This paper provides a comprehensive review of research on bilingual first language acquisition in infants and young children. It examines the central theoretical debate — the Bilingual Deficit Hypothesis versus the Bilingual Advantage and Differentiated Language System Hypotheses — and evaluates empirical evidence bearing on each. Drawing on landmark studies by researchers such as Oller, Petitto, Pearson, Volterra and Taeschner, and others, the paper surveys vocabulary development stages, the utility of the MacArthur Communicative Development Inventory, key linguistic milestones, lexical identifiers, translation equivalents, interlocutor sensitivity, language choice, codemixing, and parental discourse strategies. The overall finding is that bilingual children generally achieve early language milestones on a timetable comparable to monolinguals, and that parental involvement and environmental input are decisive factors in successful bilingual development.
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The paper exemplifies systematic literature synthesis: rather than simply summarizing individual studies, it groups findings thematically (deficit vs. advantage, timing of milestones, parental influence) and uses each study as evidence for or against a named theoretical position. This technique allows the writer to build a cumulative argument rather than a mere list of summaries.
The paper opens with a framing introduction that defines the bilingual paradox, then devotes separate sections to the deficit, unitary, advantage, and differentiated language system hypotheses. It transitions to empirical ground through sections on vocabulary development, the MacArthur CDI, and linguistic milestones. Later sections address progressively finer phenomena — lexical identifiers, translation equivalents, language choice, and codemixing — before closing with parental strategies, early consonant cluster constraints, and a synthesizing conclusion.
It is common knowledge that all over the globe young children seem to effortlessly acquire two or more languages at one time. Yet some uphold the belief that children who are exposed to multiple languages too early may experience developmental language delays and/or confusion — the dividing line as to what constitutes "too early" remaining a gray area. Scientific research has attempted to examine whether young bilinguals can ascertain that they are acquiring two separate and distinct languages early on (Watson, 1996). This paper examines the criteria for measuring early language development, the concepts associated with gauging a child's developmental progress, the research supporting the prevailing theories, and the significant findings inherent in credible research in these areas.
The line of thought that most closely resembles popular opinion about whether or not children should be raised bilingual from birth is called the Bilingual Paradox. It is a paradox because there are conflicting contemporary theories about whether or not simultaneous bilingual exposure is the preferred method of introducing two languages. While much research shows that children exposed to two languages simultaneously between the ages of zero and three achieve major milestones within the same timeframe and with similar results as monolinguals, the prevailing popular view has been to teach a child one language early on and reserve the introduction of a second language for the later school years. The thinking behind this latter position is that introducing two languages early on will confuse the child, or that the child will be disadvantaged in mastering one or both languages. There are, however, steps parents can take early on — grounded in proven research — to give their child an advantage in learning language skills. Methodologies, tools, timing mechanisms, and support strategies have been developed through testing and observation that aid children in successful and timely language development, specifically in the adoption of multiple languages.
One of the theories regarding bilingualism and its potentially detrimental effects on children's language development is called the Bilingual Deficit Hypothesis. According to this theory, being exposed to and learning two languages equally during language development requires more effort and causes more stress for children. Because of this, there is concern that bilingualism delays children's initial production of the precursors to speech. One notion associated with this concept is the "babbling drift" hypothesis, which holds that when children begin to babble in their first two years of life, their sounds resemble the phonemes of their caregivers' language duality without regard for substance. While some researchers have found predictable phonemic differences among babies of caregivers with different language backgrounds, others have found no evidence of acoustic differentiation among such babies (Oller et al., p. 408).
The Bilingual Deficit Hypothesis conveys the notion that children who are simultaneously exposed to two or more languages early on suffer language delays and confusion. The theory further assumes that these children suffer a disadvantage in lacking the same mastery over one language as a monolingual child. Concerns center on "language mixing" and a lack of appropriate usage and development of syntax for each language. The Unitary Language System Hypothesis details a specific assumption about how early bilingual representation can cause developmental delay.
