There's a period in the development of toddlers when they seem to learn a new word every day.  There are different theories about why young children undergo what experts call a word spurt. But one psychologist believes it may have to do with how young children learn difficult words.   VOA's Jessica Berman reports.

Bob McMurray is a professor of psychology at the University of Iowa.

"And it would be very tempting to say, 'Wow, look at the rate of acquisition has sped up,' right?  'It took them a year to learn mommy but only a month to learn daddy.' But in actuality that child was probably working on that word daddy all along.  They really spend a year and a month to learning the word daddy," he explained.

McMurray says once toddlers begin speaking their first few words, it is only a matter of time before they seem to learn words at a faster rate.  According to the psychologist, the first word spurt usually occurs around 18 months of age.

A toddler learns the word for 'sheep'

But according to a computer model McMurray created, toddlers have "word spurts" after they have gathered enough easy, similar sounding words - like mama, dada, dog and cat.  Once they have mastered these words, by repeating them over and over again, they then tackle more difficult words.  McMurray says the exposure to more difficult words is a key factor in the word spurt.

"The critical factor is actually how many words are hard and how many words are easy.  So, as long as you have, and this is what we were able to prove mathematically, more difficult words than easy words, you're always going to get some kind of acceleration in word learning," he added.

"Kids are keeping track of lots and lots of words at the same time," he explained.  "They may not be using all of the words they've learned, but as they are hearing words, they are soaking [them] up.  They are not waiting until they are done learning one word before moving on to the next."

The study on toddlers' word development is in the journal Science.