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Psychiatric diagnoses are not reflections of the aetiology of the disorder, but rather lists of symptoms with considerable overlaps, which hamper research and may cause confusion. The diagnoses of autism spectrum disorder, attention deficit hyperactivity disorder and tic disorder are often comorbid along with a number of other symptomatic syndromes. Individual immune responsivity is possibly involved in pathophysiological mechanisms. Multiple environmental factors may contribute to the clinical phenotypes. Recent research supports to some extent the involvement of dietary and nutritional factors in ADHD. In spite of impressive progress in the molecular biological understanding of the pathophysiology of these disorders, treatment options are still limited and more research is warranted.