The Unitary Language System Hypothesis holds that, until the age of three, a child exposed to two languages has a single fused linguistic representation. This implies that a language delay is inherent in the need to sort out the two languages once the child is able to differentiate between them. The genesis of this line of thought was a study conducted in 1978 by Volterra and Taeschner. The study examined bilinguals in the one-word stage (approximately one year old) and noted that there were few semantically corresponding words across the two languages. The classic example is: if a child uses the word "ball" in one language, he or she will not use the equivalent of "ball" in the second language to express the same idea — implying that the children are not drawing on both languages for the same concepts at the same time (Caputi, 1986).
Essentially, the theory states that if the child already has a verbal concept in one language, they will not be motivated to express it using a second language until they begin to recognize that they are using two distinct languages. Volterra and Taeschner cite the example of one child who insisted that a hairpin was not a molletta (Italian) but a klammer (German). In their observations of three children, they reported that the children almost entirely lacked doublets — two words representing a single object or concept — until a seven-month period had elapsed. This contradicts Petitto's conclusion that the capacity to differentiate between two languages is established prior to first words. Volterra stated that once the seven-month period had passed, the children began using translation equivalents (also known as doublets) in 30% to over 90% of all words. Since Volterra studied one-year-olds, the indication is that the children were well past the age of one when the production of translation equivalents matured (Dromi, 1987). The contradiction with Petitto's findings appears to be primarily one of timing, given that the average monolingual child produces a first word at approximately one year of age.
Clark's Principle of Contrast (1987) supports Volterra's concepts and asserts that young bilinguals "reject cross-language synonyms in their earliest lexicons." Clark claims that during the first stage of vocabulary development, the child assumes each word has a separate and distinct meaning. Clark's theory further states that the child will tend to accept the word from the language they grasp most quickly, or at least until their vocabulary reaches 150 words. Parents can be mindful from the outset of their own patterns of language usage and whether they tend to naturally favor one form of language expression over the other, since a child is highly susceptible to the parent's linguistic influence.
The other prevailing theory, the Bilingual Advantage Hypothesis, holds that children raised in a bilingual environment have higher intellectual flexibility due to richer language input and therefore have an easier time producing speech sounds (Oller et al., p. 409). Evidence to varying degrees exists to support both theories.
According to the research of Kimbrough Oller, some of the fears about the detrimental effects of early bilingual exposure may be allayed (Oller, 1997). In Oller's observations of English- and Spanish-speaking children, he found that bilinguals were no less intelligible than monolingual children and in some cases were more intelligible. He further states that by the time bilinguals reach school age, they have a larger conceptual space and their vocabularies are at least as large as — if not more diverse than — those of monolinguals, giving them an advantage both socially and in communicating with a larger population.
The Differentiated Language System Hypothesis challenges the unitary view. In this framework, language mixing occurs according to regular grammatical patterns and mirrors sociolinguistic factors — that is, the child's surroundings and tendency to mirror the language emphasis of their parents or primary caregivers. The crux of this premise is that language mixing is not the result of biological overload, confusion, or inability, but rather a demonstration that children are exhibiting distinct syntactical nuances at an early age. Evidence suggests that children mirror the emphasis of their primary caregiver, whether that be a parent, grandparent, childcare worker, or sibling.
Earlier empirical data on the two prevailing hypotheses were premised on studies beginning at eighteen months, focusing on multi-word combinations. By the age of eighteen months, early language milestones such as first word, first fifty words, and first two-word combinations have already occurred. More recent studies have examined these hypotheses from the perspective of infants and very young children.
Four stages of vocal development have been identified as occurring within the first year of life. In the first two months, the phonation stage takes place, during which infants produce quasi-vowels — verbal signals such as crying or laughing. The primitive articulation stage, occurring in months two and three, finds the infant producing cooing sounds. In the expansion stage, sounds develop into full vowels and marginal babbling. In the canonical stage, infants produce consonant- and vowel-like syllables in rapid format transitions (Jusczyk, 1999). It is in this fourth, canonical stage that parents can begin to determine whether there is an indication of delay in speech development. It is important to recognize these stages of vocal development within the first year of life and to address any perceived delays promptly. Infants use multiple cues to determine word boundaries between the ages of six and twenty-four months. As early as 7.5 months of age, infants can detect stress patterns in speech and, later, phonotactic nuances (i.e., acceptable consonant clusters at the beginning of a word).
