Why bilingual development is not easy (2024)

The publisher's final edited version of this article is available at Adv Child Dev Behav

4.1. In early development, children exposed to two languages acquire linguistic knowledge at a rate comparable to monolinguals, but the bilinguals’ knowledge is distributed across two languages

We measured the productive vocabulary and grammar in English and in Spanish of 47 children with bilingual exposure and the productive vocabulary and grammar in English of 50 children with monolingual exposure when they were 22, 25, and 30 months old. The measurement instruments were the English and Spanish versions of the MacArthur-Bates communicative development inventories (Fenson et al., 2007; Jackson-Maldonado et al., 2003). These are widely used and validated parent report instruments which assess children’s productive language in this age range. This sample was recruited using the criterion that both English and Spanish were used at home, and all children had a parent who was able to report on their child’s speech in each language. We found that the sum of the bilinguals’ English and Spanish scores on the vocabulary measures was not different from the monolinguals’ scores in English alone, as is shown in Fig. 1. (We did not try to create a combined grammar score, although others have [].)

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Fig. 1

The English CDI productive vocabulary score is plotted for children from monolingual homes (n = 58) and the English CDI + Spanish IDHC productive vocabulary score is plotted for children from homes where Spanish is also spoken (n = 47). Based on data reported in Hoff, E., Core, C., Place, S., Rumiche, R., Señor, M., & Parra, M. (2012). Dual language exposure and early bilingual development. Journal of Child Language, 39, 1–27.

The finding that children exposed to two languages do not learn less than children exposed to one has been observed in other studies as well. This finding is frequently referred to in support of the claim that bilingualism causes no delay in language acquisition. However, the assertion that bilingualism causes no delay because bilingual children have equivalent amounts of language knowledge—just distributed across two languages—is misleading in two ways. First, it obscures the fact that on single language measures bilingual children lag behind monolingual children (see next section). Second, this equivalence in total linguistic knowledge must be a temporary state. Knowing the same amount, only distributed across two languages, cannot be the end state of bilingual development. Ultimately, bilinguals know more than monolinguals—even if their proficiency is not monolingual-like in both their languages.

4.2. When bilingually developing children are compared to monolingual children with respect to their growth in a single language, the bilingual children lag in development

If the bilinguals are comparable to monolinguals in their total vocabulary, then they cannot be comparable, on average, when only one language is being considered. In these same children whom we studied from 22 to 30 months, we found that their growth in English vocabulary and grammar lagged that of the monolinguals, as is shown in Fig. 2. The size of the difference between the monolinguals’ and bilinguals’ English skills, averaged over the period from 22 to 30 months, was equivalent to 21 percentile points in vocabulary and 17 percentile points in the average length of the children’s three longest utterances. If we ask whether bilingualism caused a lag in reaching the milestone of combining words—the leap from single to multiword utterances, the answer is not clear. On average, the bilingual children reached that milestone in English 3 months later than the monolinguals. However, almost all of these children were considerably stronger in one language than the other, and if you take their best language, then there was no difference between the monolinguals and bilinguals in the age at which this milestone was reached in at least one language. A better test of whether bilingual exposure delays the achievement of this milestone would be to look at perfectly balanced bilinguals. But perfectly balanced bilinguals are vanishingly rare.

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Fig. 2

Growth in English for monolingual children (n = 56) and growth in English and Spanish for bilingual children (n = 47) from 22 to 30 months on measures of productive vocabulary, mean length of the three longest utterances, and percent of children combining words, based on caregiver report on the MacArthur-Bates inventories. Based on data reported in Hoff, E., Core, C., Place, S., Rumiche, R., Señor, M., & Parra, M. (2012). Dual language exposure and early bilingual development. Journal of Child Language, 39, 1–27.

4.3. Among children acquiring a minority and majority language simultaneously, growth in the majority language overtakes growth in the minority language

One of the most reliable findings across different studies and including different immigrant groups and language pairs, is that children exposed to a minority and majority language from an early age reliably acquire the majority language while minority language acquisition is more variable. In our studies of children from 22 months to 10 years, we see the gap between monolinguals and bilinguals in English narrowing and the gap between bilinguals’ English skills and their Spanish skills widening. This majority language takeover has also been observed elsewhere. Children in Wales who were monolingual Welsh speakers as toddlers become English-dominant as older children and adults (). Among children in Belgium who have early exposure to French and Dutch, only 75% were described by their parents as bilingual by the time they were 10 and 11 years old (De Houwer, 2007).

This majority language takeover is no doubt due in part to children’s increasing exposure to the majority language as they get older and interact more with the world outside their homes (). This happens later for children whose first majority language exposure comes with school entry, but it happens for them as well (). But there is also a dynamic process at work, as children’s skills have effects on further development. Pearson (2007) described an input—proficiency—use cycle in which more input leads to more proficiency, which leads to more use, which in turn leads to more input, and so on. Our longitudinal data suggest such a process contributes to increases in English proficiency and to attendant declines in Spanish proficiency. When we analyzed the relation between growth in English and growth in Spanish from 2½ to 4 years, we found that individual differences in children’s Spanish skill were unrelated to their future English growth. In contrast, individual differences in the same children’s English skill were a significant negative predictor of their future Spanish growth. The stronger the children’s English skills, the less they grew in Spanish skill over the subsequent 6 months (; ).

