In this selection from his Floating University lecture, Professor Steven Pinker deduces the nature of language acquisition by examining the generative use of grammar in children.

Children are Hard-Wired with Universal Grammar

Another important contribution of Chomsky to the science of language is the focus on language acquisition by children. Now, children can’t memorize sentences because knowledge of language isn’t just one long list of memorized sentences, but somehow they must distill out or abstract out the rules that go into assembling sentences based on what they hear coming out of their parent’s mouths when they were little.  And the talent of using rules to produce combinations is in evidence from the moment that kids begin to speak.  

Children create sentences unheard from adults

At the two-word stage, which you typically see in children who are 18 months or a bit older, kids are producing the smallest sentences that deserve to be counted as sentences, namely two words long.  But already it’s clear that they are putting them together using rules in their own mind.  To take an example, a child might say, “more outside,” meaning, take them outside or let them stay outside.  Now, adults don’t say, “more outside.”  So it’s not a phrase that the child simply memorized by rote, but it shows that already children are using these rules to put together new combinations.  

Another example: A child having jam washed from his fingers said to his mother, "all gone sticky." Again, not a phrase that you could ever have copied from a parent, but one that shows the child producing new combinations.  

Past tense rule

An easy way of showing that children assimilate rules of grammar unconsciously from the moment they begin to speak, is the use of the past tense rule.

For example, children go through a long stage in which they make errors like, “We holded the baby rabbits” or “He teared the paper and then he sticked it.”  Cases in which they overgeneralize the regular rule of forming the past tense, add ‘ed’ to irregular verbs like “hold,” “stick” or “tear.”  And it’s easy to show… it’s easy to get children to flaunt this ability to apply rules productively in a laboratory demonstration called the Wug Test.  You bring a kid into a lab.  You show them a picture of a little bird and you say, “This is a wug.”  And you show them another picture and you say, “Well, now there are two of them.”  There are two and children will fill in the gap by saying “wugs.”  Again, a form they could not have memorized because it’s invented for the experiment, but it shows that they have productive mastery of the regular plural rule in English.  

And famously, Chomsky claimed that children solved the problem of language acquisition by having the general design of language already wired into them in the form of a universal grammar, a spec sheet for what the rules of any language have to look like.