Beliefs in conspiracy theories –referring to hidden groups that cooperate to pursue malevolent goals– have a significant impact on decision-making, individual and collective behaviours and can lead to political instability. Due to these potential impacts, it presents a primary challenge for policymakers and a compelling interest for psychological sciences. However, despite various predictors being identified in the literature, only few studies delve into the underlying cognitive mechanisms. This project proposes cheating detection as a fundamental cognitive process underlying beliefs in conspiracy theories. Following the theoretical proposal by Aubert-Teillaud and Vaidis (2023), we argue that beliefs in conspiracy theories can be modelled using signal detection theory and error management theory. For this purpose, a specific paradigm has been developed prior to the project and allows measurement of the existing bias using the decision criterion (i.e., individuals' tendency to prefer false positives or false negatives) and sensitivity (i.e., the ability to effectively perform the task). The procedure of the cheating detection task and the preliminary studies are described in the project. To test the theoretical hypotheses that the cheating detection bias is responsible for belief in conspiracy theories, we planned 9 studies. Several studies are built on mediation analysis with identified factors as predictors, conspiracy theory belief as the dependent variable, and the cheating detection bias as the mediator. In Axis 1 (WP1), we will examine socio-political factors. In Task 1, we will manipulate group belonging (Study 1), group competition (Study 2), and entitativity (Study 3). In Task 2, we will manipulate the informational status about corruption (Study 4). Finally, because mediation analyses present limits concerning the causal link, Task 3 will manipulate the cheating detection bias (Studies 5 & 6). In Axis 2 (WP2), we will examine personological factors. In Task 4, we will scrutinise the link between paranoid ideation (Studies 7 & 8) and conspiracy theories via cheating detection bias, while in the last study, we will focus on hypersensitivity to agency (Study 9). The project adopts an open and solid science approach, incorporating systematic power analysis and utilising the registered reports format. These measures strengthen the validity of the methods and results. From a cross-disciplinary perspective, the project is made possible through an integrative approach incorporating theories and measures from different subfields of psychological sciences. To ensure comprehensive expertise, we have brought together a consortium of researchers specialising in various methodological areas, such as social psychology (e.g., intergroup relations) and cognitive psychology (e.g., signal detection). Additionally, we have enlisted recognized experts in the field of conspiracy theories. Finally, we anticipate the compilation of a policy brief for the general public and policymakers offering practical recommendations to address the challenges posed by conspiracy theories. Altogether, the realization of the project would permit both a scientific and societal breakthrough in the study of conspiracy theory beliefs.