A new study conducted at the MIT Language Acquisition Lab provides new insight: sentences contain subtle cues in their grammar that tell young children the meaning of new words. The finding, based on experiments with 2-year-olds, suggests that even very young children are capable of absorbing grammatical cues from language and exploiting that information to acquire new words.

“Even at a surprisingly young age, children have sophisticated knowledge of the grammar of sentences and can use it to learn the meaning of new words,” said Athulya Aravind, associate professor of linguistics at MIT.

The new insight contrasts with a previous 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 much children react 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,” said 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 i Psychological science. The authors are Brody; Roman Feiman, Thomas J. and Alice M. Tisch Assistant Professor of Cognitive and Psychological Science 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.

Focus on focus

Many researchers have believed that young children, when learning new words, have an innate bias against mutual exclusivity, which may 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 types of objects, while any object can be described by countless many words. For example, a rabbit can be called not only a “rabbit” or a “rabbit”, 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 shown to be a strong tendency in children’s word learning.

What Aravind, Brody, and Fieman suggest is that children have no such tendency, but instead rely on so-called “focus” cues to determine what a new word means. Linguists use the term “focus” to refer to how we stress or emphasize certain words to signal some kind of contrast. Depending on what is focused on, the same sentence can have different implications. “Carlos gave Lewis one 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’ experiment manipulated focus in three experiments with a total of 106 children. Participants watched videos of a cartoon fox asking them to point to various objects.

The first experiment determined how focus affects children’s choice between two objects when they hear a label, such as “toy,” that could in principle correspond to either of the two. After giving a name to one of the two objects (“Look, I point to the look”), the fox said to the child, “Now you point to the toy!” The children were divided into two groups. One group heard “toy” without emphasis, while the other heard it with emphasis.

In the first version, “the look” and “toy” probably refer to the same object. However, in the second version, the added focus, through intonation, means that “toy” is contrasted with the previously discussed “look”. Without focus, only 24 percent of respondents thought the words were mutually exclusive, while with focus created by emphasizing “toy,” 89 percent of participants thought “the look” and “toy” referred to different objects.

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

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

She adds: “The particular claim we make is that there is no inherent bias in children towards mutual exclusivity. The only reason we draw the corresponding conclusion is that the focus tells you that the word means something other than another word. When the focus goes away, kids don’t make these exclusivity conclusions anymore.”

The researchers believe that the whole set of experiments sheds new light on the question.

“Earlier explanations of mutual exclusivity introduced a whole new problem,” says Feiman. “If children assume that 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 rabbit, and children have to learn both at some point. Our finding explains why this is not actually a problem Children will not believe that the new word excludes the old word by default, unless adults tell them it is – all adults must do if the new word is not mutually exclusive just say it without focusing on that, and of course they will if they think of 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 linguistic focus concept to address a question of interest in both fields.

“We hope this will be a paper that shows that small, simple theories have a place in psychology,” says Brody. “It’s a very small theory, not a grand 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 of how children use new words correctly.

“An influential idea in language development is that children can use their existing knowledge of language to learn more language,” says Aravind. “We kind of build on that idea and say that even in the simplest cases, aspects of language that children already know, in this case an understanding of focus, help them understand the meaning of unfamiliar words.”

The researchers acknowledge that more studies could further advance our knowledge of the issue. Future research, they note in the paper, could reexamine previous studies on mutual exclusivity, record and study naturalistic interactions between parents and children to see how focus is used, and examine the issue in other languages, particularly those that mark focus in alternative ways, such as for example as word order.

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