To answer these questions, David Lightfoot looks closely at young children. A child develops a grammar on exposure to some triggering experience. A small perturbation in the trigger may entail a different grammar in the next population of speakers, with dramatic effects. This "sensitive dependence on initial conditions" is the key to explaining how languages change, and why they change in fits and starts.

The "cue-based" approach to language acquisition presented here is a radical departure from formal models of language learning. Lightfoot challenges conventional understanding by showing that language change is essentially contingent - unpredictable but explainable; and he contests how far natural selection enables us to understand the evolution of the language faculty in the species.