Background and aims: Bilinguals have been argued to show superior executive functioning (EF) skills compared to monolinguals, due to long-term experience in dual-language management. Despite extensive investigation during the past twenty years and a persistent debate regarding the existence of a bilingual advantage in the EF skills of bilingual children, the results remain mixed; namely bilinguals might perform better, worse, or similarly in EF tasks compared to their monolingual counterparts. The current thesis investigates the EF skills as well as the language skills of Greek-English bilingual children attending a Greek supplementary school in England. The cognitive performance of the bilingual children is compared to that of Greek monolingual and English monolingual control groups. This thesis has three aims: i) to investigate the performance of Greek-English bilingual children in EF tasks compared to monolingual Greek and monolingual English children after closely matching them on a large number of relevant variables, ii) to propose a novel approach to evaluate the EF performance of bilingual and monolingual children holistically, and iii) to explore the variables that might affect scores in language tasks of expressive/receptive vocabulary and receptive grammar of the Greek-English bilingual children attending a Greek supplementary school in England and investigate if and how the exposure to a supplementary school setting affects these scores.

Methods: A total of 109 children took part in this study, namely 39 Greek-English bilingual children, 45 Greek monolingual children and 25 English monolingual children aged 63-153 months. Bilingual Greek-English children were recruited from a Greek supplementary school in the north-west of England. We use an array of executive functioning tasks which tap into inhibition, updating and shifting, as operationalised by Miyake et al. (2000). We use k-means nearest neighbour methods to match bilingual to monolingual children on a wide array of control variables and frontier methodologies which allow us to jointly consider multiple tests and metrics in a new measure; the technical efficiency (TE).

Results: In the first study (Chapter 3) the results suggest that bilinguals’ accuracy on executive function tasks is at par to their monolingual peers. However, bilinguals are faster in the inhibition and the working memory task compared to the English monolingual control group and were comparable to the Greek monolingual control group. In the second study (Chapter 4), bilinguals seem to have superior technical efficiency than their monolingual counterparts. We find bilinguals to be around 6.5% more efficient than their monolingual counterparts in executive function. Overall, we find that the TE analysis utilises the information in a more efficient way; and can thus yield similar results to the more complex MANCOVA analyses while using fewer resources. In the last study (Chapter 5), the results suggest that language use significantly predicts performance in Greek vocabulary and grammar tasks whereas age in months significantly predicts performance in English vocabulary and grammar tasks. Years in supplementary school do not significantly predict neither scores in the Greek tasks nor in the English tasks.

Conclusions: This thesis has both methodological and educational implications. Namely, we contribute to the literature in three distinct ways. Firstly, we take into consideration the majority of potential variables such as age, non-verbal intelligence, years of education in the supplementary school, years of education in a Greek medium school, language proficiency in both languages, language use, socioeconomic status (SES), music ability. To the best of our knowledge no studies have controlled for both languages of the bilingual group of children. In the first research study (Chapter 3) we control for both languages, Greek and English, using factor analysis to take as many variables as possible into consideration. Secondly and directly following the first study, we propose a novel approach to evaluate performance in the EFs of bilingual and monolingual children. In this way we deal with the extended array of executive function tasks and metrics used across the literature. Lastly, we investigate variables that affect the performance in vocabulary and grammar tasks in Greek and English and if the exposure to a supplementary school setting influences the performance in both the heritage (Greek) language and the medium of mainstream education (English). In this way, we investigate the role of the educational setting in bilingualism, namely a supplementary school, a topic that has attracted minimal attention by the relevant literature.