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Business Economics · Profit maximisation under imperfect competition

Market Structures and Imperfect Competition: Types, Features and Examples

Updated 11 October 2026 · Fact-checked

Market structure describes how many firms sell in a market, how alike their products are, and how easily new firms can enter. Perfect competition is the benchmark. Monopoly, monopolistic competition and oligopoly are imperfect competition, where firms have some market power and can influence price. To classify a market, test number of firms, product type and entry barriers.

Understand Market Structures and Imperfect Competition

A market structure is a way of classifying a market by its competitive features. Three features do most of the work: the number and size of firms, whether products are identical or differentiated, and how easy it is to enter or leave.

Perfect competition is the benchmark. There are many small buyers and sellers, the product is homogeneous, entry and exit are free, and buyers and sellers have full information. Each firm is a price taker. It cannot change the market price. Its demand curve is horizontal at the market price, so price equals marginal revenue.

Imperfect competition is any structure where at least one of these conditions fails, so firms have some market power. Market power is the ability to set price above marginal cost. A firm with market power faces a downward-sloping demand curve. To sell more, it must lower the price, so marginal revenue is below price.

There are three main types. A monopoly has one seller, no close substitutes and high barriers to entry. Monopolistic competition has many firms selling differentiated products with free or easy entry, such as restaurants or clothing brands. An oligopoly has a few large firms, high barriers to entry and interdependence: each firm must think about how rivals will react. Examples include telecom, airlines and cement, though always check how the exam describes the market.

Market power comes from barriers to entry: economies of scale, legal protection such as patents and licences, control of key inputs, strong brands, high start-up costs, and aggressive behaviour by existing firms. Product differentiation also gives power, because buyers see the product as having fewer perfect substitutes. The more power a firm has, the less elastic its demand is.

Key rules to remember

Profit-maximising rule
MR = MC (with MC cutting MR from below)
Applies in every market structure. What changes is how MR relates to price.
Price taker
P = AR = MR
Holds only under perfect competition, where the firm faces a horizontal demand curve.
Firm with market power
MR < P = AR
Holds for a firm facing a downward-sloping demand curve that sells at a single price.
Linear demand and MR
If P = a − bQ, then MR = a − 2bQ
The MR curve has the same intercept and twice the slope. Use it for monopoly-type calculations.
Lerner index of market power
L = (P − MC) ÷ P
Zero for perfect competition. Higher values mean more market power. For a profit-maximiser, L = 1 ÷ |elasticity of demand|.
Supernormal profit
(AR − AC) × Q
Positive when price exceeds average cost. Entry can remove it in the long run if barriers are low.

How to solve Market Structures and Imperfect Competition questions

Use this method for any question that asks you to identify, compare or analyse a market structure.

  1. 1Read the description and note the number of firms, their size and any dominant firm.
  2. 2Decide whether the product is identical, differentiated or unique.
  3. 3Judge the barriers to entry and exit: scale, legal rights, inputs, brands, costs.
  4. 4Match the features to a structure: perfect competition, monopoly, monopolistic competition or oligopoly. If the market sits between two, say so and explain.
  5. 5State the demand curve the firm faces: horizontal for a price taker, downward-sloping otherwise, and how elastic it is.
  6. 6Apply MR = MC to say how price and output are set, and whether long-run supernormal profit is likely given entry.
  7. 7Link your answer to the question: efficiency, consumer welfare, pricing power or strategy. Support each claim with a feature from the description.

Quickest way: Three-question classification

When to use it: Use for multiple-choice questions that ask you to identify a structure or choose a true statement.

  1. Ask: how many sellers? One points to monopoly. A few large ones points to oligopoly. Many points to the next question.
  2. Ask: is the product identical? Yes with free entry means perfect competition. Differentiated with free entry means monopolistic competition.
  3. Ask: do firms react to rivals' decisions? If yes, it is an oligopoly.
  4. Check the options for the rule that P > MR only applies to firms with market power, and eliminate any option that gives a price taker a downward-sloping demand curve.

Common mistakes in Market Structures and Imperfect Competition

  • Saying a monopoly can charge any price it likes.

    Students forget the demand curve limits the firm.

    Fix: A monopolist chooses price or output, not both. Higher price means lower quantity demanded. It sets output where MR = MC and reads price off demand.

  • Treating oligopoly and monopolistic competition as the same because both have differentiated products.

