The interaction engine is a label for the specialized capacities involved in human social interaction. The interaction engine comprises a suite of specific abilities that enable communication, for example, the capacity for rapid conversational turn-taking using multimodal signals, with systematic contingency between initiating signal and response—in which that contingency is often inference based rather than conventional in character. The properties of informal interaction subsumed within the broader “interaction engine” label appear to be largely universal, in contrast to the diversity of languages and many other aspects of social behavior. They seem to provide a platform that makes it possible for children to acquire their particular language and culture.