I think it was mostly 'nurture' and the development of 'character' - breeding, education and the like, with no hard-wiring or biological basis. The earliest attribution of a specific deficit to a brain injury is of course by Broca in the mid 1800s (we still have Broca's area in the brain, involved in speech). Following this initial breakthrough, many different deficits were 'mapped' to different regions of the brain., although in a rather simplistic, modular (non-networked) fashion. After that it was Luria in Russia (around WWII?), but his work did not get the exposure it deserved because of language issues, war etc. - some of it did not come out in English until the late 1980s! There is one very cool notion in all this history. Most of the 'easily' studied neurological deficits involve the left side of the brain, which controls the 'higher' functions and constitutes a more recent evolutionary addition. The more 'primitive', evolutionarily older right hemisphere, we share with our animal cousins turns out to be much harder to study, because it deals with very fundamental brain processes. Often, a patient with right-brain problems is not even aware of the problem, although it ma be as clear as day to everyone around him! For anyone interested, I would recommend "The Man Who Mistook His Wife For A Hat" by Oliver Sacks. It is an easy read that's extremely entertaining. Not a tome that has to be plowed through with grim determination, but short, very well written vignettes about problem brains. Of course, my favorite model of the brain remains that proposed by one of the old Greeks (Aristotle, I think). He ventured that the brain was a 'cooling organ'. I agree that the brain is overrated - so many people seem to get along fine without one that I suspect that it remains a squishy air-conditioner.