The Strengths and Difficulties Questionnaire (SDQ; Goodman, 1997) is one of the most frequently used methods for assessing mental health in children and adolescents from a self- and parent perspective. In addition to the version for children and adolescents, there is also a version for adults, the SDQ 18+. The SDQ assesses mental health problems and resources of children and adolescents with 5 items each on five subscales: behavioral problems, emotional problems, hyperactivity, peer problems, and prosocial behavior. It has good psychometric properties, is economical, and has been widely used in research (Goodman, 2001).

The Patient Health Questionnaire scale 9 (PHQ-9; (Kroenke et al., 2001), is a self-report questionnaire designed to assess the severity of depressive disorder in adolescents and adults. The PHQ-9 has been shown to display good psychometric properties and has been validated in autistic adolescents and adults (Arnold et al., 2020).

The Generalized Anxiety Disorder scale 7 (GAD-7; Spitzer et al., 2006), is a self-report questionnaire designed to assess the severity of anxiety in adolescents and adults. The GAD-7 has been validated in adolescent and adult populations, displaying good psychometric properties (Löwe et al., 2008). The GAD-7 is strongly correlated with anxiety measures including the Beck Anxiety Inventory (Spitzer et al., 2006) and provides a valid measure also of other anxiety disorders, e.g. social anxiety and panic disorder (Kroenke et al., 2007).

The Perceived Stress Scale (PSS-10; Schneider et al., 2020) is a 10-item questionnaire commonly used to assess stress levels in adolescents and adults 12 years and older. It assesses the extent to which a person has perceived life as unpredictable, uncontrollable, and overwhelming in the past month.

3-item Loneliness Scale (Klein et al., 2021): Loneliness is also measured by the 3-item Loneliness Scale. The scale is an economic measure derived from the R-UCLA Loneliness Scale (Russell et al., 1980) that captures basic aspects of loneliness: A sense of isolation, disconnectedness, and not belonging. Items are rated on 5-point Likert scales. Responses are summed for a total score ranging from 0 to 12, with higher scores indicating higher levels of loneliness. Autism related adaptations of the instrument was made according to recent research (Grace et al., 2023).

The Reflective Function Questionnaire for Youth-5 (RFQ-Y; Sharp et al., 2022) measures mentalizing ability in adolescents. The English version showed satisfactory psychometric properties (Sharp et al., 2022).

The Frith-Happé Animations Test (Livingston et al., 2021), which represents interactions between triangles in animations, is an online instrument for measuring Theory of Mind (ToM). The web-based version of this test provides a quick and objective measurement of the construct ToM using multiple-choice questions. The instrument demonstrated good psychometric properties in a clinical as well as a non-clinical sample (Livingston et al., 2021).

The Difficulties in Emotion Regulation Scale (DERS; Gratz and Roemer, 2004) was used to measure difficulties in emotion regulation. Although there are different versions, this study we will use the short version with 8 items as suggested by Penner & Sharp, 2022. The DERS-8 has shown good psychometric properties (Penner & Sharp, 2022).

The quality of dyadic Parent-Adolescent Communication was assessed with the German translation of the Parent-Adolescent Communication Scale (PACS; Barnes & Olson, 1982). The original measure consists of an adolescent and a parent version with 20 identical items that are rated on a scale from 1 ("strongly disagree") to 5 ("strongly agree"). The German version of the instrument has shown acceptable psychometric properties (Zapf & Boettcher, 2024)