The current thesis investigated the topic of cross-linguistic influence (CLI) at three
different developmental stages of adult third language (L3) acquisition of Norwegian by
sequential first language (L1) Spanish – second language (L2) English speakers (n = 18). Using
a mixed methods approach consisting of a grammaticality judgment task and a closed-ended
questionnaire on linguistic proximity, the study tested the acquisition of four Norwegian
morphosyntactic properties: (i) post-nominal possessives gender agreement, (ii) number
concord on definite articles, (iii) adjective placement, and (iv) subject pronoun expression
(SPE). Based on the subtracted language groups experimental design (Westergaard et al.,
forthcoming), L3 learners’ performance was compared to the ones of Spanish (n = 5) and
English (n = 13) L2 learners of Norwegian. The study also counted with a native group (n =
15). Predictions were grounded on five main L3A models, all which diverge in terms of the
source (L1, L2, or both) and nature (holistic or property-by-property) of CLI. Altogether, results
were consistent with the L1 Factor (Hermas, 2010, 2014), as findings indicated L3 learners’
performance was solely influenced by their L1 Spanish, whereas both the linguistic proximity
and psychotypology were overridden. In two of the conditions, the L1 Spanish groups
performed significantly different from the L1 English group, having outperformed the latter in
the treatment of possessives gender agreement, indicating facilitative CLI from L1 Spanish,
and being outperformed by the same in the judgement of SPE sentences, indicating nonfacilitative CLI from L1 Spanish. On the other two conditions, all learning groups performed
alike, showing overall either high or low rates of accuracy. These last findings suggested the
linguistic complexity and frequency of input of individual properties to be an important
triggering factor of CLI. Finally, the L3 proficiency level was found to be a strong factor in
CLI, as transfer effects were observed to be more salient at beginner (A1) and elementary (A2)
L3 proficiency stages as compared to the pre-intermediate (B1) level