Oller's research was aimed at providing empirical data to support parents seeking reliable information on which to base their child-rearing decisions. His methodology involved a study contrasting and comparing children raised in a monolingual English environment with children raised in a bilingual Spanish-English environment. All children were two months old at the onset of the study, and the bilingual children were exposed to both languages equally. Forty-four monolingual and 29 bilingual subjects participated. A scale of 1–5 was assigned to parental education, parental employment patterns, and family stability in order to account for sociological influences, and the average scores for the two groups were comparable.
During the first year, monthly laboratory visits took place. Between visits, parents journalized the children's vocalizations — especially their use of repetitive, well-formed syllables, which were considered canonical babbling. Through training, parents were able to consistently identify specific features of their infants' sounds, such as when canonical babbling began and the unique characteristics of the spoken sounds (Oller et al., p. 412). Scientists then reviewed the vocalizations and categorized sounds as having either a quasi-vowel or a full vowel, and as including or excluding a consonant. True canonical babbling consisted of a full vowel and at least one consonant-like sound joined with a well-formed transition and was considered more advanced than utterances lacking one or more of these features.
The level of difficulty increased greatly between the ages of three months and ten months, after which the rate of improvement decreased. There were no significant differences between the monolingual and bilingual children, disproving the Bilingual Deficit Hypothesis. The only significant finding was that children of higher socioeconomic status had higher volubility — more utterances per minute. Otherwise, there were no significant differences in the onset of canonical babbling, the ratio of canonical syllables to total syllables produced, vowel ratios, or utterances per minute between monolingual and bilingual children (Oller et al., p. 420).
Documentation for standards in developmental word lists was absent until MacArthur developed the Communicative Development Inventory (CDI) in toddler and infant forms (Jackson-Maldonado & Bates, 1988). Prior to this, word lists were recorded by linguists on mostly small samples of children — some of whom were the biological children of the researchers themselves — which varied greatly in what was recorded, the timing of records, and the methodology employed. Earlier studies included those by Volterra and Taeschner (1978), Taeschner (1983), Leopold (1939), Vogel (1975), Vihman (1985), Jekat (1985), Mikes (1990), Yavas (1991), and Quay (1993). (See De Houwer, 1990, for a review.) Direct comparisons across these sources were difficult at best given their many unique differences.
MacArthur standardized an inventory tool in 1989 for children aged eight months to two years and six months, allowing parents to easily record the words their child uses at each sampling interval in more than one language. The infant form in English consists of 395 words representing those most frequently produced during the ages of eight months to one year and three months, arranged in twenty-two semantic categories. The form asks parents to mark words comprehended in one column, and words both comprehended and produced in another. The toddler form contains 679 words and is used for children between ages one and two, for whom the only scoring parameter is word production. The Spanish version of the CDI follows the English format, with 428 infant words and 732 toddler words drawn from research studies. The CDI does not purport to be a comprehensive historical record of every word a child might say, but rather a representation derived from a controlled sample.
The validity of the CDI as a reliable controlled sample was tested with hour-long samplings of the productive vocabulary of 45 children. The results — compiled in the LEX Database — were in agreement with the CDI (Dale & Fenson, 1993). This standardized tool thus allows for the identification of translation equivalents in a child's vocabulary structure and serves as a comparative reference point for language development. It would be useful for parents to know in advance the most commonly articulated words, the stages at which they appear, and the timing according to which they are expected to occur. While each child develops at an individual rate, these parameters provide useful benchmarks for gauging whether the child is progressing within a normal range. The parent embarking on raising a bilingual child should have an understanding of significant milestones so that they can intervene appropriately if additional assistance is required at any given stage.
A review of available research regarding bilingual first language acquisition is telling on many levels. The following observations can be drawn from the research presented:
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