A new study carried out at the MIT Language Acquisition Lab offers a novel insight into the early acquisition of vocabulary—sentences contain subtle hints in their grammar that tell young children about the meaning of new words. The finding, based on experiments with two-year-olds, suggests that even very young kids are capable of absorbing grammatical cues from language and leveraging that information to acquire new words.
Even by age one, many infants seem to think that if they hear a new word, it means something different from the words they already know. But why they think so has remained subject to inquiry among scholars for the last 40 years.
“Even at a surprisingly young age, kids have sophisticated knowledge of the grammar of sentences and can use that to learn the meanings of new words,” says Athulya Aravind, an associate professor of linguistics at MIT.
The new insight stands in contrast to a prior explanation for how children build vocabulary: that they rely on the concept of “mutual exclusivity,” meaning they treat each new word as corresponding to a new object or category. Instead, the new research shows how extensively children respond directly to grammatical information when interpreting words.
“For us it’s very exciting because it’s a very simple idea that explains so much about how children understand language,” says Gabor Brody, a postdoc at Brown University, who is the first author of the paper. The paper is titled, “Why Do Children Think Words Are Mutually Exclusive?” It is published in advance online form in Psychological Science. The authors are Brody; Roman Feiman, the Thomas J. and Alice M. Tisch Assistant Professor of cognitive and psychological sciences and linguistics at Brown; and Aravind, the Alfred Henry and Jean Morrison Hayes Career Development Associate Professor in MIT’s Department of Linguistics and Philosophy.
Focusing on Focus
Many scholars have thought that young children, when learning new words, have an innate bias toward mutual exclusivity, which could explain how children learn some of their new words. However, the concept of mutual exclusivity has never been airtight: Words like bat refer to multiple kinds of objects, while any object can be described using countlessly many words. For instance, a rabbit can be called not only a rabbit or a bunny but also an animal or a beauty, and in some contexts even a delicacy. Despite this lack of perfect one-to-one mapping between words and objects, mutual exclusivity has still been posited as a strong tendency in children’s word learning.
What Aravind, Brody, and Feiman propose is that children have no such tendency and instead rely on so-called focus signals to decide what a new word means. Linguists use the term focus to refer to the way we emphasize or stress certain words to signal some kind of contrast. Depending on what is focused, the same sentence can have different implications. “Carlos gave Lewis a Ferrari” implies contrast with other possible cars—he could have given Lewis a Mercedes. But “Carlos gave Lewis a Ferrari” implies contrast with other people—he could have given Alexandra a Ferrari.
Aravind explained: “The particular claim we’re making is that there is no inherent bias in children toward mutual exclusivity. The only reason we make the corresponding inference is because focus tells you that the word means something different from another word. When focus goes away, children don’t draw those exclusivity inferences anymore.”
“Earlier explanations of mutual exclusivity introduced a whole new problem,” Feiman says. “If kids assume words are mutually exclusive, how do they learn words that are not? After all, you can call the same animal either a rabbit or a bunny, and kids have to learn both of those at some point. Our finding explains why this isn’t actually a problem. Kids won’t think the new word is mutually exclusive with the old word by default, unless adults tell them that it is—all adults have to do if the new word is not mutually exclusive is just say it without focusing it, and they’ll naturally do that if they’re thinking about it as compatible.”
Learning Language from Language
The experiment, the researchers note, is the result of interdisciplinary research bridging psychology and linguistics—in this case, mobilizing the linguistics concept of focus to address an issue of interest in both fields.
“We are hopeful this will be a paper that shows that small, simple theories have a place in psychology,” Brody says. “It is a very small theory, not a huge model of the mind, but it completely flips the switch on some phenomena we thought we understood.”
If the new hypothesis is correct, the researchers may have developed a more robust explanation about how children correctly apply new words.
“An influential idea in language development is that children can use their existing knowledge of language to learn more language,” Aravind says. “We’re in a sense building on that idea, and saying that even in the simplest cases, aspects of language that children already know, in this case an understanding of focus, help them grasp the meanings of unknown words.”
