Introduction
 
Many children around the world acquire more than one language during the preschool years. For some children, acquisition of two or more languages occurs simultaneously, such as when parents regularly use two languages with their child from birth. Alternatively, it can occur successively, such as when children are exposed to and speak only one language at home during the first one or two years of life and then attend daycare, preschool or elementary school programs where another language is used.
 
There are many important reasons for raising children bilingually. First and foremost are personal and family reasons. If members of the immediate or extended family include those who speak different languages, then it is a benefit for everyone if children speak these languages as well. In addition, research has shown that children, and even adults, who have advanced levels of proficiency in two languages have cognitive advantages when compared to monolinguals (Bialystok & Martin, 2004). As well, individuals who are bi- or multilingual can take full advantage of the opportunities afforded by globalization, given the enhanced value of being multilingual in the job market, and having the ability to use the Internet to seek out additional sources of information.
 
Despite the numerous advantages associated with bilingualism, parents, educators, and professionals often express concerns about raising children bilingually (see Genesee, 2009). In this Encyclopedia entry, research that addresses some common concerns about raising children bilingually is discussed. This entry focuses on children who grow up learning two languages from birth, known as simultaneous bilinguals. The findings reviewed do not necessarily apply to children who acquire a second language after the first language has been established, around 2 to 3 years of age.
 
Key Research Questions
 

1. Do children who are raised bilingually from birth go through an early stage in development when their two languages are fused?
2. Is child bilingual code-mixing a sign of confusion?
3. Are simultaneous bilinguals delayed in their language development and is their competence compromised?
4. How much exposure is enough for bilingual children to be competent in two languages?
5. Should parents use the one-parent, one-language rule with young bilingual children?
6. Are children with language impairment or at risk for language impairment at greater risk for language development when learning two languages than when learning one?

 
Recent Research Results
 
Do children who are raised bilingually from birth go through an early stage in development when their two languages are fused?
Early theories of bilingual acquisition claimed that simultaneous bilingual children go through an initial stage in development when their two languages are combined to form a single system.  According to these theories, they only separate their two languages later in development, around 3 years of age (Volterra & Taeschner, 1978). However, research indicates that there is no evidence that children go through such a stage; rather, they acquire distinct languages from the beginning. When bilingual children’s language use is carefully examined, it is noticeable that even two-year old children in the earliest stages of bilingual development use their two languages differentially and appropriately with others (e.g., Genesee, Paradis, & Nicoladis, 1995). For example, bilingual children as young as two years of age know to use the mother’s language with the mother and the father’s language with the father. They also use the appropriate language with monolingual strangers.
 
Yet another source of evidence against the fused language hypothesis comes from an in-depth examination of bilingual children’s acquisition of phonology (sounds), vocabulary, and grammar in their two languages. The results from this research show that they acquire distinct phonological, vocabulary, and grammatical systems in each language (Genesee & Nicoladis, 2006), even when the two languages are very different from one another (like English and Estonian). Bilingual children who are infrequently exposed to one of their languages may not be completely competent in that language and may use the sounds, words, or grammar of their stronger language to express themselves when speaking their weaker language. They may also score lower than monolinguals on standardized language tests (Thordatdottir, 2011). We discuss the importance of exposure in more detail later.
 
Is child bilingual code-mixing a sign of confusion?
Mixing languages in the same sentence or conversation is referred to as code-mixing. Adults who hear young bilingual children code-mix are often concerned that this is a sign that their two languages are mixed up, or fused, and that the child is linguistically confused. Code-mixing has been studied extensively in both adults and children. Research on child bilingual code-mixing indicates that it is not a sign of confusion (Genesee, 2003). As we noted in the preceding section, we know that these children are not confused because they are able to use their two languages appropriately with different people.
 
