A quick definition: There is no commitment to a way of thinking and no concern about that lack of commitment.
Combined with ADD: Picture that lack of an inner core of Who I Am, smothered in a host of defensive behaviors, well practiced, to ward off criticism.
It's a normal stage of development, generally resolved in the teenaged years. I think it's a stage that some ADD simply never gets around to. Too distracted, too... busy fighting off criticism, or self-medicating, or whatever.
Combined with some rapid-fire vacillating between seeing significant others as either all-good (You agree with me!) or all-bad (You said No!?!!?), this can come across as extraordinarily narcissistic. Except I don't think it is, really. The fewer limits placed on the individual during the formative years, as in my husband's case, the more closely it resembles narcissism; but in him, there is none of the desire to tear down the one who frustrates him, simply for the sake of punishing that person. I've known pathological narcissists. They are quite sadistic.
I believe that the "something else" in the picture along with ADD, in some cases, is borderline personality. It's the neurotic who sometimes takes a walk on the psychotic side. I've seen the psychotic side more often in the past year because of the stresses of operating his own business, combined with overmedication on adderall and only God knows what else.
Just a bit of what I've learned over the past weekend. Maybe someone else would want to research and could relate.