3. Which came first: the chicken or the egg?

The studies attempting to show that "ADHD" exists have focused on trying to identify differences in the brains or central nervous systems of children with "ADHD" and without it. The problem is that even if such a finding were made, it would be impossible to interpret what it means. It is believed that when people run they produce chemicals in their body that cause "runner's high". But how do we know if the running produced the chemicals, or the chemicals caused them to run? 

It is not medically possible to determine levels of brain chemicals in a living person. Whenever someone is told they have a "chemical imbalance" it is based on theory and "symptoms", not laboratory studies. So if we found differences in brain chemicals between a person who feels depressed and one who doesn't, it would be impossible to determine if the chemicals caused the depression or the depression caused the chemicals. Similarly, if there ever were a discovery of a consistent difference in brain functioning in children with "ADHD", the question would still remain whether the behaviour caused the difference or the difference caused the behaviour.