ABSTRACT
IT IS COMMONLY THOUGHT that children learning two languages simultaneously during infancy go through a stage when they cannot differentiate their two languages. In fact, virtually all studies of infant bilingual development have found that bilingual children mix elements from their two languages. Researchers have interpreted these results as evidence for an undifferentiated or unitary underlying language system. In this chapter I will examine the empirical basis for these claims and I will argue that they are questionable because of serious methodological shortcomings in the research. I will then offer some tentative evidence based on speech perception studies and re-analyses of selected bilingual case studies that young bilingual children are psycholinguistically able to differentiate two languages from the earliest stages of bilingual development and that they can use their two languages in functionally differentiated ways, thereby providing evidence of differentiated underlying language systems.
