— Young children become chatterboxes within months of barely being able to speak a few words. Now one scientist thinks he knows why.
Children do not need any specialised learning to suddenly improve their vocabularies, says language psychologist Bob McMurray at the University of Iowa in Iowa City, US. Instead, their behaviour can be described by a simple mathematical rule of thumb.
Parents of small children will be familiar with the so-called "word spurt", the slightly disconcerting stage of a child's life when they go from hardly talking to suddenly uttering hundreds of new words, sometimes after hearing them only once.
At 18 months, for instance, the average child can say 50 words, but by age two, they have learned up to 350 words; just half a year later, that has doubled to 600.
Scientists have proposed various theories to explain the phenomenon. For instance, perhaps learning a few basic words helps a child learn others. The theory of "naming insight", for instance, suggests that at around 18 months children suddenly realise that each object has a specific name.
Another theory, called "fast mapping", suggests that children quickly understand that groups of objects are related, and therefore they learn unfamiliar words describing objects within familiar groups more quickly.
According to McMurray, however, there is a much simpler explanation. The acceleration in a child's learning will inevitably happen due to the way most languages are structured.
All languages, he says, contain a distribution of words, where most are of medium difficultly to learn, while fewer are either very easy, or very difficult. And children always learn a number of words in parallel. He factored these parameters into a computational model, which then simulates how long it takes to learn 10,000 new words.
On each simulation the model produced the same characteristic acceleration in learning. Essentially learning one new word makes learning another new word even easier. This allows a child to move through words of medium difficulty more quickly. "Acceleration is an unavoidable by-product of variation in difficulty," he says.
"Mathematically this may be true," says speech expert Lisa Gershkoff-Stowe at the University of Indiana at Bloomington, US. But she cautions that it isn't the first time a researcher has tried to explain the spurt with a computational model, and that McMurray's idea "doesn't get to the heart of why kids learn faster."
The model also doesn't explain why older people learning a second language don't show a similar acceleration in their learning, she says. "They ought to show it," he says, "and I'm not sure why they don't."
Journal reference: Science (vol 317, p 631).