Are two languages at a time too much for the infant mind? According to a recent study, babies as young as 20 months can understand a lot more than you'd think.
In a new study, a team of researchers, including those from Princeton University, reported that bilingual infants as young as 20 months of age efficiently and accurately process two languages.
The study found that infants can differentiate between words in different languages. "By 20 months, bilingual babies already know something about the differences between words in their two languages," said co-author Casey Lew-Williams.
"They do not think that 'dog' and 'chien' [French] are just two versions of the same thing," Lew-Williams said. "They implicitly know that these words belong to different languages."
To determine infants' ability to monitor and control language, the researchers showed 24 French-English bilingual infants and 24 adults in Montreal pairs of photographs of familiar objects. Participants heard simple sentences in either a single language ("Look! Find the dog!") or a mix of two languages ("Look! Find the chien!"). In another experiment, they heard a language switch that crossed sentences ("That one looks fun! Le chien!"). These types of language switches, called code switches, are regularly heard by children in bilingual communities.
The researchers then used eye-tracking measures, such as how long an infant's or an adult's eyes remained fixed to a photograph after hearing a sentence, and pupil dilation. Pupil diameter is an involuntary response to how hard the brain is "working" and is used as an indirect measure of cognitive effort.
The researchers tested bilingual adults as a control group and used the same photographs and eye-tracking procedure as tested on bilingual infants to examine whether these language-control mechanisms were the same across a bilingual speaker's life.
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They found that bilingual infants and adults incurred a processing "cost" when hearing switched-language sentences and, at the moment of the language switch, their pupils dilated. However, this switch cost was reduced or eliminated when the switch was from the non-dominant to the dominant language, and when the language switch crossed sentences.
Lew-Williams suggested that this study not only confirms that bilingual infants monitor and control their languages while listening to the simplest of sentences, but also provides a likely explanation of why bilinguals show cognitive advantages across the lifespan. Children and adults who have dual-language proficiency have been observed to perform better in "tasks that require switching or the inhibiting of a previously learned response," Lew-Williams said.
"Researchers used to think this 'bilingual advantage' was from bilinguals' practice dealing with their two languages while speaking," Lew-Williams said. "We believe that everyday listening experience in infancy, this back-and-forth processing of two languages, is likely to give rise to the cognitive advantages that have been documented in both bilingual children and adults."
The study is published in the journal Proceedings of the National Academy of Sciences.