Why do they code-mix then? When children code-mix, it is often to fill in gaps in their vocabulary in one language. Bilingual children rarely know the exact same words in both languages. As a result, they might use a word from one language while speaking the other language because they do not know the word in the language they are using. Sometimes bilingual children code-mix because there are specific concepts or meanings that can be expressed easily or only in one language. If a child wants to express that meaning or concept, he/she will have to use the word from the language that it belongs to even if it means mixing languages. In most cases, this strategy works because often the adults in the child’s life know both languages too. As bilingual children get older, they mix languages in order to express their bilingual identity when they are with other bilinguals. Bilingual adolescents and adults also often mix with other bilinguals because it reflects their identity.
 
Even more compelling evidence that child bilingual code-mixing is not a sign of confusion comes from studies of the grammaticality of their mixed utterances. This research has shown that bilingual children’s mixed utterances are grammatically correct most of the time (Paradis, Nicoladis & Genesee, 2000). If a child uses words from both languages in the same sentence, he/she will insert the words in the sentence so that the sentence is grammatically correct. For example, when a French-English bilingual child says “J’aime chocolate cake beaucoup,” he/she has switched from English to French and back again at points in the sentence that make the sentence grammatically correct. Bilingual code-mixing is a way for young bilingual children to use all of their linguistic resources while they are still acquiring linguistic competence and they can do it without violating the grammatical constraints of either language.
 
Are simultaneous bilinguals delayed in their language development and is their competence compromised?
Adults often think that learning two languages is harder than learning one and that it will take children longer to learn both languages. Contrary to this belief, research shows that bilingual children achieve the same basic milestones in language acquisition at about the same age as children who learn only one language (Genesee & Nicoladis, 2006). More specifically, bilingual children begin to babble at the same time as monolingual children (Maneva & Genesee, 2002). They say their first words at the same age as monolingual children; and they start to produce multi-word sentences at the same time (see Paradis, Genesee & Crago, 2011, for a review).  However, there are differences between bilingual and monolingual children. For example, it has been found that 15-month-old infants learning two languages may be delayed by 2 or 3 months in their ability to distinguish new words that differ in small ways (e.g., “bit” vs. “bet”) in comparison to monolingual children (Byers-Heinlein, Burns, & Werker, 2010). In the long run, this short delay has an insignificant effect on bilingual children’s overall language ability. Researchers suggest that this delay is a kind of flexibility that allows bilingual children to be open to the great diversity of words to which they are exposed in both languages.
 
It is also common to find that preschool bilingual children know fewer words in each language than monolingual children, when each language is examined separately.  However, if you examine their total vocabulary in both languages by counting all known words for different concepts, bilingual children have vocabularies that are the same size or larger than those of monolingual children (Pearson, Fernández, & Oller, 1993). Differences in vocabulary between bilingual and monolingual children may be due to several factors, none of which are signs of deficit or impairment. Children have limited memory capacities and, thus, all children initially have limited vocabularies. Because bilingual children must share their limited memory with two languages, they can store fewer words in each language than monolingual children, but the same number of total words between the two languages. Bilingual children may not have completely equivalent vocabularies in both languages because they often learn each language from different people. As a result, the child will learn vocabulary that is specific to the topics discussed in each language. Even where differences in vocabulary persist into adulthood, it is important to remember that bilinguals are able to communicate and function in two languages whereas monolinguals can function in only one.
 
How much exposure is enough for bilingual children to be competent in two languages?
We do not have definitive scientific evidence to tell us the minimum amount of exposure to language required to ensure that bilingual children acquire the same linguistic competence as monolingual children. Researchers generally find that there is a positive correlation between bilingual children’s exposure to each language and their proficiency in that language (e.g., Thordardottir, 2011). Moreover, if bilingual children have very limited exposure to one of their languages (e.g., 10, 20, or 30% of their waking hours), this will likely result in incomplete acquisition of that language and even patterns of development that resemble those of second language learners rather than native speakers. Research by Thordardottir (2011) suggests that bilingual children need at least 40% of waking hour exposure to a language if their competence in that language is to be comparable to that of monolingual children, when assessed using standardized tests.
 
