There is virtually complete agreement among competent researchers that the development of linguistic knowledge in human beings is dependent upon the complex interplay of two factors: the learner's inborn capacities and the linguistic environment. Because it is not possible to directly observe the nature of the learner's mental capacities, there has been a great deal of controversy over the exact role that they play in the language acquisition process. Forced as we are to make complicated inferences about the type of language acquisition mechanisms that a child needs for success in the linguistic environment, there will probably
always remain serious disagreements over the precise contribution which properties of human nature make to the acquisition of language.
O'Grady, W. D., & Gibbons, D. E. (1980). Basic spelling competence in adults. Calgary Working Papers in Linguistics, 6(Spring), 1-10.