Market Structures, Profit Maximisation and Game Theory

Market Structures, Profit Maximisation and Game Theory

The structure of a market — the number of firms, the degree of product differentiation, and the ease of entry and exit — profoundly affects how firms behave and what outcomes consumers and society receive. This lecture examines the four main market structures, the universal profit maximisation rule, strategic interaction among firms (game theory), and how markets can fail through externalities.

The Profit Maximisation Rule: MR = MC

All profit-maximising firms, regardless of market structure, follow the same rule: produce the level of output at which marginal revenue equals marginal cost (MR = MC).

• Marginal revenue (MR) is the additional revenue from selling one more unit of output
• If MR exceeds MC, producing one more unit adds more to revenue than to cost — profit increases by expanding output
• If MC exceeds MR, producing one more unit costs more than it earns — profit increases by reducing output
• Profit is maximised at the output level where MR = MC

The Four Market Structures

Markets are classified by four key characteristics: number of firms, product differentiation, barriers to entry, and pricing power.

1. Perfect Competition — many firms selling identical (homogeneous) products; no single firm has market power; price equals marginal revenue; entry is free and easy. In the long run, economic profits attract new entrants who expand supply until price falls to minimum average total cost, eliminating economic profit. Outcome: allocative and productive efficiency; P = MC = minimum ATC. Examples: agricultural commodity markets, some financial markets.
2. Monopolistic Competition — many firms selling differentiated products; some pricing power due to product differences; easy entry and exit. In the long run, entry eliminates economic profits as in perfect competition, but firms produce at less than minimum ATC (excess capacity). Examples: restaurants, hairdressers, clothing brands, cafes.
3. Oligopoly — a few large firms dominating the market; significant interdependence (each firm's decisions significantly affect others); high barriers to entry (economies of scale, capital requirements, brand loyalty). Firms in oligopoly may collude (formally or tacitly) or compete fiercely depending on circumstances. Examples: airlines, banking, telecommunications, car manufacturing, supermarkets.
4. Monopoly — a single seller with no close substitutes; significant barriers to entry prevent competition; the monopolist sets price above marginal cost (P greater than MC), producing an output level below what a competitive market would provide; earns economic profits in the long run because barriers prevent new entry. Monopoly is allocatively inefficient — it produces a deadweight loss.

Monopoly vs Perfect Competition

The key difference: in perfect competition, P = MR = MC (no pricing power); in monopoly, P greater than MR = MC (pricing power). The monopolist maximises profit by restricting output below the competitive level and charging a higher price, creating a deadweight loss — a welfare loss to society from the reduction in output and consumption that would have occurred under competition.

Game Theory and Strategic Behaviour

Game theory is the study of how rational decision-makers interact when each player's outcome depends not just on their own choices but on the choices of others. It is particularly relevant for oligopoly markets where firms are strategically interdependent.

Key concepts in game theory:

• Dominant strategy — a strategy that is the best choice for a player regardless of what the other players do. If a dominant strategy exists, a rational player will always choose it.
• Nash equilibrium — a situation in which each player chooses the best strategy given the strategies chosen by all other players; no player has any incentive to change their strategy unilaterally. The Nash equilibrium may not be the outcome that maximises total welfare.

The Prisoner's Dilemma

The classic game in game theory is the Prisoner's Dilemma, which illustrates why individually rational behaviour can produce a collectively worse outcome. In a business context, the equivalent is a pricing or advertising dilemma between oligopolists: if both firms cooperate (maintain high prices or restrain advertising), both benefit; but each firm has a dominant strategy to defect (cut prices, increase advertising), because doing so yields a higher payoff regardless of what the other firm does. The Nash equilibrium is for both firms to defect, producing a worse outcome for both than cooperation would have achieved.

The prisoner's dilemma explains why cartels are unstable: even when firms have agreed to cooperate and fix prices, each has an individual incentive to cheat on the agreement.

Externalities and Market Failure

Markets produce efficient outcomes only when all costs and benefits are internalised — that is, borne by the parties to the transaction. When third parties outside the transaction are affected, a market failure occurs through an externality.

• Negative externality — a cost imposed on third parties not involved in the transaction (e.g. pollution from a factory imposing health and environmental costs on nearby residents). The market overproduces the good because producers do not bear all costs. Government remedy: tax the producer to internalise the external cost (Pigouvian tax), making the producer face the full social cost of production.
• Positive externality — a benefit conferred on third parties (e.g. the benefits to society from someone receiving a university education extend beyond the individual to employers, communities, and public institutions). The market underproduces the good because private demand does not capture all social benefits. Government remedy: subsidise the good to encourage production closer to the socially optimal level.

ESG and Business Strategy

ESG (Environmental, Social, and Governance) frameworks integrate economic, ethical, and societal considerations into business decision-making. ESG recognises that firms operating in markets with negative externalities may face reputational, regulatory, and legal risks if they fail to internalise their external costs. Conversely, firms that create positive externalities through education, community investment, or environmental leadership may build stronger relationships with stakeholders, attract capital from ESG-focused investors, and secure competitive advantage through trust and reputation.