Helen Pfaff, Saveria Toscano

Children who grow up bilingual show deficits in both their languages.


Why do children grow up learning more
than one language?

Nowadays, thanks to factors like globalization, bilinguals may represent around half the world’s population and the number of children being raised in bilingual homes is thriving. Children who grow up learning two languages from birth are considered simultaneous bilinguals – as they are exposed to two languages simultaneously. Early research on bilingualism held the view that these children who learn two languages from birth on will be confused and unable to differentiate between their languages. Code-switching, which occurs when a speaker alternates between two languages, was seen as evidence that bilinguals are unable to separate their languages. In that sense, bilingualism could result in delays in language development and possibly even incomplete competence. Meantime, a large body of research has refuted these early assumptions, generally ascribing cognitive advantages to bilingualism. But what about the well-known myth that bilingual children show deficits in both their languages? This blogpost will shed light on some important issues.


Learning two languages as ‘first languages’. A person who is a simultaneous bilingual goes from speaking no language at all directly to speaking two languages.

Learning one language after already knowing another. This is the case for all who become bilinguals as adults, as well for many who become bilingual earlier in life. 

Learning two languages simultaneously from birth stretches the limits of infants’ ability to acquire language and they therefore will be confused and unable to differentiate between languages if their parents use both at home. 

Speech Perception

Children are exposed to language even before their birth and thus show preference for the language used by their mothers. When mothers plan to raise their children in a bilingual environment and consequently speak more than one language during pregnancy, these infants show equal preference for both languages. At birth, infants can rely on rhythmic properties to discriminate between languages. While early research suggested that bilingual children’s language distinction would take place later in development, more recent findings have shown that they are not delayed compared to monolinguals. Hence, the current evidence in speech perception suggests that the same learning mechanisms are available to both monolingual and bilingual acquisition and that bilinguals perform as accurately as monolinguals in their first year of life.

Speech Production

Research on speech production shows that phonetic accuracy is generally determined by the children’s developing articulatory abilities and their knowledge of the phonological system. The findings were inconclusive. Bilingual children were delayed relative to monolingual children in the acquisition of the vowel length contrast but resembled monolingual children in the acquisition of the L2 vowels. These different results may be reconciled by considering the phonological systems of the two languages. Another reason why bilinguals differ in their speech production scores might be language dominance. Monolinguals hear and produce only one language at the same time and thus, can involve more capacity for it. Bilinguals performed very similar to monolinguals in their dominant language. Furthermore, the age of acquisition is one of the most important variables when it comes to speech production: Accuracy in monolingual and bilingual groups of children correlated with age of acquisition whereby early-acquired sounds were more accurate than middle- and late-acquired sounds. As a result, research showed no clear indication of bilinguals having a disadvantage in speech production compared to monolinguals. 

Word Learning

One central question in word learning deals with understanding the techniques children use to build their lexicon. Monolingual children, for instance, often rely on the application of the mutual exclusivity constraint, assuming, that a new label refers to an unknown object. Bilingual children in contrast do not comply with the mutual exclusivity constraints, which suggests that they apply flexible learning strategies in consequence of the linguistic context they are raised in. But do these different strategies in word learning result in varying vocabulary sizes? It is indeed a typical finding that bilingual children have smaller vocabularies in each of their languages when compared to age-matched monolingual children. Such research, however, is most often based on the assessment of only one language. Naturally, children who are exposed to two languages receive less input in each of their languages – and it’s evident that crucial factors contributing to word learning are quality and quantity of input. In consequence, bilingual children’s lexicon in each language is smaller than this of a child receiving input in only one language. Unsurprisingly, researchers who compared the language development of bilingually and monolingually developing children have found that bilinguals are in fact disadvantaged on measures of vocabulary in single language comparisons. When both languages were taken into account, both groups were comparable in terms of total vocabulary size, though. In consequence, as input contributes essentially to word learning, it seems perfectly normal that bilingual children are delayed in the rate at which they acquire each of their languages.

Morphology & Syntax

Studies have shown that throughout the stages of early language acquisition there is a tight relationship between developing grammar and the size of the lexicon in monolinguals, which is not predicted by other factors such as general cognitive abilities. Here, the question arose whether bilingual children acquire the grammars of each their languages separately, relying on each language’s vocabulary, or if they use their total vocabulary from both languages, which could lead to deficits in their grammatical abilities. In such studies it has been shown that bilingual children indeed acquire the grammar of each their languages separately, treating their two languages mainly as independent and autonomous systems. Since there is a positive correlation between grammar and the size of the lexicon – and bilingual and monolingual children differ in that size during early language acquisition – deficits or delays showed by bilingual children in their grammatical abilities during that stage should not be of concern.  

Cognitive Benefits

Although the consensus in early research was that bilinguals have difficulties in learning as they were outperformed by monolinguals in cognitive tasks, meantime a considerable body of evidence suggests that bilingualism actually holds a number of cognitive benefits, with the greatest advantages for early bilinguals. For instance, bilinguals are more readily able to control their attention, which can be explained by the regular use of two languages that requires inhibiting one language at a time and select the target language. They demonstrate greater metalinguistic awareness – the ability to think about language – as a result of acquiring and maintaining two languages. Moreover, bilinguals show enhanced skills in abstract or symbolic reasoning and creative and divergent thinking, problem solving, or metacognitive awareness, to name just a few examples.


It is evident that there is large variability in early lexical development among monolingual, bilingual, or even impaired learners, as language acquisition is dependent on several individual factors such as learning environment or quantity and quality of input. When comparing across groups, however, simultaneous bilingual children achieve the same fundamental milestones in language development as monolingual children. The emergence of first words and first word combinations takes place within the same time frame although bilinguals usually experience less exposure to each language than monolinguals. Comparisons between language specific impaired monolinguals and impaired bilinguals have shown that even under this condition both groups exhibit similar patterns of grammatical development. Still, the language proficiency profiles of bilinguals will also in adulthood differ from those of monolinguals, since acquisition and use of each language is tied to specific contexts while monolinguals use the same language in all contexts. Nevertheless, children who learn two languages simultaneously can acquire full competence in both languages.

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Hoff, E., Core, C., Place, S., R