Market structures

1. Core Concept Overview & Intuition

Market structure refers to the organisational and competitive characteristics of a market that determine how firms behave, how prices are set, and how resources are allocated. The four canonical structures — perfect competition, monopoly, monopolistic competition, and oligopoly — are distinguished by four structural criteria: the number of sellers, the degree of product differentiation, the conditions of entry and exit, and the extent of control over price. These criteria jointly determine the shape of the demand curve facing an individual firm, which in turn governs the firm's profit-maximising output and price.

The central analytical thread running through all four structures is the profit-maximisation condition. Every firm, regardless of market structure, maximises profit by producing the output level at which marginal revenue equals marginal cost. What differs across structures is the relationship between price and marginal revenue. Under perfect competition, the firm is a price taker, so price equals marginal revenue. Under monopoly, monopolistic competition, and oligopoly, the firm faces a downward-sloping demand curve, so price exceeds marginal revenue. This single distinction generates the entire taxonomy of market outcomes.

The intuition behind the spectrum from perfect competition to monopoly is straightforward. As the number of sellers falls and products become more differentiated, each firm gains increasing control over its price. Greater price control implies a larger gap between price and marginal cost, which in turn implies a larger deadweight loss relative to the perfectly competitive benchmark. The perfectly competitive market is therefore the efficiency benchmark against which all other structures are evaluated.

A critical temporal distinction applies to all structures: short-run versus long-run equilibrium. In the short run, fixed factors of production prevent entry or exit, so firms can earn supernormal profits or incur losses. In the long run, entry and exit drive profits toward zero in competitive and monopolistically competitive markets, but barriers to entry allow monopolists and colluding oligopolists to sustain supernormal profits indefinitely.

2. Mathematical Formalization & Core Definitions

> Definition (Market Structure). A market structure is the set of characteristics — number of buyers and sellers, nature of the product, conditions of entry and exit, and degree of price control — that determines the competitive environment in which firms operate.

> Definition (Perfect/Pure Competition). A market structure characterised by a large number of small buyers and sellers, a homogeneous product, free entry and exit, perfect information, and perfect factor mobility. Each firm is a price taker and faces a perfectly elastic demand curve.

> Definition (Monopoly). A market structure in which a single seller supplies a product for which there are no close substitutes, and barriers to entry prevent the entry of potential competitors.

> Definition (Monopolistic Competition). A market structure with many sellers, differentiated but highly substitutable products, free entry and exit, and profit maximisation as the firm's goal in both the short run and the long run.

> Definition (Oligopoly). A market structure with few sellers, where products may be identical (pure or perfect oligopoly) or differentiated (differentiated oligopoly), and where each firm's decisions are interdependent with those of its rivals.

The profit-maximisation condition common to all structures is:

$$
\max_{Q} \pi(Q) = TR(Q) - TC(Q)
$$

where $\pi$ denotes profit, $TR$ denotes total revenue, $TC$ denotes total cost, and $Q$ denotes output. The first-order condition yields:

$$
\frac{d\pi}{dQ} = MR - MC = 0 \implies MR = MC
$$

where $MR$ denotes marginal revenue and $MC$ denotes marginal cost. The second-order condition requires that $MC$ cuts $MR$ from below.

Under perfect competition, the firm is a price taker, so:

$$
P = MR = AR
$$

where $P$ denotes price and $AR$ denotes average revenue. The firm's demand curve is horizontal at the market-determined price.

Under monopoly, monopolistic competition, and oligopoly, the firm faces a downward-sloping demand curve, so:

P > MR

and the firm has market power, measured by the Lerner index:

$$
L = \frac{P - MC}{P}
$$

where $L$ denotes the Lerner index of market power. Under perfect competition, $P = MC$, so $L = 0$. Under monopoly, $L > 0$.

The relationship between marginal revenue and the price elasticity of demand $\varepsilon_d$ is:

$$
MR = P\left(1 - \frac{1}{\lvert \varepsilon_d \rvert}\right)
$$

where $\lvert \varepsilon_d \rvert$ denotes the absolute value of the price elasticity of demand. This relationship holds for any firm facing a downward-sloping demand curve.