How well does teacher talk support incidental vocabulary acquisition?

Opportunities for incidental vocabulary acquisition were explored in a 121,000-word corpus of teacher talk addressed to advanced adult learners of English as a second language (ESL) in a communicatively-oriented conversation class. In contrast to previous studies that relied on short excerpts, the corpus contained all of the teacher speech the learners were exposed to during a 9-week session. Lexical frequency profiling indicated that with knowledge of 4,000 frequent words, learners would be able to understand 98% of the tokens in the input. The speech contained hundreds of words likely to have been unfamiliar to the learners, but far fewer were recycled the numbers of times research shows are needed for lasting retention. The study concludes that attending to teacher speech is an inefficient method for acquiring knowledge of the many frequent words learners need to know, especially since many words used frequently in writing are unlikely to be encountered at all.