Multilingual advantage for adult learning of morphosyntax and the effect of processing modality Martha Black Western University Abstract This study reports on a multilingual advantage detected in adult Spanish learners’ (N = 24) performance with grammatical gender during a structured and self-paced written recognition task and a speeded oral picture description task. Metacognitive awareness and self-reported proficiency were examined to determine to what extent these factors may contribute to differentiated performance between learners with multilingual experience as compared to those with exclusively bilingual experience. Findings suggest that even novice-level multilingual exposure beyond the native language (English) and the current target language (Spanish) affords significant advantage in performance with grammatical gender as a problematic area for adult learning of morphosyntax during speeded (i.e. online) language processing with no planning time. Furthermore, this multilingual advantage does not appear to be directly related to the additional language typology, target language proficiency, nor metacognitive awareness. Rather, the observed multilingual advantage appears to point to potentially differentiated processing cues and language representation for language learners with novice-level multilingual experience. Keywords: multilingual effect; multilingualism; adult second language acquisition; instructed language learning; morphosyntax; Spanish; English 1. Introduction Most of the world's population is bilingual or multilingual. However, research on monolingualism has largely been prioritized over multilingualism as monolinguals have traditionally served as a sort of benchmark or control by which to compare bi- and multilinguals (Cummins, 2007; Cummins, 2009). Nonetheless, it is essential to recognize that multilingualism is a dynamic cognitive system that is qualitatively different from the cognitive systems of monolinguals (Cummins, 2007; Cummins, 2009). This difference is relevant for language acquisition research since bilingual and multilingual speakers have an empirically-demonstrated advantage when it comes to the task of learning a third (L3) or additional (Ln) language in lexical (e.g., Kaushanskaya & Marian, 2009), phonological (e.g., Tremblay & Sabourin, 2012), phonetic (e.g. Antoniou et al. 2015), and syntactic (e.g., Klein, 1995) domains. This multilingual advantage has been explained in terms of metalinguistic awareness, learning strategies, linguistic repertoire (e.g., Cenoz, 2013), increased language aptitude with experience (Thompson, 2013), affective factors such as increased confidence and reduced anxiety (Dewaele, Petrides & Furnham, 2008), and general changes to the cognitive-linguistic system (Hirosh & Degani, 2018) that are associated with multilingualism during subsequent language learning. The present study is motivated by a notable lack in previous research that specifically examines the effect of multilingualism on acquiring a specific target language (TL) grammatical structure. This study therefore aims to examine how novice-level knowledge of an additional language beyond L1 English and the current TL (Spanish) may affect accuracy scores with grammatical gender as a problematic feature of morphosyntax in adult learners (N = 24) of varying linguistic backgrounds (bilingual and multilingual) in the instructed context during the initial stages of TL acquisition. Furthermore, I examine if processing modality (i.e., offline vs. online) may modulate, to some degree, the effect of multilingual language experience on TL grammatical performance. I report a positive and significant effect of novice-level multilingual experience and further explore the underlying factors of this effect through an examination of the linguistic features inherent to grammatical gender, processing modality in differentiated task performance, self-reported proficiency, metacognitive awareness, and the typology of the additional language reported to determine if and to what extent these factors may elucidate the nature of the observed multilingual advantage. Grammatical gender was chosen as the target structure because it is particularly problematic for adult learners of Spanish, leading to persistent and fossilized errors despite overall increasing proficiency (e.g., Fernández-García, 1999; McCarthy, 2008; Montrul, Foote, & Perpiñán, 2008) and because it presents several inherent features—namely morphological marking, noun gender class, and domain of agreement (i.e., determiner and adjective)—with which to examine differentiated performance not only overall, but with