No Abnormality Has Been Found
If one has relinquished all claims of a physical entity and has accepted it is psychological or psychiatric, then and only then, would it be appropriate to speak of contributing psycho-social causes such as home, school, community, peer relations, etc., none physical. And yet NICE concludes that medication should be used as a first line treatment in ‘severe’ ADHD as if “severe ADHD” was a proven, diagnosable disease, when it too, along with all other classifications of ADHD, is not.
The American Academy of Pediatrics (AAP) ‘ diagnostic guideline’ referred to ADHD as the most common ‘neurobehavioral’ disorder of childhood [3]. I responded “‘Neurobehavioral,’ implies an abnormality of the brain, a disease [4]. And yet, no confirmatory, diagnostic, abnormality has been found…It is apparent that virtually all professionals of the extended ADHD ‘industry’ convey to parents, and to the public-at-large, that ADHD is a ‘disease’ and that children said to have it are ‘diseased’-‘abnormal.’ This is a perversion of the scientific record and a violation of the informed consent rights of all patients and of the public-at-large. The wording of the AAP Guideline should be changed, forthwith, to reflect the scientific and medical facts of the matter.” Nor has there been proof of the existence of ADHD as a disease from that date to the present.

I know that the author of this piece is specifically referring to neurobehavioral disorders, but there are plenty of disorders with no known physical cause. Look at IBS or fibromyalgia. Just because we haven't figured out what causes a disease doesn't mean it's not real.
And on another note, there is evidence to show that people with ADD/ADHD have, on average, a different brain structure than those without symptoms. The most noticeable difference is in the average weights of certain structures in the brain.