25 Price Determination Under Different Market Forms

25.1 What is a Market?

A market in economics is not a physical place but the whole system of buyers, sellers and the mechanism by which they exchange a good. Cournot’s (1838) classic statement: “By the term market, then, economists understand not any particular market place in which things are bought and sold, but the whole region in which buyers and sellers are in such free intercourse with one another that the prices of the same goods tend to equality, easily and quickly” (cournot1838?).

The structure of the market determines how much power a single seller has over price. Four classical market forms anchor the discussion: perfect competition, monopolistic competition, oligopoly, and monopoly.

Four Classical Market Forms — Compared at a Glance

Feature	Perfect Competition	Monopolistic Competition	Oligopoly	Monopoly
Number of sellers	Very large	Large	Few	One
Product	Homogeneous	Differentiated	Homogeneous or differentiated	Unique
Entry / exit	Free	Free	Restricted	Blocked
Knowledge	Perfect	Imperfect	Imperfect	Asymmetric
Price	Given (price-taker)	Some control	Mutual interdependence	Price-maker
Demand curve facing firm	Horizontal	Steep, downward	Kinked / strategic	Market demand
Long-run profit	Normal only	Normal only	Possibly super-normal	Super-normal
Examples	Agricultural staples (idealised)	Restaurants, branded clothing	Cement, telecom, automobiles	Indian Railways (passenger), historic IRCTC ticketing

25.2 Equilibrium Rule Common to All Forms

The first-order condition for profit maximisation is the same across every market form:

\[ \frac{d\pi}{dQ} = MR - MC = 0 \quad \Rightarrow \quad MR = MC \]

The second-order condition requires MC to be rising and to cut MR from below. What differs across market forms is the shape of the demand and MR curves the firm faces.

25.3 Perfect Competition

25.3.1 Features

Six Features of Perfect Competition

Feature	Working content
Very large number of buyers and sellers	Each is small relative to the market
Homogeneous product	Buyers are indifferent to source
Free entry and exit	No barriers
Perfect knowledge	Everyone knows prices and costs
Perfect mobility of factors	Resources flow to highest-valued use
No transport cost	One uniform price

25.3.2 Demand and revenue curves

Each firm is a price-taker: it sells any quantity it likes at the prevailing market price. The firm’s demand curve is therefore horizontal at the market price. For the firm:

\[ P = AR = MR \]

The market demand curve, by contrast, slopes downward in the usual way.

25.3.3 Short-run equilibrium

The firm chooses output where $MC = MR = P$. At that output it may earn:

Super-normal profit if AR > AC,
Normal profit if AR = AC,
Loss (operates if AR ≥ AVC; shuts down if AR < AVC — the shut-down rule).

25.3.4 Long-run equilibrium

In the long run, free entry erodes super-normal profit; firms exit if there is loss. Equilibrium settles at:

\[ P = AR = MR = MC = SAC = LAC \text{ at minimum} \]

Each firm earns only normal profit. Production happens at the minimum point of the LAC — the firm uses an optimum-sized plant at its optimum capacity.

25.3.5 Supply curve

The firm’s short-run supply curve is its MC curve above the minimum AVC. Below that point, the firm shuts down rather than produce.

25.4 Monopoly

25.4.1 Features

A monopoly is a market with a single seller and no close substitutes. Causes include statutory grants (patents, licences, exclusive franchise), control of an essential input, natural monopoly (one firm can supply at lower cost than several), strategic deterrence, and economies of scale.

25.4.2 Demand and revenue

The monopolist faces the market demand curve, which slopes downward. Therefore $AR > MR$ for all positive output, and:

\[ MR = AR \left(1 - \frac{1}{|E_p|}\right) \]

The MR curve lies below the AR (= demand) curve and (for a linear demand) has twice the slope of AR.

