Imagine a 9 year old boy in school. He exhibits some signs of ADHD, such as: fidgeting, inability to concentrate, frustration leading to occassional outbursts, and variable quality of school work. He is clinically tested, is found to have a definite, measurable physiological condition consistent with the medical definition of ADHD and is diagnosed as having the disorder.
Further imagine that this 9 year old boy lives at home with his divorced mother and her new husband and their 2 year old child. His mother is a "permissive" style parent, who lets him do what he wants to do unless he does something she really doesn't like, then she usually just yells at him and sometimes calls him names. She rarely praises him or rewards him for good work, but will inconsistently punish him for bad behavior, sometimes disproportionately. His father shows up maybe once a year for a few hours, just long enought to remind him that he used to have a father. His mother's new husband refuses to have anything to do with him and reminds him that he's not his son and tells him to go away whenever the boy seeks his attention. He spends most of his free time watching TV and playing video games and very little time outside. He exhibits signs of "acting up" consistent with children who are emotionally neglected, such as: outbursts of temper, inability to complete tasks, poor quality of school work and random incidents of disruption.
Now suppose on one particular day in school, this kid gets up out of his chair and throws his textbook across the room. Was it caused by the known condition of ADHD or was it caused by his "acting up" for attention that is known to result from living in an emotionally neglected environment?
You see the problem. There is an overlap of telltale behavior resulting from each of the two causes, one biological, the other environmental, and it's not reasonable to simply blame one cause and ignore the other, because it could be either, or both.
Incidently, this isn't a hypothetical example. This situation describes the boy who lived next door to me for eight years. He was put on Ritalin when he was 10, and he showed improvements in behavior in the areas of fidgeting and concentration, as well as a slight improvement in school work. His mother told everyone how she was so glad there was a drug that could calm him down but of course she did nothing to improve her own parenting. He eventually became violent toward other children and began to destroy private property. After he was arrested for trying to set his neighbor's house on fire, his mother gave him up to state custody, where he now lives in a group home at the age of 13.