    Both can involve branding.

    Fix: Separate them by number of firms and interdependence. Monopolistic competition has many firms and free entry. Oligopoly has few firms, high barriers and strategic reactions.

  • Writing P = MR for all firms.

    The perfect competition result is memorised without its condition.

    Fix: P = MR only for a price taker. For any firm with a downward-sloping demand curve, MR < P.

  • Assuming firms with market power always earn supernormal profit.

    Market power is confused with guaranteed profit.

    Fix: Profit depends on whether price exceeds average cost. In monopolistic competition, entry removes supernormal profit in the long run.

  • Using the monopoly MR formula with the wrong slope.

    Students halve or copy the demand slope.

    Fix: For P = a − bQ, MR = a − 2bQ. Check by testing a point.

  • Naming a market as a monopoly based on one famous firm.

    Brand size is mistaken for lack of substitutes.

    Fix: Test for close substitutes and barriers. A dominant firm with rivals may be an oligopolist.

Worked examples

Example 1

A market has 8 large firms selling similar but branded products. Entry needs very high capital. Each firm watches rivals' prices before changing its own. Identify the structure, give two sources of market power and state the shape of the firm's demand curve.

Show the solution
  1. Number of firms: a few large firms, so not perfect competition or monopoly.
  2. Product: branded but similar, so differentiated.
  3. Barriers: very high capital requirements, so entry is difficult.
  4. Behaviour: firms react to rivals, which is interdependence, the key feature of oligopoly.
  5. Sources of market power: high capital cost barrier and brand differentiation.
  6. Demand: each firm faces a downward-sloping demand curve, since it has some control over price.

Answer: The market is an oligopoly. Sources of market power are high entry barriers (capital cost) and brand differentiation. Each firm faces a downward-sloping demand curve, and MR is below price.

Example 2

A firm faces demand P = 100 − 2Q and has constant marginal cost ₹20. Find the profit-maximising output, price, and the Lerner index.

Show the solution
  1. Total revenue = P × Q = 100Q − 2Q².
  2. MR = 100 − 4Q.
  3. Set MR = MC: 100 − 4Q = 20.
  4. 4Q = 80, so Q = 20.
  5. Price: P = 100 − 2 × 20 = 60, that is ₹60.
  6. Lerner index L = (P − MC) ÷ P = (60 − 20) ÷ 60 = 40 ÷ 60 = 2/3.
  7. Check: elasticity at this point = (1/2) × (P ÷ Q) = (1/2) × (60 ÷ 20) = 1.5 in absolute value. 1 ÷ 1.5 = 2/3, which matches.

Answer: Output is 20 units, price is ₹60 and the Lerner index is 2/3 (about 0.67), showing substantial market power.

Exam tips

  • Always give the three tests: number of firms, product type and entry barriers. Examiners award marks for linking features to the structure.
  • In written answers, state that MR = MC applies to every structure and that only the shape of the demand curve changes.
  • For MCQs, watch conditions: P = MR is true only for price takers, and MR = a − 2bQ only for linear demand with a single price.
  • Name specific sources of market power rather than writing just 'barriers'. Use scale, legal rights, input control, brands and capital cost.
  • Do not rely on real-world examples alone. Describe the features, because the exam may use a hypothetical market.

Practice questions from Profit maximisation under imperfect competition

Market Structures and Imperfect Competition in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Market Structures and Imperfect Competition: frequently asked questions

What is the difference between perfect and imperfect competition?

Under perfect competition, many firms sell identical products, entry is free and each firm is a price taker. Under imperfect competition, at least one of these fails, so firms have market power and face a downward-sloping demand curve. Price is then above marginal revenue.

What are the types of imperfect competition?

The main types are monopoly, monopolistic competition and oligopoly. They differ in the number of firms, product differentiation and barriers to entry. Monopoly has one seller, monopolistic competition has many differentiated sellers, and oligopoly has a few interdependent sellers.

What gives a firm market power?

Market power comes from barriers to entry and from product differentiation. Common sources are economies of scale, patents and licences, control of key inputs, strong brands and high start-up costs. These reduce the number of close substitutes and make demand less elastic.

Do firms in monopolistic competition earn supernormal profit?

They may in the short run. In the long run, free entry attracts new firms and shifts demand for each existing firm until price equals average cost, so supernormal profit is competed away.