To learn English, bilingual children need robust vocabulary from parents and caregivers

The way a highly proficient speaker of English talks to a child is different from the way a less proficient speaker does. Even for two-year-olds, these differences matter in how they acquire and develop language skills. Early language exposure provides the foundation for children's language development. Therefore, the factors that shape that language exposure, shape language development.
A new study by researchers at Florida Atlantic University and collaborators at The George Washington University digs deep into these differences and finds that differences in the vocabulary used while talking to children affect how useful that speech is to the child trying to figure out the patterns of language. The bottom line? The quality of child-directed speech depends on the language proficiency of the speaker. Children who hear a rich vocabulary acquire a rich vocabulary and children who hear a rich vocabulary in full sentences acquire the ability to put all of their words together in full sentences.
Findings from this study, published in the Journal of Child Language, have broad implications for immigrant parents' language choices at home and for staffing practices in early care and education centers.
Many children learn language, in part, from the speech of non-native speakers who vary in their language proficiency. Even though there is evidence for the effects of language input from native and non-native parents on bilingual children's speech such as acoustic properties, no research to date has investigated the effects of vocabulary and grammar on a child's acquisition of English. Vocabulary size is the most reliably observed difference between native speakers and even quite proficient non-native speakers.
"Just because a parent speaks English better than their two-year-old, doesn't mean that they speak English well enough to maximally support the child's acquisition of English," said Erika Hoff, Ph.D., lead author and a professor of psychology in FAU's Charles E. Schmidt College of Science. "Children can't learn what they don't hear. So if their parents or other adults speak to them with a very simple, limited vocabulary that is all that they can learn."
For the study, Hoff and collaborators compared three groups of mothers: mothers who were native speakers of English; mothers who were not native speakers of English but rated themselves as highly proficient; and mothers who were not native speakers of English and rated themselves as having limited proficiency. All of these mothers talked to their children in English on a regular basis.
Results of the study provide evidence suggesting that the child-directed speech of native speakers and non-native speakers with good proficiency provide a richer database for language acquisition than the child-directed speech of speakers with limited proficiency. Native speakers consistently showed the highest level of the supportive properties of input in their child-directed speech, followed by the non-native speakers with good proficiency, and the non-native speakers with limited proficiency who consistently showed the lowest levels.
The speakers with limited proficiency illustrated a smaller vocabulary for their children to learn, used those words in shorter utterances, and their utterances contained less diversity in the lexical items that fill grammatical roles, compared to native English speakers.
The study also found being educated from an English-speaking college or university also matters even if the non-native speaker has advanced degrees from another country. Hoff suggests that one way to become proficient in a language and proficient in a way that translates into a benefit for the child is to be educated in that language.
"Even if you are highly educated from another country and you are not proficient in English, when you speak to your child in English it will show. There really is something about being proficient in English that helps you talk to your child in a way that benefits his or her language," said Hoff. "Parents who are more proficient in English use a richer vocabulary and produce longer sentences, even when they are talking to a toddler. Clearly, all of the mothers know as much as their two-year-olds, but the difference between what you know when you can barely speak a language and what you know when you're highly proficient in a language translates into a difference that matters in children, and that's really the key point of this study."
Because differences also are observed among native speakers of English in their talk to adults and children, the authors suggest providing more educational opportunities for native speakers as a route to closing other language gaps, such as those associated with socioeconomic status.
To observe speech between mother and child for the study, researchers videotaped them playing together. They transcribed everything that the mothers said to their two-and-a-half year-old children using a special software program as well as manual transcription to count the number of different words and utterances, and to measure the length of sentences.