One of the most well-known mistakes children make when learning to speak is the past tense overregularization (eg. "run" becomes "runned," or "drink" becomes "drinked"). Because children rarely hear adults make these errors, these mistakes show that language acquisition is more than just imitation, and that children must have and use (and, in some cases, overextend) grammatical rules; a recent study by Murray (1996) posits a model explaining the mental rules used in language acquisition, and how they lead to past-tense overgeneralization.

The first element of the model states that people have a default rule that tells them to add -"ed" to the end of a word to make it past-tense; this rule is used whenever the memorized past-tense cannot be recalled (due to failure of memory or the fact that it has never been stored). The second component states that past-tense forms are stored in memory, and since memory is not perfect, recall of irregular verbs is not perfect. Finally, the form of an irregular verb stored in memory always takes precedence over the default rule. The default rule is used only when the past-tense form cannot be recalled.

According to the rule-and-memory model, adults and children use similarly structured grammars; the difference in their speech exists because children have had less exposure to words of all kinds, which means they have fewer irregular verbs stored in their memory.

There are many pieces of evidence that support this theory. First of all, even adults overregularize when using words that are not familiar with (eg. "strive" as "strived"). Second, children overregularize only about 4% of the time, which means that these errors are not due to inaccurate grammar; rather they are more likely to retrieval errors or unfamiliarity with words. Third, the more often parents use an irregular verb, the more likely their child is to correctly use its past-tense. This provides evidence for the theory that the incorrect use of past-tense is due to the failure of a child to remember the correct past-tense, and therefore use the (incorrect) default rule.

Another piece of evidence supporting this theory is that these errors change gradually; if the mistakes were due to grammatical errors, they should disappear suddenly when the child learns the correct grammatical rule.

References Murray, G.F. (1996). Why do children say "breaked"? Current Directions in Psychological Science, 5, 81-85.

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