The clear physical cause is something of a misleading question. What matters more in this context is that whatever multiple causes most likely result in a simil

The clear physical cause is something of a misleading question. What matters more in this context is that whatever multiple causes most likely result in a similar systemic behavior (or failure). With ADD that seems to be pre-frontal cortex impairment and is backed up by statistical analysis of various types of neural imaging studies. In short, though the causes may be as varied as genetic in origin to pre-frontal cortex injury in a collision (injuries sustained in front-end crashes which significantly damage the PFC can lead to ADHD-like symptoms), they all lead to similar systemic failures. These similar failures appear to respond quite similarly to stimulant treatment. The key here is that we're treating a systemic problem rather than addressing a specific origin.