A new study carried out at the MIT Language Acquisition Lab offers a novel insight into the matter: Sentences contain subtle hints in their grammar that tell young children about the meaning of new words. The finding, based on experiments with 2-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 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 Fieman 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.

The researchers’ experiments manipulated focus in three experiments with a total of 106 children. The participants watched videos of a cartoon fox who asked them to point to different objects.

The first experiment established how focus influences kids’ choice between two objects when they hear a label, like “toy,” that could, in principle, correspond to either of the two. After giving a name to one of the two objects (“Look, I am pointing to the blicket”), the fox told the child, “Now you point to the toy!” Children were divided into two groups. One group heard “toy” without emphasis, while the other heard it with emphasis.

In the first version, “blicket” and “toy” plausibly refer to the same object. But in the second version, the added focus, through intonation, implies that “toy” contrasts with the previously discussed “blicket.” Without focus, only 24 percent of the respondents thought the words were mutually exclusive, whereas with the focus created by emphasizing “toy,” 89 percent of participants thought “blicket” and “toy” referred to different objects.

The second and third experiments showed that focus is not just key when it comes to words like “toy,” but it also affects the interpretation of new words children have never encountered before, like “wug” or “dax.” If a new word was said without focus, children thought the word meant the previously named object 71 percent of the time. But when hearing the new word spoken with focus, they thought it must refer to a new object 87 percent of the time.

“Even though they know nothing about this new word, when it was focused, that still told them something: Focus communicated to children the presence of a contrasting alternative, and they correspondingly understood the noun to refer to an object that had not previously been labeled,” Aravind explains.

She adds: “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 any more.”

The researchers believe the full set of experiments sheds new light on the issue.

“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.”

The scholars acknowledge that more studies could further advance our knowledge about the issue. Future research, they note in the paper, could reexamine prior studies about mutual exclusivity, record and study naturalistic interactions between parents and children to see how focus is used, and examine the issue in other languages, especially those marking focus in alternate ways, such as word order.

The research was supported, in part, by a Jacobs Foundation Fellowship awarded to Feiman.