This article looks at the general effects of bilingualism on cognitive development and highlights the specific effects of bilingualism on third language acquisition. First, it examines the effects of bilingualism on cognitive development, metalinguistic awareness and communicative skills and then, it focuses on the specific effects of bilingualism on third language proficiency by discussing the results of several research studies conducted in different settings and contexts. An important distinction is made between studies focusing on the acquisition of general proficiency in the third language and studies that examine specific aspects of third language proficiency. The effects of bilingualism on third language acquisition are discussed as related to the experience acquired by third language learners in the process of second language acquisition and the effects of bilingualism on metalinguistic awareness and communicative skills. These effects are also discussed with reference to the concepts of additive bilingualism(Lambert, 1974), the threshold hypothesis(Cummins,1976) and the distinction between control of attention and analysis (Bialystok, 1991).
In this study of the placement of sentence negation in third language acquisition (L3), we argue that there is a qualitative difference between the acquisition of a true second language (L2) and the subsequent acquisition of an L3. Although there is considerable evidence for L2 influence on vocabulary acquisition in L3, not all researchers believe that such influence generalizes to morphosyntactic aspects of the grammar. For example, Håkansson et al. (2002) introduce the Developmentally Moderated Transfer Hypothesis (DMTH), which incorporates transfer in Processability Theory (PT). They argue against syntactic transfer from L2 to L3. The present study presents counter-evidence to this hypothesis from two groups of learners with different L1s and L2s acquiring Swedish or Dutch as L3. The evidence clearly indicates that syntactic structures are more easily transferred from L2 than from L1 in the initial state of L3 acquisition. The two groups behave significantly differently as to the placement of negation, a difference that can be attributed to the L2 knowledge of the learners in interaction with the typological relationship between the L2 and the L3.