The Causes of ADHD
A multitude of research studies have examined the causes of ADHD. Even with this wealth of data, experts remain uncertain as to the exact cause of the disorder. At this point, the research suggests that ADHD is primarily a neurological or brain-based disorder that is either present at birth, or that develops early on in childhood. Although environmental factors can play a role in increasing the severity of a person's symptoms, the environment does not seem to be the primary cause of the disorder.
This site offers a number of different causes.