Why Do Children Experience A Vocabulary Explosion At 18 Months Of Age?

"The field of developmental psychology and language development has always assumed that something happens at that point to account for this word spurt: kids discover things have names, they switch to using more efficient mechanisms and they use their first words to help discover new ones," said Bob McMurray, assistant professor of psychology in the UI College of Liberal Arts and Sciences. "Many such mechanisms have been proposed."

In an article about his study on the topic being published Aug. 3 in the journal Science, McMurray writes that children may still engage those specialized mechanisms. But a series of computational simulations that he conducted suggest that simpler explanations - such as the repetition of words over time, the fact that children learn many words at the same time and the fact that words vary in difficulty - are sufficient to account for the vocabulary explosion.

"Children are going to get that word spurt guaranteed, mathematically, as long as a couple of conditions hold," McMurray said. "They have to be learning more than one word at a time, and they must be learning a greater number of difficult or moderate words than easy words. Using computer simulations and mathematical analysis, I found that if those two conditions are true, you always get a vocabulary explosion."

McMurray's simulations are analogous to a series of jars of different sizes, each representing a word, with more difficult words represented by larger jars. As individual units of time passed, a chip is dropped into each jar. Once the jar is filled, the word is learned.

McMurray's mathematical analysis suggests that the word spurt is largely driven by the number of small jars (easy words) relative to large jars (difficult words). As long as there are more difficult words than easy ones, the vocabulary explosion is guaranteed.

Few words in any language are used an overwhelming number of times in ordinary speech. So, if frequency of use is considered as a measure of degree of difficulty, languages have many more difficult than easy words, McMurray said.

Experts have long thought that once a child learns a word, it is easier for him or her to learn more words. Or in the case of McMurray's simulation, the jars become smaller. But McMurray also simulated a model in which the jars became larger once a word was learned and found that the vocabulary explosion still occurred.

"If we see the same word spurt when we model the inverse of accepted thinking, then clearly the specialized mechanisms aren't necessary," he said. "Our general abilities can take us a lot farther than we thought."