There is of course no correlation whatever between the sound of a language or dialect and the level of intelligence or sociability of its speakers. The only reason we might think otherwise is because for generations people were told so by their – for want of a better word – ‘betters’. Traditionally, the English aristocracy looked down on provincial speech, considering it harsh and rough. Nobles who had received a good education would naturally think of provincials as ignorant and lazy, and would associate their lack of knowledge with the way they talked. It is then a short step from saying that people are ignorant to saying that they are unintelligent.
People do hold the most extraordinarily strange views about writing and speech, apparently thinking of the latter as a poor and thoroughly imperfect reflection of the former, as if letters had come first and primitive man had slowly learned to turn them into utterable sounds.