At present, many parents are faced with the dilemma of whether to raise their children bilingually or not. There is concern about possible consequences. Families from mixed backgrounds and parents who know different languages may desire to pass on the languages to their children for many reasons: To give children access to both languages, maintain the parent’s heritage or promote cognitive advantages.
Research shows that levels of native-like proficiency in bilingual individuals is determined by the age of acquisition (AoA) (Johnson and Newport, 1989). The critical period hypothesis explains that language sensitivity is at peak before adolescence, thus early exposure is advised. Lenneberg (1967) indicates that the ability to learn languages declines rapidly at about thirteen years of age. This does not imply that after such age it is impossible to learn languages. Lenneberg’s point is that during childhood languages are learnt almost automatically with little effort, but that after this stage learning requires a conscious effort. Speaking to infants in the different languages since birth enhances their learning of languages. For example, Eilers et al. (1982) found that infants exposed to bilingual input maintained the phonemic recognition of the two languages which was lost in their monolingual counterparts. This discrimination of linguistic sounds is lost at about ten months of age if the input for that particular language is absent.
The most researched cognitive advantage of bilingualism is metalinguistic awareness – the ability to bring linguistic knowledge and skills into conscious awareness, therefore, being able to transfer these across languages. This is evident in code-switching and translation. Children do not get confused with multiple languages and can differentiate between the them. Inference (shifting elements of one language onto another one) does not mean that the child cannot tell one language from another. It just means that proficiency levels are higher in the language being shifted, e.g. the lexical item label is only known or is more accessible, due to higher frequency of usage, in the other language.
However, some theorists still hold that learning another language comes at a cost, especially for children developing their first language. It is argued that the consequences are detrimental and have a long-lasting effect. For instance vocabulary is affected. Macnamara (1966) examined various comparative vocabulary tests and concluded that monolinguals have higher vocabulary levels compared to their bilingual counterparts. Therefore those that only speak one language have, presumably, better knowledge of that language. Other research, such as Pearson et al. (1993), consistently find similar results that support this perspective. However, the problem with these forms of testing is that they normally just test one of the languages of the bilingual (normally the culturally dominant one) and it does not take into account that the total size of vocabulary is larger in bilinguals if both languages were tested.
If children can have access to many languages at home parents or carers should definitely raise them bilingual simultaneously (at the same time), preferably since birth. It is best to provide dual language exposure with as much frequency and consistency as possible. The most effective method is the one parent one language rule. This is that each parent strictly speaks in one language only while the other parent uses the other without mixing the languages.
Altarriva, J. & Heredia, R. R. (Eds.). (2008). An Introduction to bilingualism principles and processes. New York, NY: Taylor & Francis Group
Bialystok, E. (2001). Bilingualism in development: language, literacy, & cognition. United States: The Press Syndicate of the University of Cambridge.
Hamers, J. F. & Blanc, M. H. A. (Eds.). (1995). Bilinguality and bilingualism. Cambridge: Cambridge University Press