Market Structures

Market Structures

🟢 Lite — Quick Review (1h–1d)

Market Structures describe how industries are organized based on the number of firms, product nature, entry conditions, and price control. Four types dominate: Perfect Competition (price-taker, homogeneous product, free entry/exit), Monopoly (single seller, price-maker, entry barriers), Monopolistic Competition (differentiated products, many sellers, free entry), and Oligopoly (few interdependent firms, high barriers).

Key formulas to memorize: Profit (π) = TR − TC, where TR = P × Q. In perfect competition’s long-run equilibrium: P = MR = AR = MC = AC and abnormal profit = 0. In monopoly: MR = MC but P > MR. The Lerner Index measures monopoly power: (P − MC)/P = 1/|Ed|. The Herfindahl-Hirschman Index (HHI) = Σ(Market Share)² of all firms (in %²).

RPSC RAS tip: questions often ask to compare two market forms, identify features from a given scenario, or calculate profit/HHI. The kinked demand curve (price rigidity in oligopoly) and normal vs. abnormal profit are frequently tested in MCQs and short answers.

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🟡 Standard — Regular Study (2d–2mo)

Perfect Competition

In this model, a large number of buyers and sellers trade a homogeneous product with free entry and exit. No single firm influences market price — all firms are price-takers. The firm’s demand curve is perfectly elastic (horizontal) at the market price. Revenue curves relate as: AR = MR = P (constant). In the short run, a firm can earn abnormal profit when P > AC, or abnormal loss when P < AC. In the long run, with free entry/exit, abnormal profits attract new firms, supply increases, price falls, and equilibrium reaches P = MC = min AC with normal profit only.

Monopoly

A single seller with no close substitutes and significant entry barriers defines monopoly. The monopolist is a price-maker facing the market (downward-sloping) demand curve. Since P must fall to sell more output, MR < AR/P, and the profit-maximizing condition is still MR = MC. The monopolist can earn abnormal profit even in the long run because entry is blocked. Price discrimination (first, second, third degree) allows the monopolist to capture consumer surplus. The Lerner Index measures monopoly power: L = (P − MC)/P = 1/|Ed| — higher L means greater power.

Monopolistic Competition

Many sellers offer differentiated but imperfect substitutes (e.g., soaps, restaurants). Free entry/exit exists, but product differentiation gives each firm a downward-sloping demand curve. Short-run abnormal profit is eliminated in the long run as new entrants drive P = AC at zero economic profit — the defining long-run equilibrium condition: MR = MC and P = AC simultaneously.

Oligopoly

Few firms dominate the market with mutual interdependence — each firm’s actions directly affect rivals. Products may be homogeneous (steel, cement) or differentiated (automobiles, smartphones). High entry barriers persist. The kinked demand curve model explains price rigidity: if a firm raises price, rivals don’t follow (elastic segment above the kink); if it cuts price, rivals match (inelastic segment below). This creates a discontinuity in the MR curve between the kink points, so marginal cost changes within this gap leave the profit-maximizing output unchanged. Concentration Ratio (CRn) measures market power by summing the market shares of the top n firms.

Key Exam Patterns

RPSC RAS frequently asks: (1) distinguishing features of each structure with a numerical or descriptive scenario, (2) equilibrium conditions in the long run for perfect competition vs. monopolistic competition, (3) calculating Lerner Index or HHI from given market share data, and (4) explaining why oligopolistic prices are sticky using the kinked demand curve.

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🔴 Extended — Deep Study (3mo+)

Herfindahl-Hirschman Index (HHI) — Calculation and Interpretation

The HHI sums the squares of all firms’ market shares (expressed as percentages): HHI = Σ(Market Share)². A purely monopolistic market with one firm holding 100% gives HHI = 10,000. Perfect competition with infinite equal firms approaches HHI → 0. The US DOJ uses HHI to classify markets: HHI < 1,500 = unconcentrated (competitive), 1,500–2,500 = moderately concentrated, > 2,500 = highly concentrated (oligopoly/dominance). RPSC RAS may provide market shares of 4–5 firms and ask you to compute HHI and comment on market structure — so practice: if Firm A = 40%, B = 30%, C = 20%, D = 10%, then HHI = 1,600 + 900 + 400 + 100 = 3,000 (highly concentrated oligopoly).

The Kinked Demand Curve — MR Discontinuity Proof

At the current price P₀ and output Q₀, the demand curve has two segments:

• Above P₀: if the firm raises price and rivals don’t follow, it loses many customers → demand is elastic (|Ed| > 1).
• Below P₀: if the firm cuts price, rivals match the cut, so it gains few customers → demand is inelastic (|Ed| < 1).

Since MR = P(1 + 1/Ed), a more elastic segment gives a higher MR, and a less elastic segment gives a lower MR. The jump between these two MR values at Q₀ creates a vertical discontinuity in the MR curve. Any shift in MC that stays within this gap leaves the profit-maximizing Q unchanged — explaining price rigidity without assuming explicit collusion.

Common Mistakes to Avoid

1. Confusing AR with MR: In perfect competition AR = MR = P; in monopoly AR ≠ MR because the demand curve slopes downward.
2. Forgetting that normal profit is included in TC (economic cost includes normal profit as the opportunity cost of the entrepreneur’s resources), so zero economic profit means the firm is still covering all costs including the entrepreneur’s best alternative.
3. Applying the perfect competition long-run equilibrium condition P = MC = AC to monopolistic competition — the correct long-run condition for monopolistic competition is P = AC (not P = MC) because the firm’s demand curve is downward-sloping, so the tangency point between d and AC lies above the MC curve.
4. Misinterpreting HHI: a higher HHI indicates less competition, not more. Always square the percentages before summing.

Practice Prompts

1. Given market shares of five firms: 35%, 25%, 20%, 12%, 8%, calculate the HHI and CR₄. Classify the market structure and explain what policy implication a regulator would draw from your HHI value.

2. A monopolist faces demand P = 100 − 2Q and has TC = 50 + 10Q. Find the profit-maximizing output, price, and profit. Then calculate the Lerner Index. (Hint: MR = 100 − 4Q; set MR = MC → Q = 22.5; P = 55; profit = (55 × 22.5) − (50 + 10 × 22.5) = 1,237.5 − 275 = 962.5; Lerner Index = (55 − 10)/55 ≈ 0.818)

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