Children follow a consistent pattern when they acquire language. Instead of learning the most common words first, they start by learning a disproportionate number of nouns. In the youngest talkers nouns form up to 60 percent of their vocabulary, compared to just 40 percent of the vocabulary of a typical 2 and a half year-old (who now knows over 600 words).
This pattern applies in many different languages, even Mandarin and Korean, where verbs appear in more prominent positions in sentences. The phenomenon is so universal that it has led some theorists to speculate that acquisition of non-noun forms is simply beyond the cognitive ability of infants — they have to “grow into” using verbs. But there’s another possible explanation: it could be that anyone learning a language is going to learn nouns first, simply because that’s the easiest way to learn language. In this view, the early dominance of nouns has nothing to do with general cognitive ability and everything to do with the process of language learning.
One way to distinguish between the two explanations could be to look at second-language learners. An older child — or even an adult — learning a language clearly has the cognitive ability to understand all types of words. But people learning second languages usually learn them differently from infants: they receive formal instruction; they memorize lists of words. Babies are never “taught” language — they acquire it naturally by watching others speak.
A team led by Jesse Snedeker realized that one population might be able to cast some light on the issue: Children adopted internationally. More than 20,000 kids are adopted in foreign countries and brought to the U.S. each year. If they’re adopted before reaching school age, they learn language in the same way as infants: by watching others speak, rather than formal instruction. Snedeker’s team located 27 recently adopted children between two and a half to five and a half years old and tested their language abilities every three months until they had been in the U.S. for 18 months.
Parents recorded themselves playing with the kids for an hour, and checked off words their kids could produce from a list of 680. Their results were compared to a population of non-adopted American children. This graph shows the number of words they had learned after specified durations in the U.S.:
The adopted children learned words quickly, with many children who’d been in the U.S. for just 12 months possessing the vocabulary of a 30-month-old. Clearly these older children learn language faster than infants — but what about the proportion of nouns in their vocabulary? Take a look at this chart:
As you can see, the proportion of nouns in the adopted children’s vocabulary declines in nearly the identical pattern to the infant controls. Both infants and older children acquire nouns first, followed by other word types.
The researchers also analyzed sentence complexity based on the recordings of parents playing with their kids. Here are those results:
Both the infants and the older kids produced more complex sentences as their vocabulary increased. Again, the pattern of the results was nearly identical — the only difference between language acquisition for infants and older kids was the rate of learning: older kids learn faster.
So the reason that infants learn nouns faster than verbs and other parts of speech isn’t necessarily that they aren’t generally smart or mature enough: older children follow precisely the same pattern of language development.
Snedeker, J., Geren, J., Shafto, C. (2007). Starting Over: International Adoption as a Natural Experiment in Language Development. Psychological Science, 18(1), 79-87.