After reading An Anthropologist on Mars, by Dr. Oliver Sacks, I observed that several other neurological conditions have some similarity to ADD, most notably Tourette's Syndrome. The initial response when I say this to someone is that I'm crazy (which may be true, but for other reasons). Tourette's Syndrome, like most medical conditions ends up being horribly simplified in the popular culture, even among people who should know better, however for the sake of space, I'm going to be forced to simplify Tourette's syndrome into a dreaded one sentence description: A person with Tourette's often has nearly (or completely) uncontrollable urges to do certain things -- touch things, say things, bizarre rituals, etc. Now, I see a certain similarity in this description to a portion of ADD. Look at a hyperactive child in a classroom. They will often shout out, get out of their seat, and do other activities they know (if asked) are inappropriate, but they happen without premeditation or planning... much like people with Tourette's. Even in non-hyperactive people like myself, I notice I often do small, strange things that I can't explain to myself why I did them, much less to anyone else. Like a person with Tourette's, an ADD person can function very well and very normally under pressure or mental stimulus, it is when things are quiet that the curious quirks show themselves.
The popular conception of illness, and perhaps even reinforced by the formal medical business, is that most medical conditions are very cut-and-dried. For example, the flu. A particular virus has invaded your body, set up shop, and your body responds in various ways due to the action of the flu virus and your body's defense systems. However, many conditions are very different. In some cases, yes, a single "flaw" or variation results in a particular problem (for example, the lack of a clotting agent results in hemophilia), but in most, doctors have a set of symptoms, and a pile of defined "disorders" (their words), and they diagnose the patient by matching the symptoms to the pre-defined disorder that fits best. There are often tremendous overlaps in disorders and diagnosis's. The human body and brain are an incredible machine, the operation of which science only barely understands. What medicine primarily studies is the effects, not the causes, and often the effects are so far removed from the root causes that the root causes are not recognized. I doubt ADD is one simple "disorder" or condition, traceable to one simple genetic or other factor. Most likely, it is a combination of variations, when put together in one way, cause a person to be called "normal" (i.e., no noticed symptoms), another way, ADD, another way Tourette's, another way, perhaps Parkinson's.
Now, I was rather surprised to hear from an relative that yes, a university out west is actually investigating a connection between ADD and Tourette's... I may not be so off base after all!