25.4.3 Equilibrium

At $MR = MC$, the monopolist sets $Q^*$ and reads off $P^*$ from the demand curve. Since $P > MR = MC$, the monopolist charges a price above marginal cost — the source of monopoly’s allocative inefficiency.

flowchart LR
  EQ[MR = MC] --> Q[Profit-maximising<br/>output Q*]
  Q --> P[Read price P*<br/>from AR curve]
  P --> Pi[π = AR.Q − AC.Q]
  style EQ fill:#FFEBEE,stroke:#C62828
  style Pi fill:#E8F5E9,stroke:#2E7D32

25.4.4 Long-run

Because entry is blocked, the monopolist can earn super-normal profit even in the long run. Long-run equilibrium needs only $LMR = LMC$.

25.4.5 Price discrimination

When a monopolist charges different prices to different buyers of the same good, it is engaging in price discrimination. Three classical degrees were distinguished by Arthur C. Pigou (pigou1920?):

Pigou’s Three Degrees of Price Discrimination

Degree	Meaning	Example
First-degree (perfect)	Each buyer charged the maximum she is willing to pay	Customised B2B contracts; haggling
Second-degree	Different prices for different quantities	Bulk discount; tiered electricity tariffs
Third-degree	Different prices to different market segments	Student vs adult cinema tickets; weekday vs weekend

Conditions for price discrimination: (i) monopoly power; (ii) separable markets — buyers in the cheap market cannot resell in the dear market; (iii) different elasticities across markets — the dearer market must be the more inelastic one.

The third-degree result: $\frac{P_1}{P_2} = \frac{1 - 1/|E_2|}{1 - 1/|E_1|}$. In the more inelastic market the price is higher.

25.5 Monopolistic Competition — Chamberlin (1933)

Edward Chamberlin’s Theory of Monopolistic Competition (1933) and Joan Robinson’s Economics of Imperfect Competition (1933) — published in the same year — recognised that most real markets sit between perfect competition and monopoly (chamberlin1933?; robinson1933?).

25.5.1 Features

Five Features of Monopolistic Competition

Feature	Working content
Large number of sellers	Each firm small relative to the market
Product differentiation	Real or perceived; brands, packaging, design, location
Free entry and exit	Long-run drives profits to normal
Selling costs	Advertising and promotion are central
Imperfect knowledge	Consumers face search cost

25.5.2 Demand curve

Each firm faces a highly elastic but downward-sloping demand curve, reflecting differentiation. Chamberlin distinguished two demand curves: a firm’s own demand (steep) and a share-of-the-market demand (flatter).

25.5.3 Equilibrium

Short run: $MR = MC$ generates profit — possibly super-normal, possibly loss.

Long run: free entry drives profits to normal. The firm’s tangency equilibrium is:

\[ P = AR = AC > MC = MR \]

The tangency happens on the falling part of the AC curve — so production occurs at less than the optimum scale. This generates excess capacity — the famous “wastes” of monopolistic competition.

25.5.4 Wastes of monopolistic competition

Excess capacity (firms produce below LAC’s minimum).
Resources spent on advertising for differentiation rather than productive efficiency.
Multiplicity of brands with marginal real differences.

25.6 Oligopoly

25.6.1 Features

Six Features of Oligopoly

Feature	Working content
Few sellers	Two (duopoly) to a handful
Product can be homogeneous (cement, steel) or differentiated (cars, smartphones)
Mutual interdependence	Each firm’s actions affect rivals’ decisions
Barriers to entry	Capital, technology, brand, regulation
Price rigidity	Prices are sticky relative to marginal cost
Non-price competition	Advertising, R&D, customer service

25.6.2 Cournot model — 1838

Augustin Cournot’s pioneering duopoly model assumes each firm chooses its quantity, taking the rival’s quantity as given. The reaction functions of the two firms intersect at the Cournot-Nash equilibrium. For a linear demand and zero marginal cost, each firm produces one-third of the competitive output, and combined output is two-thirds — between monopoly (one-half) and perfect competition (the whole) (cournot1838?).

