Psycholinguists estimate that the average English-speaking six-year-old knows 10,000 words, and the average high-school graduate around 60,000 words. Paul Bloom describes how this estimate can be made.

Words are taken from a large unabridged dictionary, including only those words whose meanings cannot be guessed using principles of morphology or analogy. . . . Since it would take too long to test people on hundreds of thousands of words, a random sample is taken. The proportion of the sample that people know is used to generate an estimate of their overall vocabulary size, under the assumption that the size of the dictionary is a reasonable estimate of the size of the language as a whole. For example, if you use a dictionary with 500,000 words, and test people on a 500-word sample, you would determine the number of English words they know by taking the number that they got correct from this sample and multiplying by 1,000.

Children generally begin to produce their first words around the age of one. Bloom calculates that between the ages of one and 18 we would have to learn approximately ten words every day to have a vocabulary of 60,000 words. It’s worth pointing out, I think, that this figure just takes into account the words that we have stored (fully or partially) in our mental lexicon, and not the words – perhaps an infinite number of them – that we can create by using rules of word formation. We will return shortly to our knowledge of word formation rules and its relation to our mental lexicon. First, however, we will look more closely at how we acquire our mental lexicon.