At around one year of age, most babies utter their first word.  A short time later, a second word is spoken. Bob McMurray, a professor of  psychology at the University of  Iowa in the United States, says it took a child 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 that 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.
When the burst of language occurs, McMurray says, the conventional theory among  psychologists is that toddlers are rapidly developing specialised learning tools, such as the ability to recognise that objects have names.
However, 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.
Critical factor
"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.
McMurray believes his model demonstrates that the word spurt of toddlers is the result of their understanding of more complex words.
     "Kids are keeping track of  lots and lots of words at the same time," he says. "They may not be using all of the words they have learned, but as they are hearing words, they are soaking them up. They do not wait until they have finished learning one word  before  moving on to the next.”