This clinical study investigates the effectiveness of two psychological treatment formats for young people aged 16-25 with symptoms of depression, anxiety, stress, or interpersonal difficulties. The study compares group-based Acceptance and Commitment Therapy (ACT) with treatment as usual (TAU), most frequent that would be individual psychotherapy, both of which are established treatment approaches.

The study uses a randomized controlled design (RCT), in which participants are randomly assigned to one of the two treatment conditions. This allows for a systematic comparison of treatment outcomes between ACT delivered in a group format and standard individual therapy.

The ACT group intervention consists of a structured program in which participants meet regularly over a defined treatment period. The treatment focuses on processes such as psychological flexibility, acceptance of internal experiences, and engagement in actions aligned with personal values.

The individual therapy condition consists of one-to-one sessions with a clinician, following standard therapeutic practice. Treatment content and duration are tailored to the participant's clinical presentation and therapeutic needs.

Outcome measures include standardized assessments of mental health symptoms, functioning, and psychological processes. These assessments are conducted at baseline, during the treatment period, at post-treatment, and at follow-up time points. Data collected from these measures will be used to evaluate changes over time and differences between the two treatment conditions.

The primary aim of the study is to determine whether group-based ACT is as effective as, or more effective than, individual psychotherapy for young people receiving mental health services. The results are expected to contribute to improved knowledge about treatment options for this age group and inform future clinical practice.

Background and Rationale Mental health problems among adolescents and young adults represent a significant and increasing public health concern. A substantial proportion of young people experience symptoms of depression, anxiety, and related psychological distress during a developmental period characterized by major psychological, social, and biological transitions. These challenges may interfere with identity development, educational attainment, and social functioning, with potential long-term consequences for health and well-being .

At the same time, mental health services face increasing demand and limited capacity, resulting in prolonged waiting times and restricted access to timely care. Traditional outpatient treatment is primarily delivered as individual psychotherapy, which limits the number of patients that can be treated. There is therefore a need to evaluate alternative and potentially more scalable treatment formats that can maintain or improve clinical outcomes while increasing accessibility.

Acceptance and Commitment Therapy (ACT) is a transdiagnostic behavioral intervention that aims to improve psychological functioning by enhancing psychological flexibility. The ability to act in accordance with personal values while remaining in contact with difficult internal experiences. Rather than focusing solely on symptom reduction, ACT targets core processes such as acceptance, cognitive defusion, present-moment awareness, values clarification, and committed action.

Although ACT has a substantial evidence base in adult populations, research on its effectiveness for young people remains more limited, particularly in routine clinical settings and in comparison with standard care. Group-based formats may represent an efficient way of delivering ACT, while also offering therapeutic benefits such as peer support, normalization, and shared learning. The present study addresses important gaps in the literature by evaluating a structured group-based ACT intervention tailored for young people, compared with treatment as usual (TAU), within specialized mental health services.

Study Objectives The primary objective of the study is to evaluate the effectiveness of group-based ACT compared with treatment as usual in reducing symptoms of depression and anxiety among young people aged 16-25.

Secondary objectives include:

• Evaluating the impact of the intervention on psychological well-being, quality of life, and functional outcomes
• Assessing whether group-based ACT leads to comparable or improved remission rates relative to TAU
• Investigating mechanisms of change, particularly the role of psychological flexibility related to the ACT processes
• Exploring potential moderators of treatment outcome, including demographic and clinical characteristics

Study Design

This study is a randomized controlled trial (RCT) with two parallel treatment arms:

1. Group-based Acceptance and Commitment Therapy (ACT)
2. Treatment as usual (TAU), typically consisting of individual psychotherapy Participants meeting eligibility criteria will be randomly allocated to one of the two conditions. The study is conducted in routine clinical settings across child/adolescent (BUP) and adult (DPS) mental health services, as well as in collaboration with primary care services, to enhance ecological validity and generalizability .

