Coordinated effects is a theory of harm within competition economics and law, specifically applied in the analysis of horizontal mergers. It investigates the potential for a merger to increase the likelihood, scope, or sustainability of tacit or explicit coordination among the remaining firms in an oligopolistic market. This theory focuses on how reduced competitive pressure facilitates collective behavior that results in anticompetitive outcomes akin to collusion, representing a distinct concern from unilateral market power effects, and serves as a fundamental analytical framework for competition authorities assessing the potential for mergers to lessen competition through the facilitation of coordinated conduct.