Social functioning, fundamental to adolescent's development and mental health, may be impaired by polarizing problematic social functioning, namely externalizing symptoms manifested by Oppositional Defiant Disorder and internalizing symptoms portrayed by Social Anxiety Disorder. Despite their high prevalence and similar associated impairments, interventions targeting these disorders are differently conceived. Alternatively, Acceptance and Commitment Therapy (ACT) proposes that those apparently dissimilar social difficulties are rooted in similar processes. Though research has shown ACTs' efficacy in changing adults' internalizing and externalizing symptoms, studies on the potential of ACT in changing those problematics in adolescence are still scarce. This project proposes to conduct three clinical trials to test the efficacy and (dis)similarities of an transdiagnostic ACT intervention for changing internalizing and externalizing symptomatology in adolescents. It will amplify the transdiagnostic and evidence-based application of ACT to adolescents presenting polarizing disorders in the social functioning spectrum.

Adolescence represents vulnerability to mental health issues, with approximately 1 in 7 adolescents presenting mental disorders that, when untreated, lead to significant impairments and less long-term quality of life. Given that adolescents are hypersensitive to social stimuli and social exclusion, and place particular value in peer approval/influence, peer rejection in adolescence seems to increase the risk for developing mental disorders. Conversely, positive peer relationships may protect against mental health problems and promote resilience. This buffering effect may be absent in disorders related to adolescents' social functioning.

Oppositional Defiant Disorder (ODD) and Social Anxiety Disorder (SAD) are polarizing examples of problematic social functioning as adolescents with these diagnoses view the social world as a place of potential evaluation and rejection. ODD and SAD have alarming lifetime prevalence rates in adolescence, suggesting that these conditions are highly stable and predict later emotional and behavioral problems. Though their impact on socialization is similar (i.e., rejection, isolation), they have been differently conceptualized and intervened with (e.g., self-focused attention in SAD versus hostile attribution style in ODD).

Alternatively, Acceptance and Commitment Therapy (ACT) focuses on common underlying processes, rather than specific symptoms, as the foundation for social difficulties such as ODD or SAD. Specifically, Psychological Inflexibility (PI) - rigid attempts to control internal experiences and the inability to adapt one's behavior in the service of freely chosen values - is proposed as the basis of human suffering. PI stems from six interrelated processes: Cognitive Fusion, Experiential Avoidance, Attachment to the Conceptualized Self, Dominance of the Conceptualized Past/Feared Future, Lack of Values Clarity, and Inaction, Impulsivity or Avoidant Persistence. ACT aims to cultivate Psychological Flexibility (PF) - the ability to be in the present moment with willingness to be with the internal experience non-judgmentally and to act in a way that is congruent with ones' values. ACT relies on 6 core principles to cultivate PF, each opposing to one of the PI processes: Acceptance, Cognitive Defusion, Contact with Present Moment, Self-As-Context, Values, and Committed Action. Although there is evidence about PI processes as transdiagnostic factors underlying psychopathology and about the efficacy of ACT on adults' mental health in comparison with control conditions, research targeting adolescent samples (though emerging) is lacking.

Research has supported the efficacy of ACT interventions in adults with externalizing behavior. However, only a few works used ACT to address externalizing behavior in adolescents, though results were promising. There is evidence supporting the efficacy of ACT in treating SAD, though, again, only a few studies addressed this issue and found promising outcomes with adolescents. None of these works considered change in the processes of PI/PF and its intercorrelation with change in symptomatology, nor ACT's potential to promote similar change in these polarizing examples of problematic social functioning.

To overcome these shortcomings, we propose to take an ACT approach to adolescent ODD and SAD in a sequence of three clinical trials: A pilot single-arm clinical study of ACT applied to ODD, a Randomized Controlled Clinical Trial of ACT applied to SAD, and an equivalence clinical trial of ACT applied to ODD and SAD. These studies will be conducted to explore the (dis)similarity in efficacy of ACT for ODD and SAD, considering symptomatology, PI/PF processes and mental health as outcomes.

The research team will adapt, implement, and investigate the efficacy of a face-to-face ACT Intervention to adolescents presenting ODD or SAD via: 1. Changes in primary (i.e., disruptive behavior and anxiety symptoms) and secondary (i.e., mental health and PI/PF processes) outcomes following intervention - significant improvements are expected at post-intervention (i.e., in the SAD diagnosed intervention group in comparison with a control group and in the ODD diagnosed intervention group); 2. Examining the stability of change over time (i.e., 3- and 6-months follow-up) - improvements are expected to be maintained; 3. Comparing the efficacy of ACT between both intervention groups - Similar effects are expected on outcome measures for both intervention groups; 4. Investigating mechanisms of change following intervention in both intervention groups - Change in PI/PF is expected to predict change in outcome variables in both intervention groups.

All procedures involved in this project (described elsewhere in this form) were approved by the Ethics Committee of the Faculty of Psychology and Educational Sciences, University of Coimbra and the General Directorate of Education authorized the data collection protocol to be implemented in school contexts. Informed consent from adolescents and their parents/legal guardians will be required for all potential participants prior to any data collection. Adolescents and their parents/legal guardians will be informed that participation is voluntary and that they can decline to participate at any time during the project without any negative consequence. Moreover, they will be informed that the confidentiality of responses will be assured in all moments.

Inclusion Criteria:

Study I:

• Main diagnosis of Oppositional Defiant Disorder (ODD)
• Meeting diagnostic criteria for Social Anxiety Disorder (SAD)

Study II:

• Main diagnosis of Social Anxiety Disorder (SAD)
• Meeting diagnostic criteria for Oppositional Defiant Disorder (ODD)

Exclusion Criteria (applies to both studies):

• Impaired cognitive development based on school records
• Presence of psychotic symptoms according to the diagnostic interview
• Diagnosis of autism spectrum disorder according to the diagnostic interview
• Currently undergoing another psychological intervention for a psychiatric condition

Note: Comorbid conditions other than the co-occurrence of ODD and SAD will not be considered exclusion criteria.