There has been a tendency among the second language acquisition/learning theorists to make generalization about the stages that learners go through in learning a second language (L2). Processability Theory, developed by Pienemann (1998), is one of those theories. It is argued in Processability Theory that learners can learn an L2 in an order that they are capable of at specific times. In other words, learners acquire/learn L2s in a predictable order, which is called ‘developmental trajectory’. This article reviews Processability Theory from a critical perspective and investigates the limitations of and ambiguities in the theory through examining previous studies. The issues discussed in this paper include the hierarchical order, hypothesis space, grammar and lexicon, operational definition of language processor and its connection to Neurolinguistics and working memory, overgeneralization of features to all languages, and lexical functional grammar. Based on the review of these issues, it is proposed that Processability Theory may need some modifications and amendments in near future, as there is a need for more empirical studies.
Processability theory, processability hierarchy, second language acquisition, working memory, lexical-functional grammar, individual differences in language learning