I began this book by stating that its goal was the study of the concurrent acquisition of two languages from birth, Spanish and English, by two developing siblings, Nico and Brennan. A number of related questions addressed in the various chapters of the book have been examined in a wealth of longitudinal data from the two siblings, ranging from the age of about 1;6 until approximately 6;0. The guiding theme has been the search for answers to the question of what effects on the siblings’ bilingual development could be accounted for by conditions internal to the languages (e.g., structural complexity), and/or by such crucial external factors as the amount of exposure to and use of English and Spanish over the first six years of life. In this final chapter I summarize the major findings and explore their implications for theories of bilingual language acquisition, the nature of heritage languages, and language contact and change.
Let us first restate that the general hypothesis investigated here is that the amount of exposure to and use of the two languages of a bilingual have a direct consequence on the level of proficiency attained in different grammatical domains. Situations of unbalanced exposure and use lead to the development of a weaker language in comparison with another, stronger language, normally the predominant language at the societal level. In Chapter 2 we saw evidence that the siblings had at all times been exposed to English and had used English more than Spanish, their heritage language. Consequently, English became their dominant language and they attained similar levels of proficiency in various aspects of this language, comparable also with those of English-speaking monolingual acquirers.