4.4. Children with majority-minority dual language exposure vary in their profiles of dual language skill, but there is no evidence of a tradeoff between learning one language and learning the other

Children differ. Heterogeneity is perhaps the most widely observed characteristic of the language skills of bilingually exposed young children. We tried to find patterns in that heterogeneity among 126 children from Spanish-speaking homes at 5 years of age. We used cluster analysis to identify subgroups of children who were similar with respect to the relation between their English and Spanish expressive vocabulary skills. We identified four clusters or dual language profiles. The mean English and Spanish scores and the prevalence of each profile are plotted in Fig. 3, along with the mean English score for the monolingual group of children.

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Fig. 3

Prevalence of four different profiles of dual language skill (right axis) and mean English and Spanish expressive vocabulary scores on the Expressive One-Word Picture Vocabulary Test (Brownell, 2001) (left axis) for 126 5-year-old bilingual children and mean English scores for 29 English monolingual children. Error bars indicate +/− 1 standard error. The bilingual data are reported in Hoff, Tulloch, and Core (accepted).

There are several lessons about children’s bilingual skills to be learned from these profiles and their prevalence:

1. Bilingual children differ not only in balance but also in total level of language knowledge. This should not be surprising; monolingual children differ in their level of language skills as well.

   See Also

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2. There is no tradeoff between English and Spanish skill. Strong Spanish skills do not come at the expense of English skills or vice versa. The correlation between these children’s English and Spanish vocabulary scores was near zero. In fact, multiple studies of bilingual children, at ages ranging from 2 to 7 years, using different measures of language skill, find correlations between skill levels in their two languages ranging from weakly negative (e.g., ) to moderately positive (e.g., (), with many studies finding near zero correlations ().

3. All these profiles of bilingual proficiency are English-dominant, despite the fact that the children come from homes that are, on average, Spanish-dominant, with Spanish accounting for 58% of home language use according to parent report. This finding of majority language dominance in children from minority language homes is consistent with the literature. For example, U.S.-born children in Dominican families studied by Tamis-LeMonda et al. were, on average, balanced in their English and Spanish skills at 2 years of age, and they were strongly English-dominant at 5 years (). Some of these children even showed decline in Spanish over this period, measured in terms of vocabulary used in spontaneous speech. Other studies have found that children from monolingual Spanish-speaking homes who have minimal English skills at the age of 5 years move toward English dominance once they enter school (e.g., Collins et al., 2014).

4. Some children at age 5 show strong skills in two languages, but this profile characterizes only 11% of the sample.

5. There was no group of children with very low skills in both English and Spanish. This outcome is something parents and teachers worry about—that bilingual children will achieve only low skill levels in two languages. The children in this study were screened at the age of 2½ years when they entered the study, and any child who showed signs of being at risk for language delay was excluded. Therefore, the study likely excluded children in the very bottom tail of the distribution of language learning ability.

6. Even the group of children with the strongest English skills had significantly lower English expressive vocabulary scores than the monolingual English-speaking children, controlling for differences between the groups in the mothers’ level of education attained in English.

4.5. The degree to which bilingual children’s language skills differ from monolingual levels depends on the language domain: Bilingual children are stronger in comprehension than production, and they are stronger in communicative competence than in vocabulary knowledge, relative to monolingual children

Expressive vocabulary is the language measure that is most different in comparisons of bilinguals to monolinguals. Receptive vocabulary skills lag less. This advantage of receptive over expressive skills in bilingual development has been observed with sufficient frequency to have acquired its own name: the receptive-expressive gap. The pattern has been observed in several studies of Spanish-English bilinguals in the U.S. (; ) and also in Hmong-English bilinguals in the U.S. (). A related finding is that school-aged French-English bilinguals in an English-speaking region of Canada showed more lexical retrieval difficulty in the form of tip-of-the-tongue experiences in French than in English (). The extreme of this pattern is passive bilingualism (also termed receptive bilingualism; (Sherkina-Lieber, 2020), that is, bilingualism with respect to comprehension but monolingualism with respect to production, which is a frequently occurring pattern of dual language proficiency among adults who grew up in language minority homes (Sherkina-Lieber, 2020).

Another way in which bilinguals often differ from monolinguals in their language skill profiles is that discourse skills—the ability to carry on a conversation or tell a story—tend to be stronger than vocabulary or grammar. The narrative skills of bilingual and monolingual children drawn from our longitudinal study were assessed when the children were 4½ years old (). We elicited fictional narratives by showing the children a short cartoon about a family of mice going on a picnic, with adventures and misadventures that ensue. When we compared the bilingual and monolingual children’s scores on a standardized test of English vocabulary, the monolingual children scored a significant 1.4 standard deviations higher than bilinguals. In contrast, when we transcribed and analyzed their stories, we found no difference between the bilingual children’s English stories and the monolingual children’s English stories in story length, in number of different words used, in the completeness of their stories, or in the quality of their stories—as indexed by the number of evaluative expressions (i.e., elements such as the characters’ thoughts and intentions that need to be inferred but that drive the narrative). This finding is consistent with evidence from immigrant and refugee children learning a English as second language: they catch up to monolinguals on measures of narrative structure even as they lag in vocabulary and grammar ().

