Collective decision-making processes play a crucial role in shaping social, political, and organizational outcomes. While existing theories, such as the Social Norms Theory, Diffusion of Innovations Theory, and Bandwagon Effect, have provided valuable insights into these processes, they often fail to capture the dynamic and complex nature of collective decision- making. This paper introduces the Trend Effect Theory, a novel theoretical framework that aims to address the limitations of existing theories and provide a more comprehensive understanding of how trends influence group behaviors. The Trend Effect Theory is grounded in the notion of binary classification, categorizing actions into two types: do/don't do and support/oppose. It emphasizes the universal applicability of trends across various group sizes and highlights the dynamic nature of trend formation and evolution. The theory distinguishes between open and closed trends, explaining how external influences can alter the size and direction of a trend. It also identifies the causes of trend changes, such as internal reversal, external dilution, concurrent internal and external influences, and decrease in participation. The Trend Effect Theory focuses on aggregate outcomes rather than individual actions and acknowledges the context-dependency of trend emergence and evolution. By introducing a 70% threshold for determining trend establishment, the theory offers a clear criterion for understanding collective decision-making processes. This paper discusses the advantages of the Trend Effect Theory over existing theories, its scope and limitations, and the roles and dynamics of participants, non-participants, andTREND EFFECT 1 initiators in trend formation. The potential applications of the theory in various fields, such as marketing, political science, and organizational behavior, are also explored. The Trend Effect Theory provides a valuable framework for researchers, policymakers, and practitioners seeking to understand and influence collective decision-making processes in an increasingly complex and dynamic world.Keywords: Trend Effect Theory, collective decision-making, binary classification, trend dynamics, open and closed trends, initiators, followers, context-dependency