2 AUGUST 2007
The "vocabulary explosion" that occurs around 18 to 24 months of age
is one of the more amazing feats of human learning. After months of sipping
and snacking at language, the toddler begins to guzzle and gulp.
A typical child learns 8 to 10 words a day for years on end, until by adulthood, the "personal dictionary" can exceed 50,000 words. Such a considerable effect seems to require a considerable cause, and some specialists in language acquisition have proposed that the human brain is uniquely suited to learning language. Others have suggested various ways that word-learning builds upon itself through a kind of positive feedback process.
Photo: Michael Forster Rothbart, Courtesy UW-Madison
And now here comes something completely different: The explosion is an inevitable result of a toddler's immersion in words of varying difficulty. Bob McMurray, assistant professor of psychology at the University of Iowa, says the key is repeated exposure to words that are more or less difficult to learn.
In the vocab explosion, he says, it may actually be that "nothing is happening." McMurray bases his results on a computer model of learning that emerges from several assumptions:
A few words are easy to learn, a few are difficult, but most lie in between;
If you grant these assumptions, logic says that for any particular child, the
time needed to learn a word depends on its difficulty and the number of exposures.
Imagine a system where each exposure to a word earns you 10 cents toward "learning" it.
Some words are cheap: You can learn "dog" for just $1.00. Others are more expensive: "Peacock" may cost $2.00, while a toughie word like "elaboration" might cost $5.00.
If you are in an environment where you hear all these words, after just 10 exposures, you can buy "dog," but you'll need another 10 exposures to get "peacock," and a total of 50 to get "elaboration."
Now you can understand a critically useful sentence like, "dog elaboration of peacock."
Photo: Michael Forster Rothbart, Courtesy UW-Madison
More important, this process of learning lots of words at once is the root explanation for the vocab explosion, McMurray says. "You are working on all these at the same time, and every day, you get a step closer to learning all your words. Some have a low threshold, and you step over that quickly."
Linguistic linguini?
Others words take longer, but because so many words have intermediate difficulty, the result is the extraordinary period where the toddler gulps, not sips, words. "You only learn a couple of words a day at the beginning, but at some point, you hit the midrange, and you learn lots of words at once," McMurray says.
The beginning of the vocab explosion: Increasing exposure
to words leads to a soaring level of word knowledge. Courtesy
Bob McMurray, University of Iowa
Many other factors could influence the rate of learning, and a lot of researchers have gone to great lengths to explore them. Perhaps a toddler gets the insight that things have names, and becomes curious about them. Or learning some words may make it easier to learn the others. Perhaps the toddler will use "fast mapping," the notion that unknown words are associated with things that have unknown names.
McMurray's computer model easily accommodates these factors. For example, if
learning some words makes it easier to learn others, you simply reduce the
price on unknown words so they are "cheaper" to learn. 
When that factor is plunked into the computer model, "vocabulary acquisition is a little faster than otherwise," says McMurray.
What accounts for the rapid rise in vocabulary around age 2? Photo: Michael Forster Rothbart, Courtesy UW-Madison
Learning words could also have a cost: Perhaps they hog space in your mental "hard disk," leaving less room for future words. You can simulate this by hiking the price of unknown words, but "you still get the vocabulary spurt in my model," says McMurray.
Despite the possible role of these factors, McMurray says, the predominance of intermediate-difficulty words is what drives the vocab explosion.
Guzzling grammar?
Many linguists have maintained that the human brain has a unique ability to learn language; the most influential of them, Noam Chomsky, argues that grammar is innately programmed in the brain. McMurray, however, credits a much more generalized ability for the vocab explosion. "Babies are powerful learners, but they are not necessarily engaging in a lot of tricky, clever mechanisms" while learning language.
A
toddler checking out these Mr. Potato Head's glasses is presumably in the midst
of a vocabulary explosion. Photo: Michael Forster Rothbart, Courtesy
UW-Madison
Young humans, he observes, have exceptional learning powers in many realms, not just language but also art, music, athletics, problem-solving and social skills. "The growth in vocabulary is really a function of using a much broader, general-purpose mechanism," McMurray says. "The specialized abilities may play a role in learning some words, but the big pattern of data does not have much to do with that."
Instead, he maintains, the vocabulary explosion is an inevitable reaction to a situation where a learning machine (AKA a toddler) is repeatedly exposed to words of varying difficulty.
-- David Tenenbaum
Related Why Files
• Numbers and language.
• Learning language.
• Inventing language.
Bibliography
• Defusing the Childhood Vocabulary Explosion,
Bob McMurray, Science, 2 August 2007.