Ever since attempts were made to describe and explain normal language development, references to exceptional circumstances have been made. Variations in the conditions under which language is acquired can be regarded as natural experiments, which would not be feasible or ethical under normal circumstances. This can throw light on such questions as:
*What language input is necessary for the child to learn language?
*What is the relationship between cognition and language?
*How independent are different components of language function?
*Are there critical periods for language...