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Attention-deficit/hyperactivity disorder (ADHD) is neurodevelopmental condition characterized by symptoms of inattention and hyperactivity/impulsivity that interferes with daily functioning. Many individuals with ADHD show elevated emotional reactivity and emotion regulation challenges, and there has been a long-standing debate on whether emotional challenges should be included in the diagnostic criteria. However, the current literature is limited regarding the phenotypic and aetiological links between emotional challenges and ADHD. The aim of this thesis was to improve specificity of the association between ADHD and emotional reactivity and regulation with a focus on childhood and adolescence. 

Study I used a co-twin control design with a sample of dizygotic and monozygotic twins (N=382, 8–36 years) and found that the association between ADHD and rated emotion dysregulation remained despite adjusting for sex, age, and psychiatric co-occurrence and that their association may be influenced by genetic factors.

Study II had a cross-sectional design including two cohorts, the diagnostic cohort (N=104, 10–17 years) and the referred cohort (N=85, 13–18 years), which showed that ADHD was robustly associated with rated elevated anger reactivity, and dysregulation of anger and exuberance. 

Study III had a cross-sectional design including the diagnostic cohort with an extended recruitment period utilizing an emotional task (N=176, 10–17 years) and questionnaire data (N=94, 13–17 years). Main findings showed an association between ADHD and less self-rated and task-specific use of adaptive strategies, and more self-rated use of maladaptive strategies, although these did not remain after full adjustments. Post-hoc analyses indicated that ADHD is related to lower self-rated use of acceptance, specifically. 

Study IV used a single-case experimental design to evaluate a newly developed psychological intervention (ER-SKILLS) aiming to increase emotion regulation capacity among adolescents with ADHD (N=9, 13–17 years). ER-SKILLS was feasible and effective for some participants, although methodological constraints partly hindered determination of intervention effects. 

In conclusion, reactivity and dysregulation of anger, and possibly exuberance, as well as a lower use of acceptance, may be specific to ADHD. A shared genetic background between emotion dysregulation and ADHD was also revealed. ER-SKILLS shows promise and may be refined by adopting group-based formats and targeting motivational difficulties.