Down syndrome is the most common chromosomal disability which impacts development on a range of aspects including cognitive and linguistic abilities. Consequently, concerns have been raised as to the extent to which children with this particular language profile are capable of acquiring two (or more) languages. This thesis aimed to address this issue by establishing the linguistic profiles of children with Down syndrome acquiring two languages, specifically Welsh and English in comparison to suitably matched control groups. The linguistic setting in which this research is based (i.e. Welsh-English bilinguals in Wales) is firstly introduced and consideration is given to some of the overarching debates and theories surrounding current literature in bilingualism. Following this, the research to date concerning bilingual language development in children with a developmental disability and more specifically children with Down syndrome is presented.

Children were recruited into one of four target groups and matched on developmental age as well as other important variables such as chronological age and SES. The groups were: Welsh-English bilinguals with Down syndrome, English monolinguals with Down syndrome, typically developing Welsh-English bilinguals and typically developing English monolinguals. The children were assessed on a range of cognitive and linguistic assessments in Welsh and English order to build a comprehensive profile of the children’s language abilities. These assessments include measures of core, receptive and expressive language, phonological awareness and working memory. Four case studies of bilingual children with a dual diagnosis of Down syndrome and an Autism Spectrum Disorder are also presented.

Key findings show no negative impact of bilingualism compared to monolingual groups, both for the typically developing children and importantly, also the children with Down syndrome. Comparable profiles are evident for bilinguals and monolinguals with Down syndrome whereby corresponding patterns of strengths and weaknesses were found. Bilinguals with Down syndrome were also comparable to typically developing bilinguals in the additional language (Welsh). The results reveal that there is no evidence to support the view that children with Down syndrome should only be exposed to the majority language as the children in this study were developing appropriate language abilities which were commensurate with their linguistic backgrounds. The case studies document that even children with dual-diagnoses and substantial intellectual and linguistic challenges were developing appropriate bilingual abilities.

In the context of Wales, the findings challenge the view that children with Down syndrome should be placed in English-medium educational settings in favour of bilingual or Welsh-medium settings. On the contrary, bilingual services and provisions should be made available to this population, whilst also considering each child’s circumstances and ensuring that there is appropriate support in that setting. Overall, the results increase understanding of bilingualism in Down syndrome and contribute to the growing body of literature documenting no adverse impact of bilingualism for children with developmental disabilities. Implications arising from this research are discussed which include policy, clinical assessment, interventions, professional recommendations and family support. Future research directions are also proposed.