In contrast to monolingual child language acquisition where children grow up with one language from birth onwards, children are referred to as bilingual or multilin-gual if they are exposed to two or more than two languages respectively from birth onwards (e.g., De Houwer, 2002). The type and amount of languages acquired by children depend on the surrounding speech community. Since multilingualism is not included in the empirical part, the focus in the present section lies on bilingualism.
The degree of proficiency in both languages of a bilingual speaker is not de-fined consistently in the literature. Definitions range from a very broad specification where the competence in both languages needs to be matured to a level that enables the speaker to fulfill basic needs (Grosjean, 1989) to a highly proficient knowledge with full fluency in both languages (Bloomfield, 1961: 56). A third view put for-ward for example by Haugen (1968) and McLaughlin (1978) suggests that a bilingual speaker must be able to “produce complete and meaningful sentences” in both lan-guages (Haugen, 1968: 10) even if one language is dominant compared to the other.
This view is adopted in the present thesis as it reflects the most frequently observ-able cases in which the two languages dominate in different areas across bilingual children. Language dominance may be observed in different situations or when inter-acting with different people in the child’s environment: for example, children may use one language during meals and the other when playing and yet other children speak one language with their family members and the other with their peers. Therefore, the terms “first language” and “second language” are generally not used in a context with children who grow up with two languages from very early on. Instead, the terms
“language A” and “language α” have been in use for the past decades (based on the idea of W¨olck, 1989; see also De Houwer, 2009: 2).
In general, bilingual competence may be achieved through two types of expo-sure: “language acquisition” and “language learning”. In the former case, a language is acquired in a natural setting whereas in the latter case, a language is learned through formal instructions in a school setting, which is usually the case for older children and adults (e.g., McLaughlin, 1978: 57, 59f.). The present thesis is only concerned with bilingual language acquisition due to the bilingual children’s natural exposure to both languages from birth onwards.
Bilingual acquisition can be further divided into simultaneous and sequential first language acquisition. Simultaneous language acquisition refers to bilinguals who acquire their two languages at the same time from birth onwards and who are exposed to both languages to a more or less equivalent extent. Sequential language acquisi-tion refers to bilinguals who first acquire one of the two languages and afterwards the other one. According to McLaughlin (1978: 73), the cutoff age is at three years. The author emphasizes that sequential as well as simultaneous bilinguals can achieve full fluency in both languages. Solely the type of exposure to the two languages marks the distinction between simultaneous and sequential acquisition.
In a bilingual language environment, children acquire both languages presup-posing that they have direct access to them. The best motivation to acquire both languages is given if children have a positive emotional relation to both languages, for example because each parent speaks one of the languages (e.g., De Houwer, 2009). A common observation in bilingual children are errors based on interferences from one
language into the other. This phenomenon has been investigated in several studies, but the amount of interaction differs among those findings. In production, R¯ uk¸e-Dravi¸na (1965) found that children substitute sounds from one language into the other. In this study, the easier uvular /K/ from the dialect of Sk˚ane was produced instead of the more difficult apical /r/ in the Czech language. Other investigators noted that children tend to use words from both languages within one sentence (e.g., Burling, 1959). In a study by Leopold (1947), the recorded child used her competence in one language to generate new words in the other language. The pronunciation of the new words was adapted to the language-specific phoneme inventory. Morpholog-ical mixing occurred for example in a Swedish-Estonian child investigated by Oskaar (1970). The child produced Swedish morphemes combined with Estonian endings.
Despite the occurrences of language interference, Bahrick and Pickens (1988) as well as Mehler et al. (1988) found that newborns are able to keep their two languages apart from the beginning. These studies indicate that the ability to extract prosodic structures helps early bilinguals to form separate entries for the two languages in their mental lexicon. Infants in a very early phase have not yet established a reper-toire of consonants and vowels for their native languages on which they could rely to distinguish between the two languages, but they are able to use prosody to distin-guish between them. A study by Shafer, Shucard and Jaeger (1999) confirm these results. This study closely investigated the types of languages to be distinguished.
The results show that three-month-old infants growing up in an English-speaking environment have more difficulties distinguishing English from Dutch compared to English and Italian as the latter two languages are prosodically more distinct. The fact that bilingual children are able to establish two separate grammatical systems is embraced as the “Separate Development Hypothesis” (see confirming experimental data by De Houwer, 2009: 52ff.). A widely accepted strategy to prevent language interference is achieved if the domains of language use are clearly separated as in the one-parent-one-language approach (McLaughlin, 1978: 94). Interference between languages is enhanced in case both parents speak both languages with the child and in case the two languages are closely related.
Concerning the language acquisition rate, previous studies found that bilingual children who are exposed to both languages from birth onwards take the same de-velopmental steps in the acquisition process as monolinguals. In certain linguistic aspects, bilingual children lag behind monolinguals by three to six months on aver-age (McLaughlin, 1978: 74, 91f.). For example, the lexicon for one languaver-age develops faster in monolinguals and their error production is lower compared to bilinguals.
This is due to the fact that bilingual children have the additional task of building two separate language systems. McLaughlin states that bilinguals and monolinguals make use of the same set of strategies in language processing which serves as a basis to dissociate language-specific properties. Curtin, Byers-Heinlein and Werker (2011) reiterate this observation for perception.
The milestones in bilingual child language acquisition are outlined for example by De Houwer (2009: 5). The author states that exactly like monolingual infants, bilingual infants start to babble in syllables between six months and one year of age.
By the age of one year, bilingual babies possess the perceptional competence of a high number of words and phrases in both languages. At the beginning of the first year of life, bilinguals begin to speak their first single words that are nearly adult-like in one or even both languages. Bilingual children start combining two words by the age of two and short sentences appear between the second half of the third year and age three. Around the age of four, bilingual children are able to produce sentences including subclauses. De Houwer (2009: 5–7) reports that variation exists across bilingual children with respect to language development and even within children with respect to their language competence in both languages. The author attributes this variation in bilingual children to different amounts of speech input across the two languages. Usually, the language competence of monolingual and bilingual children is equalized when they enter school. The following section is concerned with effects that have been discussed to have an impact on the vocabulary development in child language acquisition in general.