Positive and Negative Evidence in Language Acquisition
1．Introduction ． The necessity for input in the process of second language acquisition (SLA) is a well-accepted fact, but the form and type that it needs to take for learning to occur remains a controversial issue. Those subscribing to a nativist or rationalist position of acquisition support the idea that positive evidence is all that is required for acquisition to occur (Chomsky, 1989). They believe that human knowledge develops from structures, processes and ideas that are in the mind at the birth. On the other hand, those working within the interactionist paradigm see positive evidence as insufficient and propose a role for both positive and negative evidence (Labov, 1969; Gass, 2003).
2. Positive Evidence Positive evidence is evidence that a particular utterance is grammatical in the language that the language that the child is learning. It consists of descriptive information about a form or an utterance. It consists of actually occurring sequences, i.e. sentences of the language. Various options exist for positive evidence including plentiful exemplars of the target feature without any device to draw attention to it. For example, Trahey (1996) developed materials consisting of stories, games, and exercises with the aim of simply exposing learners to the subject. In this case, acquisition occurs as a result of frequent exposure to a target feature. It involves some sort of attempt to highlight instances of the target feature, thus drawing learners' attention to it. Positive evidence can function entirely by itself. Learners can simply be asked to listen to or read texts that have been provided. It can also be accompanied by some kind of meaning-focused activity that incidentally assists learners to focus their attention on the target feature.
For example, comprehension questions that can only be answered correctly if the learners process the target feature. There are tasks that are designed to elicit production of a specific target feature in the context of performing a communicative task, and tasks
that are intended to result in learners' employing some feature that has been specifically targeted (White, 1987).
Several patterns in language have been claimed to be unlearnable from positive evidence alone. One example is the hierarchical nature of languages. For any given set of sentences generated by a hierarchical grammar capable of infinite recursion there are an indefinite number of grammars that could have produced the same data. This would make learning any such language impossible. Indeed, a proof by E. Mark Gold showed that any formal language that has hierarchical structure capable of infinite recursion is unlearnable from positive evidence alone,[ in the sense that it is impossible to
formulate a procedure that will discover with certainty the correct grammar given any arbitrary sequence of positive data in which each utterance occurs at least once. However, this does