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The overall research aim was to examine the cognitive development of Irish and English speaking developing bilingual children. Caveats from previous research indicated the importance of considering language factors such as acquisition, proficiency and use, which informed a further research aim of conducting a comprehensive examination of language background information for immersion educated children, through multiple sources. Working memory and executive control were of particular research interest as prior international " research with developing bilingual children produced mixed findings for both working memory and executive control, with a link between these processes suggested to be impacted on by the bilingual experience (Bialystok, 2009), The first study examined whether any evidence supported an examination of working memory in the developing bilinguals, given Bialystok's (2009) suggestion of equivalent working memory performance in bilingual and monolingual children and adults. As findings indicated differences in the structure and development of working memory as compared with data from Alloway, Gathercole and Pickering (2006), further investigation into language factors impacting on developing bilinguals' language experience, were conducted to provide a context within which to consider findings. Language information was used to consider subsequent findings of research examining short term memory, working memory, receptive vocabulary, non-verbal lQ and executive control in developing bilingual and monolingual children. Findings overall indicated largely equivalent cognitive and linguistic performance of bilingual and monolingual children which must be considered in light of developing bilinguals' early acquisition and additive bilingualism. Notable cognitive differences between developing bilingual and monolingual children were found and replicated on a mazes memory task of visuospatial working memory. Research implications were considered throughout.

It is often said that bilinguals are not two monolmguals in one person. But what does this really mean, especially in the context of bilingual acquisition? Despite the upsurge of case studies of bilingual children since the 1990's, the main central issue within the literature has largely remained focused on the one-vs. -two-system debate. Earlier studies focused on the question of whether bilingual children had a single/fused system or two separate/differentiated ones. There are a growing number of more recent studies focusing, instead, on the relationship between the two languages in the developing language system of the child. The study on which this thesis is based is the first longitudinal group study of lexical development of French-English bilingual children. The study aims to investigate the nature of the developing bilingual lexicon and its impact on the development of syntax. The key questions addressed in this new body of research include: are bilingual children developing in the same way and at the same rate as their monolingual peers; are there cross-linguistic influences on bilingual acquisition; are there features, patterns or processes specific to bilingual acquisition? We report findings from a longitudinal group study of 13 children between 1;4 and 3;0 who are acquiring French and English simultaneously within the one person - one language framework. The originality of this study lies in several main points. First of all, a larger number of children have been studied systematically than in traditional longitudinal studies, which are usually based on either cross-sectional sample or on single cases. Secondly, the children in this study have been systematically selected according to a set of sociolinguistic variables. This allows meaningful comparisons of the results as well as possible future replications of the study with even larger samples or with other language pairs. Furthermore, the methods used in the study are innovative in that both quantitative and qualitative methods have been used longitudinally as opposed to onLy longitudinal qualitative data or only quantitative cross-sectional data. The profiling of the bilingual lexicon reports that bilingual children's lexical categories in each language develop in a parallel manner whether or not the children are dominant in a language. The results also show that their development is very similar to previously reported data for monolingual children. Despite current theories,the evidence suggests that bilingual children produce translation equivalents before the 50-word stage. However, I attempt to bring forward the idea that cross-linguistic equivalents are different from synonyms within a language and so bilinguals cannot be compared to monolinguals in that respect. This thesis also sets the age of first word combinations for bilingual children to around 1 ;8 while claiming that this is only achieved after each language has reached the 50-word milestone. Finally, great variability is noted throughout the thesis in terms of lexical development amongst the children. Some of the differences are explained by socio-linguistic factors such as parental strategies and language exposure. Therefore, the importance of accounting for such factors when studying bilingual language development is underlined. Our understanding of bilingual acquisition centrally contributes to our understanding of language acquisition in general. Similar features of bilingual and monolingual acquisition have been highlighted throughout this thesis. Thus, the bilingual lexicon has shown to develop at a similar rate and in a similar manner as the monolingual one despite being strongly influenced by individual socio-linguistic factors.

This PhD thesis contributes new empirical knowledge to the question of what paths bilingual acquisition of sound structure can take in early simultaneous bilinguals. The issues of language differentiation and interaction are considered in their relationship to language input, crosslinguistic structure and longitudinal effects. Two Russian-Scottish English subjects aged between 3;4 and 4;5 were recorded longitudinally. Russian was spoken in their families, and Scottish English in the community (Edinburgh, UK). The family environments were similar, but one subject had received substantially more input in Russian than the other one. We addressed the detail of their production of prominent syllable-nuclear vowels in Scottish English and in Russian with regard to their vowel quality, duration and vocal effort. Language differentiation and interaction patterns were derived by accounting for the language mode, and by statistical comparison of the crosslinguistic structures to the speech of monolingual peers (n=7) and adults (n=14). Subjects' bilingual results revealed both substantial language differentiation and systematic language interaction patterns. The extent of language differentiation and directionality of interaction depended on the amount of language exposure. Its directionality did not necessarily depend on the markedness of the crosslinguistic structures, and could be bi-directional for the same properties. Longitudinally, language differentiation increased, while interaction reduced. The amount of reduction depended on both language input and the structural complexity of the languages with segmental tense/lax contract and complex postvocalic vowel duration conditioning showing more persistent language interaction effects. The results confirmed the importance of language input. We showed that in bilingual phonological development language interaction should be considered as a normal but non-obligatory process. Besides, some structurally complex processes potentially explainable by 'markedness' (applied to isolated segments) could rather be explained by lexical and phonotactic factors.