Background and aims: Perfectionism, a focused self-concept, and erroneous beliefs have been implicated in the development and maintenance of various disordered behaviors. However, researchers have yet to examine how these factors combine to explain different disordered behaviors. Herein, we addressed this gap and hypothesized a moderated-mediation model whereby perfectionism fosters the development of disordered behaviors through a focused self-concept. Critically, the effect of a focused self-concept on disordered behaviors is specific to people with erroneous beliefs about their disordered behaviors. The model was tested in the contexts of disordered gambling and disordered eating, particularly dietary restraint. Method: In Study 1, participants were community members who gamble (N 5 259). In Study 2, participants were university women (N 5 219). In both studies, participants completed self-report measures of all constructs that are both reliable and valid. Results: In Study 1, as expected, there was a positive association between perfectionism and disordered gambling, which was mediated by financially focused self-concept. This mediation was only observed among participants who scored high on illusion of control and belief in luck. Likewise, in Study 2, there was a positive association between perfectionism and dietary restraint, which was mediated by appearance focused self-concept. The mediation effect was only observed among participants who believed that maladaptive dietary restraint behaviors were safe and efficacious. Discussion and Conclusions: The findings support the transdiagnostic utility of our model, which may help explain an array of disordered behaviors, including other addictive behaviors as well as behaviors that involve rigid adherence to rules and control.  Kulcsszavak: cognitive distortions, disordered eating, disordered gambling, perfectionism, self-concept