Most parents try their best to speak clearly and fluently to their toddlers, but they shouldn't sweat it if an occasional "um" or "uh" shows up in their speech.
"In order to understand speech, babies (and adults, too) are constantly generating predictions of what the next word is going to be," explains Richard Aslin, one of the study's authors and a professor of brain and cognitive sciences at the University of Rochester. "Toddlers have learned that when adults have disfluencies it is usually followed by an unusual word."
With graduate student Celeste Kidd and post-doctoral researcher Katherine White, Aslin studied 48 toddlers ranging in age from 18 to 30 months. Each toddler sat on his or her mother's lap and watched a screen with two images. One image was a familiar object like a bed, while the other showed an unfamiliar, made-up object.
The toddlers heard a recorded voice speak a simple fluent sentence, like this one:
Or a sentence with some hesitation thrown in:
At the same time, a special camera was tracking their eye movement. At the end of a fluent sentence, the kids would look at the object that was mentioned. But if the voice stumbled, they looked immediately towards the unfamiliar object, even before it had been named.
"They are making the inference not consciously that when someone has difficulty making a word they are most likely referring to an object that is rare," says Aslin.
The youngest children in the study didn't know that disfluencies precede unknown words. But the 2 1/2-year-olds had figured out that "uh" is a signal it means that unfamiliar information will probably follow. This predictive power helps set the stage for learning.
Does this mean new parents should try to sprinkle a few extra ums and ahs into their conversations with toddlers? Of course not, says Kidd. But it's nice to know that you don't have to speak like a Shakespearean actor to help your child learn. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.