(simulposted by a much more succinct post from Raak, but...)
[BM] Well... yes and no. I'll take ADD as my example, since that is obviously what I know best.

It is clear that ADD behaviours can be severe enough to cause problems, and that there are a lot of people who carry these behaviours around with them, but it is not certain that ADD is an objectively definable disease. For instance, ADD has been described and recognised by the US psychiatric establishment for more than 30 years (although its name has been changed a few even times over that time), but the UK establishment only officially recognised ADD in the 1990s - this despite the first published work on ADD being in the UK a hundred years before. Even in the US, ADD has only recently (ie in the last ten years or so) been widely recognised as occurring in adults - it was considered a childhood affliction which was obliterated by adolescence.

Part of the problem is that labelled conditions such as ADD, OCD, etc are note discrete: a bone is broken or it is not broken, but ADD has more degrees than there are labels, and even then it is not clear that even someone whose collection of symptoms is technically pathological enough to be so labelled is actually displaying anything more than just a strong personality trait. There has been work to scan brains under ADD-expressing conditions, and there are apparently common factors to those scans, but ADD is at best a collection of possibly related symptoms. But then is personality just an expression of brain chemistry?

As I say, my belief is that those who are actually disadvantaged by their ADD behaviours are those who have been trying to use their brains in ways which don't mesh with their brain chemistry or personality, where "one size fits all" education systems teach the same learning mechanisms to everyone regardless of how they really learn best. This is one of the reasons that I talk about my having ADD behaviours; Idon't consider myself to be diseased¹.

[1] this is going in a footnote because it's not part of the core point, but one of the things which concerns me about statements that ADD is underdiagnosed is that it is then treated as a disease. At this point the children (and this is where I become most concerned - it is almost always children) are dosed up with psychostimulants in order to make them fit in rather than training them to use their brains to their best advantage: the problem is not solved, it is avoided.

Absol-bloodly-lutely. Extremely well put. It extends beyond education and learning mechanisms too, in my view.

[Dunx and Raak] Hear hear. Nicely expressed.

As Raak says, a leg is generally broken or not broken, and the difference is pretty clear and agreed upon, but you get members of any minority group into a room (ADD sufferers, autists, gays, black people) into a room and ask them to define "ADD", "autism" or, heaven help you, "gayness" or "blackness" and you will soon have a heated debate on your hands.

...just witness the endless controversies in the last few decades over the definition and meaning of "disabled".

I agree with Dunx that the problem is really one of society not being able to handle people who are different. It's terrible that children, in particular, who have a different way of learning, get straitjacketed into the traditional methods and classed as having a problem if they don't fit in. It's especially ridiculous given how much society actually prizes ADD traits in many ways - such as creativity and spontaneity - which is how people like Billy Connolly, Robin Williams, and Ozzy Osbourne got to where they are today. Bill Clinton, of course, is the classic ADD kid made good. But at the same time, I do think that there is good evidence that ADD, in particular, is a discrete and specific disorder, with specific physiological causes, rather than a handy name for a bunch of sometimes associated phenomena. Raak is wrong to say that the genetic correlations of ADD are vague: they are not vague at all, and there is very good evidence for a strictly physiological basis, although of course how that basis manifests itself will vary according to conditions. Certainly ADD behaviours do vary from person to person in type and degree, which is one of the things that make it hard to diagnose, but it doesn't follow that the underlying cause varies - at least where ADD is the correct diagnosis. We have a rather skewed view of it in the UK because, as he says, it hasn't been officially recognised here for very long, compared to the US; in fact, some of the newer treatments for it, such as Concerta (essentially slow-release Ritalin) are unheard of to many supposed experts in this country. Plus, of course, the media like to fixate on issues of medication and horror stories about it. By the same token, it would be better if society could change to accommodate those of us who are different, but given that it can't, medication is a whole lot better than nothing. I have known people whose lives have been utterly ruined by ADD as well as by OCD. If they had been diagnosed earlier and given the help they needed - medication as well as proper counselling and behavioural therapy - who knows how things might have been different.

Something happened to my paragraph marks. I do apologise - as if my tedious exhalations aren't bad enough as it is...

[BM] So what are the discrete physiological causes of ADD popularly supposed to be, then?

[Projoy] Primarily temporal lobe dysfunction, at least according to Dr Daniel Amens who was one of the first to use brain scans to try and figure out what is different in an ADD brain. I've read his book Amens' "Healing ADD" where he describes the different areas of brain activation in the six varieties of ADD which he identifies. All of them share some degree of temporal lobe dysfunction.

The temporal lobe controls your ability to concentrate, and is more active when a brain is concentrating on something. A typical pattern in an ADD brain is that when it is used to concentrate deliberately on something then the temporal lobe is actually deactivated (ie there is less neural activity). I say "deliberately" because a common ADD behaviour is 'hyperfocus' where the brain will concentrate on something novel¹ to the exclusion of all else, but this is rarely deliberate.

Note that there is another book called "Healing ADD" by Thom Hartmann, that I would also recommend, but which is entirely different. In particular, Mr Hartmann specifically attacks Amens' work in scanning brains. But there we are.

[1] another very annoying aspect of this is that I cannot go to sleep if there is speech in earshot. I'll be drifting off quite satisfactorily when my brain will latch on to the interesting noises and amplify them in my perception. Similarly, I need silence or at least white noise if I need to think about a task which I am not entirely involved in, because otherwise my mind will concentrate on the novel rather than what I'm supposed to be doing.

Right, but that still leaves Raak's point unchallenged, doesn't it? There's nothing to suggest that there's a single cause for that particular pattern of brain activity.

Quite right.

For instance, even the mechanisms by which ADD brain patterns arise in the individual are not certain. There is a high correlation between ADD in parents and in their offspring (a figure of 70% is often quoted) which might suggest a genetic link, but it could equally be due to upbringing: the brain is plastic enough that learning will change its structure (no specific references, I'm afraid - I read this recently but can't remember where), and if much of a child's early development is achieved through mimicry then it's quite possible to imagine that parental ADD behaviours might imprint on the child.

It may well be that the role of genetics in mental development is overstated. There is some work (this book, for instance) which puts forward the argument that:

1. there is a common perception that genetic factors are the largest causative agent in mental illness
2. there is almost no evidence of a correlation between mental illness in children and in their parents (the book linked to states that the strongest correlation is in schizophrenia where there is a 50% correlation; nothing about ADD though)
3. if hypothesis (1) is wrong, then it must be environmental
4. the most pervasive environmental influence on children is their parenting
5. therefore the way you are raised is why your head works the way it does.

So, ADD may be at least as much a social phenomenon as a genetic one, although I also think that any learning which has this effect would have been at such a young age that there is no element of choice to it .