Verified by 
Post written by François Grosjean.
In an earlier post, we noted that when bilinguals are communicating with others, they have to ask themselves two questions: Which language should they use? And can they bring in their other language(s). When they are in the presence of monolinguals, or of bilinguals with whom they only share one language, or with whom they do not feel they can intermingle their languages (see here), they have to use only one language and not let their other language(s) intervene.
It never fails to impress me how well bilinguals can keep to just one language. If, in addition, they speak the language fluently and have no accent in it, then they may "pass" as monolinguals. How many of us have not been taken aback to hear a monolingual, so we thought, suddenly start speaking another language we knew nothing about. The surprise is probably as great as if we had heard Natalie Wood suddenly switch into fluent Russian during the movie West Side Story. Although artistically impossible, it was linguistically feasible as she had been raised in a Russian-speaking family and was bilingual in Russian and English.
Researchers have long been interested in the way bilinguals manage to control the language they are speaking monolingually and keep out their other language(s). Cognitive scientists such as Ellen Bialystok at York University examine the executive control needed for such an operation and neuroscientists study the neural structures that supervise the selection of the appropriate language.
How "language tight" is the process? Despite the fact that bilinguals are adept at keeping out their other language(s) when they are speaking or writing monolingually (as I'm doing right now, keeping out my French), the other language(s) may sometimes seep through, most of the time in the form of interferences, that is deviations from the language being spoken (or written) stemming from the influence of the other, deactivated, language(s).
Interferences, also called transfers, follow bilinguals throughout their lives, however hard they try to filter them out. They are, in a sense, their uninvited hidden companions! Static interferences reflect permanent traces of one language on the other such as a permanent accent (see a post on this), specific syntactic structures taken from the other language, and so on.
Dynamic interferences, on the other hand, are the fleeting intrusions of the other language as in the case of the accidental pronunciation of a sequence based on the rules of the other language, or the momentary use of a word or grammatical structure from the wrong language. Bilinguals are not usually aware of these latter types of interferences. It is only when their interlocutor asks what they meant by word X, or corrects their syntax, or looks at them in a strange way that they realize, after the fact, that the other language has slipped in.
Bilinguals are often left with the feeling that they were sure that X was a correct word or structure in the language they were speaking when in fact that was not the case. Things get even more difficult when they are dealing with homographs or homophones. Einar Haugen, a pioneering researcher in bilingualism, gives the following example. When a young bilingual Norwegian American was asked by a stranger in English where his father was, he replied quite spontaneously, "Oh, he's in the stove," This was due to the fact that "stova" means a living room in Norwegian.
This said, monolinguals who live or work with bilinguals grow accustomed to language that can be influenced by the other tongue. They get used to hearing an accent, a strange sentence structure or a word that is not quite appropriate, and this often makes communication easier.
The positive side of interferences is that they often render what is said more original and less stereotypical. Thus, the prose of bilingual writers such as Joseph Conrad and Samuel Beckett, among others, is greatly enriched by the influence of their other language(s).
Eva Hoffman, the contemporary author of Lost in Translation, and herself bilingual in Polish and English, states this fittingly when she writes: "Each language modifies the other, crossbreeds with it, fertilizes it. Each language makes the other relative. Like everybody, I am the sum of my languages." (p. 273).
Note: Dynamic interferences are discussed further here.
References
Hoffman, Eva (1989). Lost in Translation. New York: Penguin.
Grosjean, François. "Speaking and writing monolingually". Chapter 6 of Grosjean, François (2010). Bilingual: Life and Reality. Cambridge, Mass: Harvard University Press.
"Life as a bilingual" posts by content area: http://www.francoisgrosjean.ch/blog_en.html
François Grosjean's website: www.francoisgrosjean.ch