4.6. Adult bilinguals are not two monolinguals in one

Eventually the children in this study will be adults, and they will provide data on the long-term outcomes of their early dual language exposure. In the meantime, however, data on other samples provide clues as to what those outcomes will be. We recruited Spanish-English bilingual college students in South Florida who met two criteria: they had been exposed to both languages from before the age of 5 years (most had been exposed to both languages from birth), and they used both languages at least on a weekly basis. We cannot know from this study how many young adults who had early dual language exposure no longer use both languages on a regular basis, but we can ask for those who do use both languages as adults what their levels of English and Spanish proficiency are. We administered a battery of tests of language and literacy skills in English and Spanish to this sample of young adult bilinguals, and we compared their English to their Spanish skills, their English skills to those of monolingual English-speaking young adults in South Florida, and their Spanish skills to those of monolingual Spanish-speaking college students in Chile.

The results are easy to describe. These bilinguals had English skills that were not different from the skills of their monolingual English peers, except on one task, whereas their Spanish skills were significantly below those of Spanish monolinguals on almost every measure. The one task where the bilinguals’ English skills differed from the monolinguals’ was in the speed with which they provided labels for pictures. These findings tell us what our data from the young children had already foreshadowed: Children from Spanish-speaking homes in the United States, if they become bilingual, become bilinguals who are much stronger in their English skills than Spanish skills. Ardila, Rosselli, Ortega, Lang, and Torres (2019) observed the same pattern of monolingual-like English skills and weaker Spanish skills among U.S.-born Spanish-English bilinguals. In contrast, the same study found that Spanish-English bilinguals living in the U.S. who were born in Latin America and had attended school in both Latin America and the U.S. had stronger Spanish skills than the U.S.-born bilinguals while also having comparable English skills. Similar findings appear in comparisons of French and Italian heritage language speakers who grow up in Germany. The French heritage speakers, who went to French language schools in Germany, performed like French monolinguals on tests of French grammar. The Italian heritage speakers, who attended school only in German, had noticeable problems with some aspects of Italian grammar (). In sum, the levels of language proficiency observed among adults who have gone to school were acquired in part through schooling.

While schooling appears to be necessary for the attainment of typical adult levels of proficiency, it seems also to be true that schooling alone is not sufficient. Children who attend school entirely in English and hear only Spanish at home do not have the same English skills as monolingual English children—at least not by the age of 11 years (). Similar findings have been obtained in a study of high school students in Ontario, Canada, where English is the majority language and French a minority language. Students who go to school conducted entirely in French and hear only English at home do not have the same proficiency in French as the students who also hear French at home (). The results from the Canadian study are the obverse of the Miami results because in the Canadian study, it was the minority language that was used in school. However, both studies point to the same conclusion: language experience both at home and at school contribute to language proficiency.

There is also evidence that continued language use is necessary for the maintenance of whatever proficiency level is achieved. The highest levels of Welsh proficiency among Welsh-English bilingual adults were among those for whom Welsh was their only language at home as young children and who, as adults, have a spouse with a similarly strong Welsh background—presumably because they continued to use Welsh as adults (). In fact, without occasion to use a language, it appears possible to completely forget even one’s native language. Japanese soldiers who were left behind in Russia at the end of the World War II and who were “found” 50 years later spoke only Russian and required translators to communicate to Japanese speakers (Kristof, 1998; Sugio, 2017). Together, these findings make it clear that language development is not complete and fixed at age 5.

In sum, the average course of language development among children with early exposure to a minority and majority language is to begin to acquire both languages simultaneously but to acquire the majority language at a more rapid rate than the minority language. The gap between majority and minority language skills is widened by school experience, where schooling is conducted only in the majority language. The growth of each language in children simultaneously learning two languages is slower than the growth of a single language in children learning only one. At some point between school entry and young adulthood, many minority-majority language simultaneous bilinguals reach monolingual levels in the majority language. Minority language outcomes are more variable. Not everyone who was a bilingual 2-year-old becomes a bilingual adult. Receptive bilingualism is one possible outcome. Among those who continue to use both languages, it is still likely that minority language skills will be lower than majority language skills if schooling was only in the majority language. Within this average pattern, however, there are individual differences. The truism that there is no average child is particularly true when talking about bilingual children. In the next section we look for the sources of individual differences in bilingual development.

