ADHD in adults: A dimensional or categorial construct?

In recent years, attention in empirical research has been drawn to the classification and structuring of mental disorders and constructs. Particular attention has been paid to the debate between the current categorical vs. a new dimensional approach. As a childhood mental disorder, ADHD seems to be a clinical picture fitting a dimensional concept. Various taxonomic studies supply empirical evidence for this in children and adolescents with ADHD. However, for adults, there has been little empirical research so far. Thus, this present study is a first exploration of the nature of ADHD in adults. Based on the findings of ADHD in childhood, we hypothesize a dimensional structure for adults as well. Two samples were used to test our hypothesis: 605 people of a healthy normal sample and 722 individuals from a clinical sample consisting of 336 individuals without an ADHD diagnosis and 386 individuals with an ADHD diagnosis. All analyzes were based on the ADHD Self-Report Scale (ADHD-SR). Primarily discriminant and factor analysis were conducted, subsequently finite mixture models were calculated using the EM algorithm. The discriminant analysis revealed that cut-off values only discriminated partially between individuals with and without ADHD. The factor analysis for all groups resulted in the same two-factor solution of ADHD, which is also proposed by the DSM-5 (inattention, hyperactivity/impulsivity). The mixed models of the different groups could not been distinguished. These findings lead to the conclusion that a dimensional structure of the disorder can be considered to be very likely. In other words, the symptoms of ADHD represent extreme forms of normal human behavior and there are no clear boundaries between people with and without an adult ADHD. Albeit its possible dimensional structure, ADHD is a mental disorder. Recording severe functional impairments could help determine whether psychotherapeutic or psychiatric treatment is required.