A duopoly forces each producer to carefully consider its rival's potential reactions to certain business decisions. When members of a duopoly compete on price, they tend to drive the product's price down to the cost of production, thereby lowering profits for both members of the duopoly.
These circumstances give duopolists a strong incentive to agree to charge a monopoly price and share the resulting profits. However, federal antitrust laws, most notably the Sherman Act, make collusive activity illegal in the United States. Additionally, each member of a duopoly still has an incentive to compete, even while colluding with the competition. An undetected price adjustment will attract customers who are buying from the competition and customers who are not buying the product at all. Price adjustments may be subtle, including better credit terms, faster delivery, or related free services.
Duopolies are most effective when the demand for the duopoly's product is not greatly affected by price. This is also why duopolies are more effective in the short term; over the long term, prices often become more elastic as consumers find substitutes for the product. Also, demand volatility may lead to disagreements within a collusive duopoly regarding outputs and prices.