25.6.3 Bertrand model — 1883

Joseph Bertrand’s critique: in reality firms set price, not quantity. Two firms with identical, constant marginal cost selling a homogeneous product undercut each other until $P = MC$ — the Bertrand paradox: just two firms suffice to reach the perfectly competitive outcome (bertrand1883?).

25.6.4 Stackelberg model — 1934

Heinrich von Stackelberg modified Cournot by allowing one firm — the leader — to choose first. The other — the follower — observes the leader’s quantity and best-responds. The leader produces more than its Cournot share and earns more profit; the first-mover advantage is the central result.

25.6.5 Kinked demand curve — Sweezy (1939)

Paul Sweezy’s kinked demand curve explains price rigidity under oligopoly. A firm believes that:

If it raises price, rivals will not follow — its demand falls sharply. The demand curve is elastic above the prevailing price.
If it cuts price, rivals will follow — its demand rises only modestly. The demand curve is inelastic below the prevailing price.

The demand curve has a kink at the prevailing price; the corresponding MR curve has a vertical discontinuity. Even when MC shifts within the discontinuity, the profit-maximising price does not change. Hence the price stickiness oligopoly is famous for (sweezy1939?).

25.6.6 Collusive oligopoly — cartel and price leadership

When mutual interdependence is recognised and exploited through agreement, the firms form a cartel. The joint-profit-maximising solution is identical to a multi-plant monopoly. Cartels are typically illegal under competition law (in India, under the Competition Act 2002). Tacit collusion and price leadership — dominant-firm leadership and barometric leadership — are alternative ways to coordinate without formal agreement.

25.6.7 Game theory note

Modern oligopoly theory is game-theoretic. A Nash equilibrium is a profile of strategies in which no firm can gain by unilaterally changing its own strategy. The Cournot equilibrium is a Nash equilibrium in quantities; the Bertrand equilibrium is a Nash equilibrium in prices. The classic Prisoner’s Dilemma explains why collusion is unstable: each firm has a private incentive to defect from the cartel.

25.7 Duopoly — A Note

A duopoly is the simplest oligopoly — two firms. The Cournot, Bertrand and Stackelberg models all originated as duopoly models and were later generalised to many firms.

25.8 Big Comparison — All Four Forms

Equilibrium and Welfare Across Market Forms

Dimension	Perfect Competition	Monopoly	Monopolistic Competition	Oligopoly
Equilibrium rule	MR = MC; P = MC	MR = MC; P > MC	MR = MC; P > MC; tangency at falling AC	MR = MC; outcome strategic
Long-run profit	Normal	Super-normal possible	Normal (entry)	Super-normal possible
Output	At minimum LAC	Less than competitive output	Less than minimum LAC (excess capacity)	Less than competitive output
Price	= MC	> MC	> MC	> MC
Allocative efficiency	Yes	No	No	No
Productive efficiency (output at min LAC)	Yes	No	No	No
Selling cost	Zero	Low	High	Often very high

25.9 Exam-Pattern MCQs

Q 01

Which of the following is not a feature of perfect competition?

AHomogeneous product
BFree entry and exit
CPerfect knowledge
DFew sellers with mutual interdependence

View solution

Correct Option: D

Mutual interdependence with few sellers describes oligopoly; perfect competition has very many sellers, none of whom is large enough to influence the others.

Q 02

Match each market form with its leading 1933 / earlier theorist:

	Market form		Theorist
(i)	Monopolistic competition	(a)	Cournot
(ii)	Imperfect competition (1933)	(b)	Bertrand
(iii)	Cournot duopoly	(c)	Edward Chamberlin
(iv)	Bertrand price-setting duopoly	(d)	Joan Robinson

A(i)-(c), (ii)-(d), (iii)-(a), (iv)-(b)
B(i)-(a), (ii)-(b), (iii)-(c), (iv)-(d)
C(i)-(b), (ii)-(c), (iii)-(d), (iv)-(a)
D(i)-(d), (ii)-(a), (iii)-(b), (iv)-(c)

View solution

Correct Option: A

Q 03

Under monopoly, the relation between price and marginal revenue is:

AP = MR
BP > MR
CP < MR
DP + MR = 0

View solution

Correct Option: B

With a downward-sloping demand curve, $MR < AR = P$ for all positive output.

