As we saw in great detail in Chapter 4, interest in the L3/Ln acquisition of morphosyntax is thriving. It is not surprising that a substantial amount of research in this emerging field has focused on transfer and/or cross-language effects (CLE) in L3/Ln learning. Examining how previous linguistic experience affects subsequent learning has been a staple topic in nonnative language acquisition for as long as people have been examining L2 acquisition seriously. This curiosity likely springs from both theoretical interests and personal reflection. Even as young lay people – before we were linguists studying this – we recall having some conscious, if not intuitive, feelings that our native languages both propelled and restricted our learning of the additional languages we were studying. Of course, research over many decades – many hundreds, if not thousands of well-designed studies – has shown how far beyond intuitive anecdote the effects that previous linguistic knowledge has on additional language learning go.

The aim of this chapter is to offer the reader a panoramic yet comprehensive view of the theoretical issues and models that have attracted the most attention within generative approaches to L3/Ln morphosyntactic acquisition, with a particular emphasis on how transfer selection from previously acquired languages is hypothesized to apply. To the best of our ability, all models will be treated in an equal fashion. This does not mean that the description of each model will have or could possibly have the same level of detail, for justifiable reasons. To begin, models have appeared at different times, which correlates with more or less temporal opportunity to have been tested and to have gathered a critical mass of evidence. Moreover, not all models have had an equal amount of support from the published literature – a detailed analysis of which is the focus of Chapter 5 – and/or have had the same level of updating by their authors over time.