The target sample size is 212 participants, accounting for expected attrition and ensuring adequate statistical power for both outcome and process analyses .

Participants

Inclusion criteria:

Age 16-25 years Moderate symptoms of depression, score of 14 and above in the DASS 21 questionnaire

Exclusion criteria:

Severe psychiatric conditions requiring alternative treatment (e.g., psychosis, bipolar disorder) Active substance dependence Acute suicidal ideation or behavior Eating disorders or developmental disorders requiring specialized interventions Participants with specific disorders for which established evidence-based treatments are indicated (e.g., OCD) will be offered those treatments outside the study.

Interventions:

Group-based ACT

The experimental intervention is a structured group program based on Acceptance and Commitment Therapy, adapted for young people using the DNA-V framework. The intervention focuses on developing skills in:

• Awareness of internal experiences
• Acceptance and willingness
• Cognitive defusion
• Values clarification
• Committed action The group format aims to promote experiential learning and peer interaction. The intervention consists of multiple sessions delivered over a defined treatment period, with each group including up to 10 participants and led by trained therapists. Sessions are manualized to ensure consistency and fidelity, and adherence will be monitored through supervision and video recordings .

Treatment as Usual (TAU) Participants allocated to TAU will receive standard care within mental health services. This typically involves individual psychotherapy delivered by a clinician. The content, frequency, and duration of treatment are determined by clinical needs and routine practice, reflecting real-world conditions.

Procedures and Assessments:

Participants will undergo a baseline assessment prior to randomization, including diagnostic evaluation using structured clinical interviews. Follow-up assessments will be conducted at multiple time points:

• Baseline (pre-treatment)
• During treatment (6 weeks)
• Post-treatment (12 weeks)
• 6-month follow-up
• 12-month follow-up

Outcome data will be collected using a combination of clinician-administered interviews and validated self-report measures assessing:

• Depression and anxiety symptoms
• General psychological distress
• Quality of life
• Functional impairment
• Psychological processes relevant to ACT (e.g., flexibility, mindfulness, values-based action) In addition to these scheduled assessments, ecological momentary assessment (EMA) will be conducted using a digital platform. Participants will complete brief daily measures during the treatment period and periodic follow-up assessments thereafter. This approach allows for detailed analysis of temporal changes and treatment processes in real-world contexts .

Outcomes

Primary outcomes:

• Changes in symptoms of depression and anxiety

Secondary outcomes:

• Psychological well-being
• Quality of life
• Functional impairment
• Remission rates

Process outcomes:

• Psychological flexibility
• Emotional regulation
• Mindfulness
• Self-compassion
• Values-consistent behavior

Statistical Analysis Data will be analyzed using appropriate statistical methods for longitudinal and randomized designs. Primary analyses will compare changes in outcome measures between treatment conditions over time.

Secondary analyses will examine:

• Mediation effects to identify mechanisms of change
• Moderation effects to identify subgroups for whom the treatment is particularly effective Missing data will be handled using established statistical techniques appropriate for repeated measures designs.

Ethical Considerations Participation in the study is voluntary, and all participants will provide informed consent prior to inclusion. Participants may withdraw at any time without consequences for their access to treatment.

The interventions evaluated in this study are based on established therapeutic approaches and are not expected to pose significant risk. However, participation in group-based treatment may be experienced as challenging for some individuals. Measures are in place to monitor and address adverse reactions, including the presence of trained therapists and access to follow-up support .

The study will be conducted in accordance with ethical guidelines and has been approved by the regional ethics committee.

Significance This study will contribute to the evidencebase for psychological treatments for young people by evaluating a scalable, group-based intervention within routine clinical practice. The findings may inform service development and improve access to effective mental health care for this population. Additionally, the study will provide insight into the mechanisms underlying therapeutic change, supporting further refinement of interventions based on ACT principles.

Inclusion Criteria:

• Score on DASS-21 depression scale ≤ 14

Exclusion Criteria:

• Psychosis