Even under optimal conditions, when bilingual children’s exposure to their two languages is divided equally (50:50), they may differ from monolinguals because their language learning experiences and their communicative needs are different from those of monolinguals. Moreover, not all families want their children to be equally competent in both, or all, languages. Consider the example of a family, living in an English-speaking community in Canada, in which the parents speak English and another language (like Italian or Chinese). This family may want to ensure that their child is fully competent in English (including speaking, listening, reading and writing), but would be satisfied with oral and reading fluency in the other language. In this case, limited exposure to the non-English language need not be a problem. In the end, however, the more exposure bilingual children have to each language, the better their competence will be. Thus, parents of bilingual children should take care to enrich their children’s language learning experiences to ensure they are as bilingually competent as desired.
 
Should parents use the one-parent, one-language rule with young bilingual children?
Parents are often told that they should use the one-parent, one-language rule because it ensures that their children will not get confused by parents who use both languages. There is no evidence that following this rule helps children keep their two languages separate. As noted before, as long as most people in the child’s life use only one language at a time, the child will also learn to use their two languages this way. The one-parent, one-language rule is a useful way for parents to manage the two languages to ensure that their child receives adequate exposure to both languages; otherwise, there may be a tendency to favor one language over the other.
 
Are children with language impairment or at risk for language impairment at greater risk for language development when learning two languages than when learning one?
It is commonly believed that children with specific language impairment should not learn two languages simultaneously. The assumption is that learning two languages at the same time will exceed their language capacity even more and that this will further jeopardize their eventual language competence. Although this belief makes a certain amount of common sense, research indicates that children with specific language impairment can become bilingual within the limits of their learning impairment (Paradis, Genesee, & Crago, 2011). Research by Kay-Raining Bird and her colleagues has shown that this also true for children with Down syndrome (Kay-Raining Bird et al., 2005). More specifically, with respect to children with specific language impairment who are learning two languages simultaneously, it has been shown that they exhibit the same language difficulties and the same severity of difficulty as monolingual children with impairment. At present, there is no evidence that children with language impairment are better off learning only one language, although available research is  limited. Therefore, individual children who have a language learning impairment and are learning two languages should be monitored closely and caregivers should provide the additional support they need to resolve their difficulties. The decision to raise children who have a language learning impairment or other learning or developmental disabilities bilingually should consider whether competence in two languages is particularly important, as in the case of children with parents who speak both languages. In the event that bilingual language learning is optional, as in the case of monolingual parents living in a monolingual community but who employ an au pair (or live-in childcare worker) who speaks another language so their children can become bilingual, the options should be carefully weighed.
 
Future Directions
           
There is a great need for more research on the identification of and effective intervention for bilingual children with learning or developmental disabilities. There is an especially critical need for research on children with autism spectrum disorder and other disabilities to inform parents, educators, and other professionals about these children’s capacity to learn two languages. In a related vein, the development of assessment instruments and strategies that can be used to assess the language abilities of bilingual children in both clinical and non-clinical (e.g., school) settings is also needed (for example, for resources for assessing children in multilingual settings, see http://www.chesl.ualberta.ca). While researchers have begun to examine the relationship between exposure/input and bilingual acquisition, more research on these issues would provide valuable information to parents and other childcare workers about the conditions that are necessary to ensure that bilingual children acquire full competence in two languages.  Comparative research on the bilingual acquisition of children who speak a minority language at home (e.g., Spanish or Arabic in Canada) and those who speak a majority language at home (English in Canada) would clarify the conditions that support or hinder bilingualism in these populations of learners. There is limited research on children who acquire more than two languages, although there is growing activity in this sphere.
 
Conclusions
 
Most children are capable of learning two languages from birth to a high level of competence – as competent as monolinguals in most cases, if the learning conditions are right. Evidence, albeit limited at this time, indicates that even infants and toddlers with significant developmental disorders are capable of acquiring two languages simultaneously, within the limits set by their disabilities. While infants and toddlers are effective language learners, time and quality of exposure are important. Some minimum amount of exposure to each language is necessary (estimated to be around 40-50% of learning time) if children are to perform within the limits demonstrated by monolinguals. The challenge for both parents and other caregivers in raising children bilingually is to ensure the learning conditions that will promote maximum development.