Dorothy V. M. Bishop, D.Phil., Department of Experimental Psychology, University of Oxford 
Published online 03-26-07 in the Canadian Language & Literacy Research Network in their Encyclopedia of Language and Literacy Development
Talking comes so naturally to most children that one seldom pauses to consider the enormous complexity of the achievement. Understanding just how the human brain manages to learn language-typically in the space of around 4 short years-is still a long way off. Perhaps as remarkable as the speed with which young humans learn language is the robustness of this process in the face of adverse conditions (Bishop & Mogford, 1993). Most children will learn to talk adequately even if they are exposed to impoverished language input from adults or are visually impaired and thus unable to see what is being talked about. Children who are unable to speak because of physical disability, and those who cannot hear what others say to them, will nevertheless learn to communicate by other means, provided they are exposed to alternative systems of communication such as sign language.
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