Most multilinguals have literacy skills in at least one other language, and a central question of both bilingualism and plurilingualism research is whether – and to what extent – language skills may be interdependent. Previous research allows for a strong argument that some skills have an interlingual basis. However, it is notoriously difficult to develop research designs to investigate clearly language interaction and allow for pedagogical recommendations. For example, when such skills are addressed in between-group designs, comparing L2 speakers to L1 speakers of the same language – which is approach in the majority of research –, results indicating parallels (and differences) of language skills suffer under questionable validity.
This is a particularly frustrating problem when investigating literacy skills in multiple languages, which may more directly reflect a shared proficiency in different languages (i.e. are transversal, cf. Berthele/Lambelet 2018) than a transfer of language skills from one language to another. Here, within-group research paradigms are called for. Such designs allow for a deeper understanding of multilingual literacy, as they can control for individual differences and potentially reveal how writing skills developed in different languages might parallel or even support each other. The paper first discusses research designs to investigate interdependence of literacy skills in different languages, differentiating between possible approaches to studying transversality vs. transfer of literacy aspects. It then presents two research projects investigating both transversality and transfer of writing skills in very different multilingual populations: university students studying an L2 and Grade 6 heritage language speakers. The results of both studies indicate that, while certain aspects and processes seem to be transversal, specific task characteristics will affect the degree and type of transversality evident in the research data.