The most frequently expressed concern is whether early bilinguals will be able to differentiate their languages. Research on child bilingualism has demonstrated that this is indeed the case. Children acquiring two languages simultaneously are able to differentiate their lexical and grammatical systems from very early on. Ocassional language mixing is not an indication of an underlying unitary system, fusing two or more linguistic competences. Rather, mixing is a performance phenomenon. Most mixed utterances are instances of code-switching, i.e. linguistic behaviour constrained by grammatical and sociolinguistic principles. Early mixes may also result from a choice of language that is unexpected from an adult perspective. Choosing the adequate language in bilingual settings requires sociolinguistic knowledge that is acquired in the course of children’s socialization. Parents can support bilingual L1 acquisition by their own linguistic behaviour. Following the ‘one person, one language’ (OPOL) principle is a method that has been applied successfully for over 100 years.

Parents often wonder whether children acquiring two languages simultaneously will attain native competences in both. In order to answert this question, we must contrast bilinguals’ language development with that of monolinguals. This requires a basic understanding of monolingual first language (L1) acquisition: It is always successful, happens at a fast rate and uniformly in that children proceed through identical developmental phases. These properties can be explained if we assume that L1 acquisition is guided by an innate Language Making Capacity (LMC), comprising general-purpose and language-specific cognitive principles. The latter make up the The Language Acquisition Device (LAD). Unversal Grammar (UG), a subcomponent of the LAD, consists of principles specifying formal properties of human languages. The simultaneous acquisition of two languages from birth exhibits the same properties as L1 development. Developmental sequences are non-distinct in bilinguals and monolinguals, and bilingual development happens at appproximately the same rate. It can thus indeed be qualified as bilingual first language acquisition.