‘I Gotta Feelee-ing’: Exploring the Effects of a Smartphone app (Feelee) to Enhance Adolescents’ Emotion Regulation in Forensic Outpatient Settings. A Multiple Single-Case Experimental Design

Abstract 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. Competing Interest Statement 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. Clinical Trial ClinicalTrials.gov NCT06509360 Clinical Protocols https://www.researchprotocols.org/2025/1/e64756 Funding Statement This study was funded by: Kwaliteit Forensische Zorg Jeugd (KFZ-J) Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethics Committee of the Amsterdam UMC Medical Ethics Review Committee (METC) gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data Availability All relevant data are within the manuscript and its Supporting Information files.