Linguistic diversity and the 'interaction engine'. In this lecture I argue that our new insights into linguistic diversity require a rethink about the foundations of language. In the first part of the lecture, I outline why strong theories of language universals now look untenable. Combining typological and phylogenetic data suggests that languages are largely structured by cultural evolution, rather than a specific ‘language instinct’. In the second part, I turn to the implications: What then is the nature of the human endowment for language? I argue that there is a substantial infrastructure for language, which is distinct from language itself, and strongly universal, the ‘interaction engine’ of the title. The infrastructure involves speech capacities of course (vocal learning, vocal apparatus), intention-recognition systems (the pragmatics of Gricean meaningnn), and ethological properties of communicative interaction (turn-taking, structured interaction sequences, multimodal signals, etc.). A working hypothesis is that this base, together with general (non-language-specialized) properties of human cognition, provides enough foundation for infants to bootstrap into their local cultural linguistic tradition.