Experimental studies of the cultural evolution of language have focused on how constraints on learning and communication drive emergence of linguistic structure. Yet language is typically transmitted by experts who adjust the input in ways that facilitates learning by novices, e.g. through child-directed speech. Using iterated language learning of binary auditory sequences, we explored how language change is affected by experts’ intention to teach the language to novices. Comparison between teaching chains and simple transmission chains revealed that teaching was associated with a greater rate of innovation which led to emergence of more expressive languages consisting of shorter signals. This is the first study to show that during cultural transmission, teaching can modify, and potentially optimise, functional characteristics of language.