Adolescents in forensic outpatient care often face a complex interplay of emotional and cognitive challenges, frequently shaped by adverse childhood experiences, increasing their risk for delinquent behavior. Current interventions show mixed results, potentially due to a mismatch between intervention demands and adolescents disrupted emotional development. The Feelee app may help bridge this gap by offering daily practice of emotion regulation skills through the collection of active and passive smartphone data. This study aimed to assess the initial effectiveness of the Feelee app as an addition to treatment as usual to enhance emotion regulation skills among forensic outpatients. A multiple single-case experimental ABA design was applied over a 2-week baseline (phase A1), 4-week intervention (phase B), and 2-week follow-up (phase A2), combining quantitative and qualitative methods. Twenty-two adolescents (aged 12–23) completed daily assessments of emotion regulation. Secondary outcomes focused on emotional developmental mechanisms and treatment-related factors, measured at pre-, post-, and follow-up. Qualitatively, semi-structured interviews with adolescents and clinicians explored experiences with Feelee and its integration into treatment. Results showed a significant reduction in emotional suppression during the intervention phase. No improvements were found in emotion recognition and impulse control, while reflection and distraction showed reversed outcomes. On secondary outcomes, the follow-up measurements revealed increased positive emotion differentiation, emotional awareness and self-reflection. Treatment motivation remained stable, while therapeutic alliance improved. Qualitative findings highlighted increased emotional insight but also pointed to technical difficulties and limited discussion of Feelee data during sessions. These results suggest that Feelee may be particularly helpful in the early stages of emotion regulation by reducing suppression, making it possible to take the crucial first step of engaging with emotions. Future research should explore longer-term use and actively involve clinicians in the integration of app data to maximize therapeutic relevance and impact.

I have read the journal's policy and the authors of this manuscript have the following competing interests: LvD is an employee of Garage2020, the foundation that provides the Feelee app.

ClinicalTrials.gov NCT06509360

https://www.researchprotocols.org/2025/1/e64756

This study was funded by: Kwaliteit Forensische Zorg Jeugd (KFZ-J)

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