5.1. The quantity of language exposure affects the rate of language growth

The quantity of children’s language exposure is a strong predictor of their rates of language development. This is true of monolingual development: children who are talked to more develop language more rapidly than children who are talked to less (Hoff, 2006). It is also true of bilingual development: children who hear two languages acquire the language they hear more at a faster rate than they acquire the language they hear less. In our studies we asked the bilingual children’s primary caregivers to estimate the relative amount of their children’s home language exposure that was in English and in Spanish. Caregiver estimates show good validity when assessed against diary records of children’s language exposure (), and multiple studies have found them to predict bilingual children’s language skills (). Measures of the absolute amount of exposure accomplished by analyzing day-long recordings of the speech children hear do a better job of predicting children’s skill levels in each language, but only slightly better (). Using caregiver estimates of home exposure as the predictor and children’s scores on a test of English and Spanish expressive vocabulary as the outcome for 151 children, 39 from monolingual English homes and 112 from Spanish-English bilingual homes, we estimated English and Spanish growth trajectories for children with different relative amounts of exposure (). Those growth curves are plotted in Fig. 4, and they show that between the ages of 2½ and 5 years bilingual children’s English and Spanish vocabulary scores vary as a function of how much English and Spanish they hear.

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Fig. 4

Estimated trajectories of English and Spanish vocabulary growth from 30 months to 60 months at different levels of language exposure, where English-dominant = 75% English/25% Spanish, balanced = 50% English/50% Spanish, and Spanish-dominant- = 25% English/75% Spanish. The model of English growth is based on children from English monolingual and Spanish-English bilingual homes (n = 151); the model of Spanish growth is based on only those children from Spanish-English bilingual homes (n = 112). Based on data reported in Hoff, E. & Ribot, K. M. (2017), Language growth in English monolingual and Spanish-English bilingual children from 2.5 to 5 years. The Journal of Pediatrics, 190, 241–245.

The import of this evidence of the effect of the quantity of language exposure on language growth is qualified by three other findings in the literature: (1) the effect of the quantity of exposure on language growth may not be linear, (2) the effect of exposure is not the same for all outcomes, and (3) the size of the effect of quantity of exposure is inflated by the correlation between quantity and quality.

Some evidence suggests that increases in the amount of exposure to a language yield diminishing returns to language acquisition as the level of exposure goes up (Thordardottir, 2011). It has even been suggested that there is a threshold after which the child has as much input as necessary for particular acquisition and no further learning occurs as a result of additional input (); Pearson, 2007). To the extent that either diminishing returns or a threshold applies to the relation of exposure to language acquisition, it would help to explain how it is possible for children to acquire two languages in less than twice the time it takes a monolingual child to acquire one—which they do. However, in contradiction to a threshold hypothesis, there is also evidence that children who experience as little as 20% of their language exposure in Spanish have lower levels of English vocabulary than children for whom 100% of their language exposure is in English (). Furthermore, if there are limits to how much exposure children can use to build language—such that at some point more exposure has no effect—those limits would certainly be in absolute amount of exposure, not relative exposure. This is a topic we will return to under the heading of open questions and future directions.

The effect of exposure is greater for expressive vocabulary than for other aspects of language development. In her study of French-English bilingual 5-year-olds in Canada, Thordardottir (2011) found that 40–60% levels of relative exposure were sufficient for the bilingual children to achieve receptive vocabulary scores within the range of scores achieved by monolingual children. In contrast, approximately 70% of exposure was required to reach the monolingual range in expressive vocabulary skill. As already discussed, the effect of exposure quantity appears to be less for the timing of basic milestones than for vocabulary size or grammatical complexity. The effect appears to be less for grammar than for vocabulary (Hoff et al., 2012). And the effect on grammatical development is different for different aspects of grammar (Unsworth, 2014).

Finally, unless controlled for, effects of the quantity of exposure are likely confounded with effects of the quality of exposure for the simple reason that households with more proficient English speakers tend to use English more than households with fewer proficient English speakers. In the data in Fig. 4, the effect of quantity of exposure on English vocabulary is quadratic—the opposite of the diminishing returns that Elin Thordardottir observed. However, in the sample that produced that quadratic function, the relative amount of exposure to English in children’s homes was positively related to the likelihood that one or both parents were a native speaker of English (). Thus, children who experienced more exposure to English also experienced more exposure to English from a native speaker, which makes a difference.

5.2. The quality of language exposure affects the rate of language growth and limits ultimate attainment

Not all language exposure is equally useful to the language learning child. This is true for monolingual development as well as bilingual development, although in bilingual environments there are more sources of variance in the quality of exposure than there are in monolingual environments. In our studies of children from Spanish-speaking families in the U.S., we asked mothers to keep week-long diaries in which they recorded which language(s) their children heard and from whom for each 30-min block of the day (, 2016). We found in two separate studies that more of the children’s English exposure was provided by nonnative speakers than by native speakers. We also found in both samples that the proportion of the children’s English exposure that was provided by native speakers was a positive predictor of the children’s English skill, over and above the effects of the quantity of English exposure, suggesting that something about exposure provided by native speakers makes it more useful to language acquisition. A related finding comes from Paradis’s (2011) study of English development in immigrant children in western Canada. That study found that the amount of time the immigrant children spent in activities with native English speakers was a positive predictor of their English skill whereas the amount that their immigrant parents spoke English at home was unrelated to their children’s English skill.