Q 04

Match each Pigou degree of price discrimination with the corresponding example:

	Degree		Example
(i)	First-degree	(a)	Student vs adult cinema tickets
(ii)	Second-degree	(b)	Customised B2B contract pricing
(iii)	Third-degree	(c)	Bulk discount on volume purchases

A(i)-(b), (ii)-(c), (iii)-(a)
B(i)-(a), (ii)-(b), (iii)-(c)
C(i)-(c), (ii)-(a), (iii)-(b)
D(i)-(b), (ii)-(a), (iii)-(c)

View solution

Correct Option: A

Q 05

The "kinked demand curve" model of price rigidity in oligopoly is associated with:

AAugustin Cournot
BJoseph Bertrand
CPaul Sweezy
DEdward Chamberlin

View solution

Correct Option: C

Sweezy's 1939 paper proposed the kinked demand curve to explain price rigidity in oligopoly.

Q 06

Long-run equilibrium under monopolistic competition involves:

AP = MC = AC at minimum
BP > MC and P = AC, with AC on its falling segment (excess capacity)
CP < MC, with super-normal profit
DP = MR = MC, with normal profit at minimum AC

View solution

Correct Option: B

Tangency at the falling portion of AC produces normal profit but excess capacity — Chamberlin's classic result.

Q 07

Arrange the following market models in order from least to most number of sellers: (i) Monopolistic competition (ii) Monopoly (iii) Oligopoly (iv) Perfect competition

A(ii), (iii), (i), (iv)
B(iv), (i), (iii), (ii)
C(i), (ii), (iii), (iv)
D(iii), (iv), (i), (ii)

View solution

Correct Option: A

Monopoly (one) → Oligopoly (few) → Monopolistic Competition (many, differentiated) → Perfect Competition (very many, homogeneous).

Q 08

Match each oligopoly model with its central feature:

	Model		Feature
(i)	Cournot	(a)	First-mover advantage; leader chooses quantity first
(ii)	Bertrand	(b)	Kinked demand curve and price rigidity
(iii)	Stackelberg	(c)	Each firm chooses quantity, taking rival's quantity as given
(iv)	Sweezy	(d)	Two firms setting price reach P = MC

A(i)-(c), (ii)-(d), (iii)-(a), (iv)-(b)
B(i)-(a), (ii)-(b), (iii)-(c), (iv)-(d)
C(i)-(b), (ii)-(c), (iii)-(d), (iv)-(a)
D(i)-(d), (ii)-(a), (iii)-(b), (iv)-(c)

View solution

Correct Option: A

Quick recall

Profit-max condition is MR = MC in every market form; what differs is the shape of the demand and MR curves.
Perfect competition: many sellers, homogeneous product, free entry, $P = AR = MR$, long-run $P = MC = LAC$ at minimum, only normal profit.
Monopoly: one seller, blocked entry, $MR < AR$, $P > MC$, super-normal profit possible long-run.
Pigou’s three degrees of price discrimination: First (perfect), Second (by quantity), Third (by segment).
Chamberlin (1933) monopolistic competition: many sellers, differentiation, long-run normal profit but excess capacity (tangency on falling AC).
Joan Robinson (1933) independently developed imperfect competition.
Oligopoly is mutually interdependent. Models: Cournot (1838) — quantity competition; Bertrand (1883) — price competition (P = MC); Stackelberg (1934) — leader-follower; Sweezy (1939) — kinked demand curve, price rigidity.
Cartel = joint-profit-maximising agreement, illegal under Competition Act 2002 in India.
Game theory: Nash equilibrium generalises Cournot’s reaction-function fixed point.
Compared welfare: only perfect competition is both allocatively (P = MC) and productively (output at min LAC) efficient.