My BS armchair psychiatric definition of ADD is that it was useful when our genetic forebears had to be "distracted" every minute or so to be aware of the need to attack or protect themselves or their social units.

It's one of those hypotheses that I don't have the intellectual honesty or rigor to follow up, but it's a simplifying lie that allows me to put it to the back burner. Selfish gene, blah blah.