Other evidence suggests that the relevant variable is not whether the source of exposure is a native speaker, but rather how proficient the speaker is. Some nonnative speakers are highly proficient. Several studies have found that proficiency differences in the host country majority language among nonnative parents predict their children’s skill in that host country language (; ; Hammer et al., 2012; ).

To ask what about exposure from proficient speakers makes it more useful for language acquisition than the speech of speakers with lower levels of proficiency, we compared the child-directed English of 21 nonnative mothers (6 who described themselves as having limited proficiency) to the child-directed English of 29 native English-speaking mothers (). All the mothers were recorded in toy play interaction with their children at 2½ years. Using transcripts of those interactions, we measured some of the properties of child-directed speech that previous research with monolingual children has identified as positive predictors of vocabulary or grammatical development, including the diversity of the vocabulary used, the diversity of nouns used as sentence subjects, the diversity of verbs, and utterance length (; Hoff, 2003; ). We found that child-directed speech of the native speakers was significantly higher on all these measures than the child-directed speech of those nonnative speakers who rated their own English proficiency as “limited.” The speech of the nonnative speakers who rated their own proficiency as “good” was in between those two groups and not statistically different from either. Another comparison of native to nonnative child-directed English in our sample found that native speakers more frequently used complex syntactic structures than nonnative speakers (). Other researchers have found that phonetic properties of nonnative child-directed speech are different from those of native speech with consequences for the phonetic properties of the children’s speech ().

Native speaker status or proficiency of the source of exposure is not the only factor that influences the degree to which speech is useful for language learning. Research with monolingual children has found that the context in which language is used has effects on its informative properties. Book reading is one context that elicits a wider vocabulary and more complex utterances than many other contexts, and that effect extends to conversations about the book, not just the verbatim reading (Hoff-Ginsberg, 1991; Snow et al., 1976). Such findings suggest that book reading as means of augmenting the quality of children’s language exposure might be particularly beneficial for children from language minority homes. That logic is supported by the results of a study of over 5000 4- to 6-year-old children in Denmark, which found that book reading at home was a positive predictor of children’s language skill and that the effect of book reading was stronger among children from language minority homes than among majority language, monolingual children (). Talk about decontextualized topics—as opposed to the here and now—is another positive predictor of children’s language growth (Rowe, 2012), and some contexts may be more conducive to such talk than others. This is relevant to the bilingual development of children from minority language homes because a language used primarily at home is likely to be used in a restricted range of contexts. School is a context in which language use is very different from language use at home, and we have already seen in studies of young adult bilinguals that languages used only at home are not acquired to the same degree as languages used in school (Ardila et al., 2019; ; ).

5.3. Speaking, in addition to hearing a language, affects language growth—Particularly growth in expressive language skills

It is not just exposure to language, but also use of language in speaking that appears to contribute to children’s language growth. Among studies of monolingual children, there are findings that more talkative children have better language skills than reticent children (e.g., Evans, 1996) and that the degree to which children participate in conversation is a positive predictor of subsequent language growth (Hirsh-Pasek et al., 2015). Studies of bilingual children have found that measures of use or combined measures of exposure and use are better predictors of growth in each language than are measures of exposure alone (Bedore et al., 2012; ).

Unlike monolingual children, bilingual children have options as to which language to use when they speak. Often to the dismay of their parents, who would like their children to acquire Spanish, many children in Spanish-speaking households in the U.S. choose to speak English over Spanish, even when they are addressed in Spanish (). We used this behavior to test the hypothesis that language use makes a unique contribution to language growth. We compared the rate of vocabulary growth of two groups of children we identified from their mothers’ descriptions of their conversational behavior: one group consistently answered English with English and also sometimes answered Spanish with English; for this group English use was greater than English exposure. Another group of children consistently answered Spanish with Spanish and also sometimes answered English with Spanish; for this group English use was less than English exposure. We found that the group with greater English use grew in English expressive vocabulary faster than the group with less English use, holding exposure constant. Interestingly, use did not have unique effects of use on a measure of general language comprehension () . This finding is consistent with observations of the outcome of foreign language instruction in schools. In the classroom setting, children receive a great deal of high-quality input, but they speak very little. The result is frequently very limited productive language abilities (Swain, 2005).

5.4. Cultural differences in communicative norms regarding children shape the language experience and language development of children who live in bilingual and, thus, bicultural environments

