Language transfer typically refers to the learner's trying to apply rules and forms of the first language into the second language. The term can also include the transfer of features from one additional language to another (such as from a second to a third language), although this is less common.
Contrastive analysis, discussed above, sought to predict all learner errors based on language transfer. As subsequent research in error analysis and interlanguage structure showed, this project was flawed: most errors are not due to transfer, but to faulty inferences about the rules of the target language.
Transfer is an important factor in language learning at all levels. Typically learners begin by transferring sounds (phonetic transfer) and meanings (semantic transfer), as well as various rules including word order and pragmatics. As learners progress and gain more experience with the target language, the role of transfer typically diminishes.
In the UG-based framework (see Linguistic universals below), "language transfer" specifically refers to the linguistic parameter settings defined by the language universal. Thus, "language transfer" is defined as the initial state of second language acquisition rather than its developmental stage.
Research on universal grammar (UG) has had a significant effect on SLA theory. In particular, scholarship in the interlanguage tradition has sought to show that learner languages conform to UG at all stages of development. A number of studies have supported this claim, although the evolving state of UG theory makes any firm conclusions difficult.
A key question about the relationship of UG and SLA is: is the language acquisition device posited by Chomsky and his followers still accessible to learners of a second language? Research suggests that it becomes inaccessible at a certain age (see Critical Period Hypothesis), and learners increasingly depended on explicit teaching (see pedagogical effects above, and age below). In other words, although all of language may be governed by UG, older learners might have great difficulty in gaining access to the target language's underlying rules from positive input alone.