Theorem: a competitive equilibrium is Pareto efficient. The Second: any Pareto-efficient allocation can be reached as a competitive equilibrium via suitable lump-sum redistribution beforehand. Since many such allocations exist, a Social Welfare Function (Bergson–Samuelson) ranks them by distribution, not efficiency alone.
Asymmetric Information. Adverse selection is hidden information before a contract — Akerlof's "market for lemons," where sellers know quality buyers cannot verify, so average pricing drives good-quality sellers out. Moral hazard is hidden action after a contract: an insured party, no longer bearing the full cost of a bad outcome, takes less care than before.
Worked example
Game theory: the Prisoner's Dilemma. Two suspects are questioned separately; each can stay silent (cooperate) or betray. Payoffs are years in prison, so lower is better.
| A \ B | B stays silent | B betrays |
|---|---|---|
| A stays silent | A: 1 yr, B: 1 yr | A: 3 yr, B: 0 yr |
| A betrays | A: 0 yr, B: 3 yr | A: 2 yr, B: 2 yr |
If B stays silent, A gets 1 year silent versus 0 betraying — betray wins. If B betrays, A gets 3 years silent versus 2 betraying — betray wins again. Betray strictly dominates for A, and by symmetry for B, so the unique Nash equilibrium is (betray, betray): 2 years each. Yet (silent, silent), 1 year each, is Pareto-superior — better for both, though neither can safely reach it alone.
Common traps
- Treating the income effect as always reinforcing the law of demand — only the substitution effect is guaranteed to; income effects can push either way, and dominate for a Giffen good.
- Confusing the law of variable proportions (short run, one factor fixed) with returns to scale (long run, all factors scaled together) — tested against each other because they sound alike.
- Assuming a Nash equilibrium is the best joint outcome — it only rules out profitable unilateral deviation; the Prisoner's Dilemma proves it can be worse for everyone than an unreachable cooperative outcome.
- Conflating Pareto-optimality with Kaldor-Hicks — Pareto needs no one worse off; Kaldor-Hicks needs only that gainers could hypothetically compensate losers.
- Swapping adverse selection and moral hazard — the first is hidden information before a contract; the second is hidden action after one.
Speed technique
- One tangency condition, three names: MRS = price ratio = MU ratio at consumer equilibrium — translate a question into whichever form is fastest.
- "Hidden information is before, hidden action is after" resolves adverse selection versus moral hazard in one glance.
- In a small payoff matrix, underline each player's best payoff against each of the other's strategies; a cell underlined for both players is a Nash equilibrium.
- Curvature carries the same meaning as with marginal utility: concave = risk-averse, convex = risk-loving, linear = risk-neutral.
Check yourself
- In the cardinal-utility approach, what condition must hold across two goods X and Y at consumer equilibrium?
Show answer
MUx/Px = MUy/Py — the equi-marginal principle. - A price fall for good X splits into two effects. Which one always moves in the law-of-demand direction?
Show answer
The substitution effect; the income effect can move either way. - If both players in a two-player game have a dominant strategy, is the resulting profile always a Nash equilibrium?
Show answer
Yes — neither player can gain by deviating, so a dominant-strategy profile is automatically a Nash equilibrium. - What is the key difference between the Pareto criterion and the Kaldor-Hicks criterion?
Show answer
Pareto needs no one worse off; Kaldor-Hicks needs only that gainers could hypothetically compensate losers, whether or not they actually do. - A used-car market where sellers know quality but buyers do not, driving good cars out, illustrates which information problem, and how does its timing differ from moral hazard?
Show answer
Adverse selection; it involves hidden information before a contract, while moral hazard involves hidden action after one.
Try it: Microeconomics: Consumer, Producer & Market Theory questions
Real questions from the NET Economics bank on exactly this skill. Pick an answer to see the full solution — the intuition, the worked steps, the faster methods and the traps.
When the price of a good rises from ₹20 to ₹25, the quantity demanded falls from 500 units to 400 units. Using the percentage method with the original price and quantity as the base, the price elasticity of demand (in absolute value) is:
Show the answer and worked solution
Answer: option D
With the original values as base, the change in quantity is = −20% and the change in price is = +25%.
Price elasticity = 20% ÷ 25% = 0.8 in absolute value, so demand is inelastic over this range.
So the elasticity is 0.80, option D.
Consider the following two statements: Statement I: In the Hicksian decomposition of a price change, the compensating change in money income keeps the consumer on the original indifference curve. Statement II: In the Slutsky decomposition, the compensating change in money income keeps the original bundle of goods just affordable at the new prices. In the light of the above statements, choose the correct answer from the options.
Show the answer and worked solution
Answer: option A
Statement I: Hicks adjusts income so that the consumer can just reach his original indifference curve at the new prices, holding real income constant in terms of utility; Statement I is true.
Statement II: Slutsky adjusts income so that the consumer can just buy his original bundle at the new prices, holding purchasing power constant; Statement II is true.
So, on Hicks versus Slutsky compensation, both statements are true, option A.
For the production function Q = 10 , a 10% increase in both labour and capital raises output by approximately:
Show the answer and worked solution
Answer: option B
In a Cobb–Douglas function, the sum of the exponents is the degree of homogeneity: 0.4 + 0.5 = 0.9, which is below 1, so returns to scale are decreasing.
Multiplying both inputs by 1.1 multiplies output by ≈ 1.090, a rise of about 9%.
So output rises by approximately 9%, option B.
Which of the following statements about a Giffen good are correct? I. It must be an inferior good. II. The income effect of a change in its price outweighs the substitution effect. III. Every inferior good is a Giffen good. IV. Its demand curve slopes upward over the relevant range of prices. Choose the correct answer from the options.
Show the answer and worked solution
Answer: option D
A Giffen good is an inferior good whose negative income effect is strong enough to outweigh the substitution effect, so I and II are true.
Because the net result of a price fall is a lower quantity demanded, its demand curve slopes upward, so IV is true; but most inferior goods have weak income effects and still obey the law of demand, so III is false.
So statements I, II and IV only are correct, option D.
Consider the following two statements: Statement I: In Chamberlin's long-run group equilibrium, each firm's demand curve is tangent to its LAC curve on the falling part of the LAC curve. Statement II: Because free entry eliminates supernormal profit, a monopolistically competitive firm produces at the minimum point of its LAC curve in the long run. In the light of the above statements, choose the correct answer from the options.
Show the answer and worked solution
Answer: option C
Statement I: free entry shifts each firm's downward-sloping demand curve until it just touches the LAC curve, and a downward-sloping line can be tangent to a U-shaped curve only on its falling part; Statement I is true.
Statement II: since the tangency lies to the left of minimum LAC, output falls short of the cost-minimising level, which is Chamberlin's excess capacity, so Statement II is false.
So, on Chamberlin's group equilibrium and excess capacity, Statement I is true and Statement II is false, option C.
Answer above — every one shows its working.