Cultures differ in the normative way of interacting with children, raising the possibility that children who are exposed to two languages may also be exposed to two different styles of adult-child communication, with potential consequences for language development. A dissertation in our lab compared the mother-child conversations of immigrant monolingual Spanish-speaking and native monolingual English-speaking mothers in the U.S. and their 2½-year-old children (Shanks, 2019). The ratio of mother utterances to child utterances was more than 5:1 for the Spanish-speaking mothers and was less than 2:1 for the English-speaking mothers. This finding survived a number of further checks, including statistically controlling for children’s language comprehension abilities and comparing only mothers with a college education (in whichever language they spoke). These cultural differences also appeared between the child-directed Spanish and English of Spanish-English bilingual mothers, who were recorded interacting with their 2½ year-old children on two separate occasions, once under the instruction to speak only Spanish and once under the instruction to speak only English. Monolingual English-speaking mothers and, to a lesser extent, bilingual mothers when speaking English ask their children questions and actively seek to elicit speech from their children, whereas monolingual Spanish-speaking mothers and, to a lesser extent, bilingual mothers when speaking Spanish, do more talking and less question asking. A similar pattern has been observed comparing American mothers to Thai mothers. American mothers more actively elicit speech from their children (), and English-Thai bilingual mothers differ in how much they elicit speech from their children depending on which language they are speaking (). To the extent that actually speaking, not just hearing the language contributes to expressive language skill, American mothers may have a style of conversing with their children that particularly advantages the development of expressive language abilities, and Spanish-speaking mothers may have a style that equally supports language acquisition, but not the language use by children that additionally contributes to the development of expressive skill. As a result, the relation between expressive and receptive skills that is characteristic of American children may not be characteristic of children from other cultural backgrounds.

5.5. Individual differences in abilities also affect bilingual development

Children vary in language learning ability. The specific abilities that constitute language learning ability have been more researched in monolingual children than in bilingual children, but abilities that have been found to predict language growth in bilingual children include speed of lexical retrieval () ; ), phonological short-term memory (; Paradis, 2011; ), and nonverbal intelligence (Collins et al., 2014; ; Hakuta, 1987).

Phonological memory is the ability to remember new, meaningless sound sequences—which is what all words are to children when they first encounter them. Phonological memory skill is typically measured using a nonword repetition task in which participants are presented with meaningless sequences of speech sounds and their accuracy of immediate repetition is scored. Previous research has shown that individual differences in phonological memory skill have a strong genetic component (Bishop, 2002) and predict a range of language learning outcomes (Gathercole, 2006). We measured phonological memory both for English-like nonwords (e.g., challoon) and Spanish-like nonwords (e.g., gañeca). We found in both of our samples that phonological memory abilities measured with two such different sets of stimuli are highly correlated, which suggests a language-general ability (). However, we also found in the younger sample at 22 months that children who heard more Spanish and were more proficient in Spanish were better at repeating the Spanish-like stimuli and children who heard more English were more proficient in English were better at repeating the English-like stimuli. In the older sample, we did not find an effect of exposure but we did find that memory for Spanish-like sequences predicted Spanish vocabulary better than did memory for English-like sequences and vice versa (Lauro et al., 2020). The degree to which phonological memory skill is modified by language exposure and/or language knowledge is an open question at his point (; Thordardottir, 2017). However, it is clear that phonological memory is an ability that differs among children and that contributes to individual differences among bilingual children in their levels of skill in the languages they hear.

Nonverbal intelligence is another ability that influences language development. In our second longitudinal study, we measured nonverbal intelligence when the children were 5 years old and found that it was related to the children’s rate of vocabulary growth from 2½ to 5 years in English, but not in Spanish. There are at least three other findings that nonverbal cognitive abilities are related to majority language skills but not minority language skills in bilingual children (Blom, 2019; Hakuta, 1987; (Wood et al., 2021). We have suggested, as did Hakuta (1987), that nonverbal intelligence may be more relevant to learning from out-of-home and less supportive environments than to learning in the arguably more supportive home environment. An alternative hypothesis suggested for Spanish-English bilingual children in the U.S. for whom English was a second language, is that using the majority language is cognitively more demanding than using their first language (Wood et al., 2021).

7.1. Implications for theories of language acquisition

The finding that bilingually developing children lag behind monolingual children and do so to a degree that is related to the quantity and quality of their language exposure clearly refutes the assertion that language development follows a maturational timetable, regardless of experience. This is a claim that was made at a time when the dominant theory of language acquisition was Chomskyan nativism (). The lag in single language development that characterizes children learning two languages simultaneously is entirely consistent with a theoretical approach that accords more influence to language exposure.

The finding that the size of the lag varies depending on the domain of language is also potentially informative with respect to the processes underlying language growth. It seems that more language exposure is required for language production than language comprehension (Bates, 1993; ; Thordardottir, 2011, 2014) and that experience not only hearing but also speaking a language is particularly important for the ability to produce a language (; Ribot et al., 2017). Because bilinguals have less exposure to each of their languages than monolinguals have to theirs, and because many bilingual children choose to speak the majority language over the home language, a common pattern of minority-majority language bilingual proficiency is to be relatively equal in the ability to understand the two languages, but to be dominant in the majority language with respect to speaking abilities.

