Baby Lab researchers examine how bilingual children juggle languages

Federica Bordoni

Casey Lew-Williams
Sameer A. Khan

Parents who speak two languages in the home face a dilemma: Should they raise their children with both languages, or will that confuse their young learners? Should they avoid using words from two languages in the same sentence? Should each parent only speak one tongue? “Even basic answers to these questions are long overdue,” says Casey Lew-Williams, assistant professor of psychology. While studies have found that bilinguals show slightly increased cognitive abilities, he says, there has been surprisingly little research into how young children overcome the complexities of learning two languages at once. 

In a study published in August in the Proceedings of the National Academy of Sciences, Lew-Williams is filling that gap, along with psychologists Krista Byers-Heinlein and Elizabeth Morin-Lessard from Concordia University in Montreal. Their research shows that babies may be better equipped to parse the differences in two languages than we realize.

Lew-Williams co-directs the Princeton Baby Lab, and in his work, he uses language as a way to understand the nature of early learning. “Language is a complex system, and the stakes for learning it are quite high,” he says. “Communication is a central part of what it means to be human.” Language learning presents a computational challenge to young children, who must sort through an overwhelming number of sounds and apply them to objects to make meaning. Researchers at the Baby Lab are trying to understand individual differences in early language learning, with the goal of preventing developmental delays and disorders. Bilingualism by definition doubles the challenge by requiring children to switch between two languages, sometimes within the same conversation or sentence. 

Lew-Williams and his colleagues examined the cost babies pay for that switching by looking at subtle changes in behavior and physiology, such as pupil dilation, which corresponds to how hard the brain is working to process surprising stimuli. The researchers studied a group of 20-month-old babies who were growing up in bilingual households, showing them pictures, for example, of a dog or a book, accompanied by sentences in English or French such as “Look at the dog!” In some cases, however, they mixed languages, in sentences such as “Look at the chien!” or presented sentences in different languages back to back.

They found that when babies heard a word in their dominant language followed by a word in their nondominant language, their pupils dilated up to one-tenth of a millimeter. That small but significant change in brain effort implies that the children recognized the two as belonging to different systems — rather than a mass of interchangeable words — and worked to decode them. “Even at a young age, they have learned the probability of certain words going together, and they prioritize the language they are hearing,” Lew-Williams says. 

That extra computational effort to make the shift implies bilingual children may experience a delay in processing, but only a small one, Lew-Williams says. In addition, the researchers found babies were able to process switches from their nondominant to dominant language with ease. Lew-Williams and his colleagues will follow up their study with additional behavioral and neuroscience research to understand exactly what is happening in the brain. 

In the meantime, however, their study may be good news for parents raising children in a bilingual environment. Past research has shown that children who are raised in bilingual households show better abilities as adults to accomplish work that requires multitasking. The research by Lew-Williams and his colleagues suggests that they may reap those benefits with very little cost while they are young. “This should be reassuring to those who might be skeptical of the human ability to learn two languages at once,” Lew-Williams says. “Children are born with the attention, memory, and pattern-detection abilities to make sense of their complicated perceptual environments, with surprisingly little confusion.”