Studies investigating the impact of non-manipulated overall instruction upon general proficiency may not provide a clear picture as to what acts as a causal factor in fostering learner's competence in second language. After all, classroom interaction involves a multiplicity of competing factors each affecting different aspects of learner's proficiency. In the midst of so many extraneous factors, one immediately finds himself in the difficult position of selecting the causal ones. For some, like Krashen, comprehensible input is the causal factor, while for others like Swain(1985), it is comprehensible output plus input. Swain claims that the concept of comprehensible input per se, is not enough to account for second language acquisition; comprehensible output should be included in any second language acquisition theory to better account for the acquisition process. According to Swain, language acquisition takes place when the learners realize how meaning is expressed accurately using their output as a means of hypothesis testing process. An L2 learner tests his hypotheses by trying them out in communicative situations. If his hypotheses prove to be successful in expressing his communicative intent then the hypotheses are confirmed; if not then they are revised and put into test again.

A strong version of Swain's output hypothesis cannot even account for first language acquisition. Brown and Hanlon 1970), for instance, have shown that parents do reinforce well-formed utterances of their children more than ill-formed ones. They found no significant correlation between parental approval and correctness of their children's utterances. Finally, it is concluded that output-based (dis)confirmations 'cannot be the forces causing the child to relinquish immature forms and adopt adult forms' and that a child revises his hypotheses not due to output disconfirmation but because of' the occasional mismatch between his theory of the structure of the language and the data he receives' (p.50), a finding which is quite in line with the predictions of the Input hypothesis. Similar results were reported by Hirsch-Pasek,Treiman and Schneiderman (1984), who replicated Brown and Hanlon's findings with a wider sample greater age range.

A striking example proving Krashen's claim that acquisition is possible even without production practice, is described in Fourcin (1975, cited in Krashen, 1985a). Richard Boydell was a fully intelligent though a linguistically impaired child, who could understand the language spoken around him while he couldn't speak normally nor could he use a sign language. At the age of thirty, he was given a foot-controlled type writer and only then he could communicate with the outside world.

He wrote:

I acquired an understanding of language by listening to those around me. Later, thanks to my mother's tireless, patient work I began learning to read and so became familiar with written as well as spoken language. As my interest developed, particularly in the field of science, I read books and listened to educational programs on radio and, later, television which were at a level that was normal, or sometimes rather above, for my age. Also when people visited us ... I enjoyed listening to the conversation even though I could only play a passive role and could not take an active part in any discussion ... As well as reading books and listening to radio and television .... I read the newspaper every day to keep in touch with current events. ( Fourcin,1975, cited in Krashen,1985, pp.11-12).

As Fourcin puts it, Boydell's writing was "elegantly phrased" although he had never written anything before. Krashen(1985a) attributes Boydell's success in expressing himself with such a vigorous style, to his previous listening and reading experience. The mere fact that he can produce such complicated sentences without any history of practice of productive skills; writing and speaking, runs counter to the predictions of output hypothesis, which bases the development of language proficiency on output as well as input.

The Role of the Receptive Skills on the Second Language Development

Listening and reading are not the driving force behind the first language development only. These skills also play a key role in second language acquisition. Accordingly, a number of studies have been done to test the effect of receptive skills on second language development.

Listening-based Studies

Asher, Kusudo and Torre (1983) compared the efficacy of TPR over traditional college foreign language instruction. The subjects taking listening-based TPR instruction were between the ages of 30 and 60. The treatment lasted for only 32 hours. At the end, they were compared to younger, adolescent college students who were assumed to be better acquirers. Then, the control group underwent 75 hours of systematic instruction in reading and writing. Although the listening group had not received any su ch instruction, the results showed that in terms of grammar knowledge and reading performance, they scored as high as the other despite of the differences in the length of instruction and age range. The results clearly showed that listening experience positively affected the development of reading ability . The same researchers replicated the research for Spanish learners and found similar results.

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