7.2. Implications for parents

In families where both parents speak the majority language and one or both parents is also a proficient speaker of another language, parents have a choice to expose their child to only the majority language or to two languages. Parents in this position frequently ask how exposure to the other language will affect their child’s acquisition of the majority language and how best to manage dual language exposure for optimal outcomes. Whole books have been written to address these questions, but there is a short answer supported by the research just reviewed: Any reduction in the children’s amount of exposure to a language is likely to extend the time it takes children to develop skill in that language, but if everything else is maximized, the delay will be minimal and overcome in the elementary school years. The “everything else” that needs to be maximized is the total quantity and quality of the speech children hear. A further implication of the research findings is that parents need to be clear about what they can expect in terms of their children’s ultimate bilingual development—and it is the heritage language where expectations are not always met. Early exposure is an important start, but the language proficiency that adults command is not all acquired in the first years of life; rather it is developed over years of using language for a range of purposes, including the purposes of school. Those skills, under typical circ*mstances for monolinguals, continue to be maintained by regular use. Adults who were exposed to language only at home or who use it little in adulthood will not have the same level of proficiency as adults who were educated in a language and use it on a regular basis (Ardila et al., 2019; ; ).

In families where both parents are native speakers of a language other than the majority language and are less than highly proficient in the majority language, the options are different. For those parents, speaking only the majority language to their children is not a good option because it helps the acquisition of the majority language only a little and comes at a large cost. The cost is to the acquisition of the parents’ heritage language, perhaps also to the quality of communicative interaction between parents and children, and—evidence suggests—to the information parents can pass on to their children. The relevant evidence here is the finding that among immigrant families in the U.S., greater use of English in the home was associated with lower cognitive test performance by the children (Winsler et al., 2014). Parents are children’s first teachers of the language they speak and also of the content they communicate through language. It is difficult for parents to provide content that promotes their children’s intellectual development when speaking a language they do not speak well.

As a practical matter, these findings tell parents, teachers, and clinicians that it is perfectly normal for a child who is learning two languages at the same time to take longer to learn each than a child learning only one. Sometimes parents or teachers notice the lag and conclude there must be something wrong with their child—or worse, that the dual language exposure caused there to be something wrong with their child. Neither conclusion is warranted.

7.3. Implications for educators

Teachers form impressions of children’s abilities based in part of the children’s abilities to express themselves. The foregoing findings tell teachers that bilingual children’s abilities to express themselves in the majority language are not the same indicator of intellectual ability as they are for monolingual children. Although bilingual children have linguistic knowledge equal to or greater than that of their monolingual peers, their acquisition of each of their languages tends to lag behind monolingual language acquisition. Even within a single language, different language skills are not linked as they are in monolinguals. Expressive vocabulary seems to show the greatest and most persistent lag; grammar less, and broad communicative skills even less. Teachers should be aware of this as they gauge children’s abilities to master material. It is also the case that children’s ability to sound native-like is not a good indicator of other aspects of language proficiency. Phonological skills can be strong, even when vocabulary and grammar are weak (Thordardottir, 2017).

Teachers are looked to for advice by parents, and teachers are well aware that children with limited skill in the majority language will have difficulty in school as a result. That awareness needs to be coupled with an awareness that advising parents with limited proficiency to speak the majority language to their children will not have the intended benefit for children. Researchers who study bilingual development agree that parents should speak to their children in the language they are most comfortable using. A better response to the need for children to be proficient in the majority language would be the development of programs to provide young children from minority language homes with extra experience interacting with highly proficient speakers of that language.

7.4. Implications for clinicians

Pediatricians pay attention to the language skills of their young patients because delayed language development is a warning sign for multiple developmental disorders, including autism spectrum disorder, intellectual disability, and developmental language disorder. To diagnose developmental language disorder (also known as primary language impairment), speech-language pathologists need to determine when a child’s language development is different from what would be expected given the child’s age and language learning experience (Gathercole, 2013). Recognizing when language development is delayed requires knowing what typical language development looks like. The foregoing research, which describes bilingual development in typically developing children, makes two clear points with implications for clinicians who need to identify language delay: (1) it is normal for bilingual children to lag behind monolingual children in the language growth, and (2) a bilingual child’s dominant language is not the same as a monolingual child’s only language. Often bilingual children’s skills in their dominant language is within monolingual norms, but it still the case that assessment of a bilingual child’s knowledge in only one of their languages will underestimate that child’s abilities (see also ; ).

The absence of a profile of very low skills in both languages by the age of 5 years, as revealed in the cluster analysis results in Fig. 3, suggests that very low skills in two languages is not an outcome of dual language exposure among typically developing children in reasonably supportive environments. Children at this age who have very low skills in both their languages may have internal sources of their language learning difficulties or inadequate levels of language exposure. These data and others (Thordardottir, 2011, 2014) argue that such a pattern of language skill should not be attributed to their bilingualism.

