What role, if any, does typological similarity play in bi-/multilingual language acquisition?  This question has been a principally guiding one in at least two important literatures seeking to understand the patterns and constraints of grammatical cross-linguistic influence and representational transfer: simultaneous or very young child bilingualism and adult third (or more) language acquisition. Although individuals who would easily qualify as heritage speakers (HS) feature prominently in the child bilingualism literature, HSs are very under-represented in the adult multilingualism literature. This is especially unfortunate when one considers the fact that HSs often acquire subsequent third or more languages—at least in a chronological sense—upon learning their 1st foreign language. Thus, they should not be grouped together with true L2 learners who are monolingual at the start in so-called L2 acquisition studies. Furthermore, one could make a case that formal learning of the HL later in life could be viewed as a special case of third (or more) acquisition (Polinsky, 2015). Does linguistic distance between what is already acquired in HSs matter the same way it does for those who have acquired an L2 and attempt mastery of an L3 (Rothman, 2015; Rothman, González Alonso & Puig-Mayenco, 2019)? If so or if not, what does this reveal for various questions of theoretical importance and, crucially, what does this entail for the needs of HSs as they learn additional languages and/or engage with formal learning of their HL in (young) adulthood? The present talk addresses these questions.

Our talk is divided in two main parts.  First, we introduce the theoretical landscape of adult multilingualism regarding linguistic distance effects in initial grammars and development of subsequent language learning. We will place an emphasis on the small, but insightful literature featuring HSs in this landscape with respect to the questions laid out above. In the second part of the presentation, we will present results from the ongoing PhD dissertation project of Sergio Miguel Pereira Soares. The study combines a mini-grammar learning paradigm (using Latin) and neuroimaging (Electroencephalography: Event Related Potential (ERP), resting state and power oscillation/mean coherence analyses) to unpack how linguistic similarity may (or may not) matter for Italian HSs dominant in German when learning an additional language as well as the role linguistic experience (i.e. how they manage the use and distribute the labor of Italian and German) plays in domain-general neurocognitive effects (Biaylstok, 2017; DeLuca et al. 2019) of HL bilingualism over their lifespan.