[andoando]

They had a control for adoptive siblings. The critic in me says, ok what was their age of adoption, do parents perhaps treat adopted children differently, do adopted children perhaps grow behaviors unindicative of ADHD, does the knowledge that its genetic influence the diagnoses themself?

[lynx97]

The age of adoption seems crucial to rule out early childhood trauma induced by the parents. I actually know mine, although it took me 35 years to get my mother to admit what happened. My point, I am very skeptical when it comes to parents reflecting on the bad influences they might have had on the development of their child. After all, bad parenting has a pretty harsh stigma in society (and it should!)

[rusk]

Yes but if the SAME behaviour emerges regardless of parenting style then that is significant