8.1. What are the optimal circ*mstances for bilingual development and what are the outcomes under optimal circ*mstances?

It has been proposed that some environments are additive with respect to bilingualism and others are subtractive. This distinction was first proposed to describe second language learning environments (Lambert, 1975). In additive environments, exposure to a second language results in learning the second language without taking away from the first. As result, children exposed to a second language end up knowing more than children exposed to only one language. In subtractive environments, language acquisition is a zero-sum enterprise in which acquiring competence in a new language takes away from competence in the first language. In the case of simultaneous bilingualism, there is not a first and second language, and whether an environment is seen as additive or subtractive with respect to simultaneous bilingualism depends on the perspective of the viewer. We have seen in the data that there need not be a tradeoff between the acquisition of one language and the acquisition of another. In simultaneous bilingual children, skill levels in each language are frequently unrelated. Looking at bilingual development over time in our sample revealed a more complex picture of the relation between two languages in development. Although concurrently Spanish and English skill levels were unrelated, strong English skills were a negative predictor of subsequent growth in Spanish skill, while strong Spanish skills bore no relation to subsequent English growth (Hoff, Giguere, et al., 2018; ). A related result comes from Cha and Goldenberg (2015), who looked at relations between home language environments and English and Spanish skill among 1400 kindergarten children in the U.S. They found that high levels of Spanish use at home were associated with additive bilingualism, as evidenced by a positive correlation between children’s English and Spanish skill levels. In contrast, high levels of English use at home were associated with subtractive bilingualism, as evidenced by a negative correlation between the English and Spanish skills of children from homes with high levels of English use. In other words, with strong home support for Spanish children in the U.S. acquire Spanish in addition to English; with weaker homes support for Spanish, English overtakes Spanish in children’s language growth.

It has been suggested that other environments are more supportive of bilingualism than is the U.S. Studies of French-English bilingual children in Canada have reported smaller or no differences between monolingual and bilingual children—in contrast to the lag typically found in bilinguals in the U.S. (Thordardottir, 2011; ). It has been proposed that the differential prestige of English and Spanish in the U.S. and the equal prestige of English and French in Canada play a role. This is certainly a topic that deserves further research, and that research needs to ask what mechanisms and intervening variables connect the prestige of a language in society to its acquisition by children. The work from Canada suggests, for example, that the use of both languages in school may play a role. Prestige might also influence the quantity and quality of exposure, the necessity to use each language, and the functional value of proficiency in both languages.

8.2. Why doesn’t it take twice as long to learn two languages as it does to learn one?

There may be several sources of savings in the acquisition of two languages that compensate for the reduced exposure that must come from distributing language experience across two languages. One is the nonlinear relation between exposure and growth, or, put differently, ceiling effects on the benefit of increased exposure. Elin Thordardottir found nonlinear relations between exposure and acquisition such that the additional benefit to language learning from increased exposure diminishes as exposure increases. Some have proposed that there are thresholds of exposure past which increases have no measurable effect (Cattani et al., 2014). However, if there are thresholds after which more exposure makes no difference, they must be thresholds defined in terms of absolute measures of exposure and, even in terms of absolute measures of language-advancing information. Thresholds defined in terms of percentages do not mean much, given how much the total quantity and quality of exposure can vary.

What would underlie thresholds? Children with more exposure to a language are likely to hear a wider range of vocabulary items and a greater variety of grammatical structures and they are likely to hear each word and each structure more frequently. Lexical and grammatical diversity in input, and frequency of exposure are positive predictors of children’s language growth (Hoff, 2003; ; ; ). However, the number of different words and the number of different structures a child might hear in a day or a week must have ceilings; increased quantity must reach a point of diminishing returns with respect to increases in quality. The effect of frequency of exposure is also subject to a ceiling, at least in some domains (Gollan et al., 2011). The consequence of these limits on the relation of the quantity of exposure to the quality of exposure and to the benefit of exposure to language proficiency is that the effects of increases in the quantity of exposure have large effects at the low end of the range of exposure children might experience, but small effects at the upper end.

Some developments may be timed by maturation, and the requisite maturational event, whatever it may be, might allow development in more than one language. Basic milestones such as production of the first word and production of the first multiword utterance are candidates for such aspects of language development. The speculation offered here is that whatever the underlying maturational event is, it only needs to happen once and applies to all languages. If major milestones are timed by prerequisite maturational processes and only slightly dependent on language exposure, this would explain some seemingly conflicting findings in the literature on bilingual development—that milestones of producing a first word and first word combination are reached on a timetable similar to monolinguals (Petitto et al., 2001) and that vocabulary size and the length of word combinations vary as function of the amount of language exposure (Hoff et al., 2012). Such an account is also consistent with evidence that monolingual children in cultures where children are talked to relatively little compared to the more studied children in WEIRD (Western, educated, industrialized, rich, and democratic) cultures also reach milestones without any apparent delay ().

Maturation could also play a role in allowing bilingual children with limited exposure to one of their languages to catch up once exposure increases. Such a phenomenon is most clearly seen in the rapid acquisition of a new language in international adoptees (). Even when a new language does not replace the old one, as it does in international adoptees, older children learn language faster than younger children (). The advantage that younger learners have is in their level of ultimate attainment, not the speed with which they build vocabularies or master grammar.

Last some aspects of language may not need to be acquired separately in each language, so that learning two languages is really not learning twice as much as learning one. Some language learning may transfer from the language where it is learned first instead of needing to be learned again, and some knowledge acquired in one language may be a language-general ability. On candidate for such an ability is the ability to build mental representations of meaning from speech, which would support language comprehension of as many languages as the speaker knows.

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