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Conjectural variation

In oligopoly theory, conjectural variation is the belief that one firm has an idea about the way its competitors may react if it varies its output or price.

Version
v1 · 2026-09-28 · History
Domain-specific #
8650
Domain group
Social Sciences
Origin domain
Economics & Finance
Subdomains
Industrial Organization, Oligopoly Theory → Economics & Finance

Core Idea

Conjectural variation is treated here as the recurring industrial organization identity summarized by this source-grounded definition: In oligopoly theory, conjectural variation is the belief that one firm has an idea about the way its competitors may react if it varies its output or price. In oligopoly theory, conjectural variation is the belief that one firm has an idea about the way its competitors may react if it varies its output or price. The firm forms a conjecture about the variation in the other firm's output that will accompany any change in its own output.

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Guessing What the Other Seller Does

Two kids sell lemonade on the same street. One kid thinks, "If I make more lemonade, what will the other kid do? Make more too? Make less? Do nothing?" That guess about how the other seller will react is a Conjectural variation.

Guessing Your Rival's Move

When only a few companies sell the same thing, each one's choices affect the others. Conjectural variation is the belief a company has about how its rivals will react if it changes how much it makes or what it charges. One common assumption is that the rivals won't change at all, so the company treats their amounts as fixed. Another is that the rival will cut back by exactly as much as the company adds. Which guess a company holds changes the choices it makes and the prices that result.

Firm's Rival-Response Belief

In oligopoly theory, which studies markets with only a few sellers, conjectural variation is a firm's belief about how its competitors will change their output or price in response to a change in its own. Formally, it is the change the firm expects in a rival's output to accompany each change in its own output. In the classic Cournot model, each firm treats the other firms' output as fixed when choosing its own, which is a conjectural variation of zero, sometimes called the Nash conjecture because it underlies the standard Nash equilibrium. Other conjectures are possible: a Bertrand conjecture of -1 means the firm expects its rival to reduce output by one unit for each extra unit it produces. With two firms selling the same good, where price depends on their combined output, different conjectures lead to different market outcomes.

 

Conjectural variation, in oligopoly theory, is the belief a firm holds about how competitors will respond if it varies its output or price. In quantity form, the firm conjectures a value for the change in a rival's output that accompanies a change in its own output, and it optimizes taking that expected response into account. The Cournot model assumes each firm treats rivals' output as given, a conjectural variation of zero; this is called the Nash conjecture because it underlies the standard Nash equilibrium concept. In a homogeneous-good duopoly where the market price depends on the two firms' combined output, as in Cournot's original 1838 water-duopoly account, one can instead posit other conjectures, such as the Bertrand conjecture of -1, under which a firm expects each unit of its own additional output to be offset by a one-unit reduction by the rival. The conjecture is thus a parameter of firm beliefs that shapes the equilibrium, not an observed reaction. The concept's identity is specifically this belief about rival reactions in oligopoly, not strategic interaction in general.

Scope of Application

  • Consistent conjectures. For example, in the standard Cournot model, the conjecture is of a zero reaction, yet the actual slope of the Cournot reaction function is negative.

  • Consistent conjectures. What happens if we require the actual slope of the reaction function to be equal to the conjecture?

  • Consistent conjectures. Bresnahan's consistency was a local condition that required the actual slope of the reaction function to be equal to the conjecture at the equilibrium outputs.

  • Consistent conjectures. This first order optimization condition defines the reaction function for the firm, which states, for a given CV, the optimal choice of output given the other firm's output.

  • Consistent conjectures. Note that the Cournot-Nash Conjecture is \phi=0 , in which case we have the standard Cournot Reaction function.

Clarity

A clear use of Conjectural variation names the carrier, the operative relation, and the conditions under which the source treats the identity as present. The minimal definition is In oligopoly theory, conjectural variation is the belief that one firm has an idea about the way its competitors may react if it varies its output or price.

Manages Complexity

Conjectural variation compresses multiple industrial organization details into a stable diagnostic relation. The source shows both the central mechanism—some economists argued that we could pin down the conjectures by a consistency condition, most notably Timothy Bresnahan in 1981.—and the practical consequence—the market price P is given by the linear demand curve. This compression makes cases comparable while leaving parameters, conventions, exceptions, and evidential quality explicit.

Abstract Reasoning

  1. Type the carrier. Identify the industrial organization entities to which the claim applies.
  2. State the relation. Use the source-grounded identity: In oligopoly theory, conjectural variation is the belief that one firm has an idea about the way its competitors may react if it varies its output or price.
  3. Check operation and conditions. To solve for a symmetric equilibrium, where both firms have the same CV, we simply note that the reaction function will pass through the x=y line so that.
  4. Demand recognition evidence.

Knowledge Transfer

Within the home domain. Knowledge about Conjectural variation transfers literally when a new case preserves the same carrier type, relation, and recognition test. For example, in the standard Cournot model, the conjecture is of a zero reaction, yet the actual slope of the Cournot reaction function is negative. What happens if we require the actual slope of the reaction function to be equal to the conjecture? Beyond the home domain. No canonical parent is asserted for Conjectural variation.

Neighborhood in Abstraction Space

Conjectural variation sits in a sparse region of the domain-specific corpus (66th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Classical & Trade Economic Theory (20 abstractions)

Nearest neighbors

Computed from structural-signature embeddings · 2026-10-08