Five experiments examined the extent to which speakers' alignment (i.e., convergence) on words in dialog is mediated by beliefs about their interlocutor. To do this, we told participants that they were interacting with another person or a computer in a task in which they alternated between selecting pictures that matched their 'partner's' descriptions and naming pictures themselves (though in reality all responses were scripted). In both text- and speech-based dialog, participants tended to repeat their partner's choice of referring expression. However, they showed a stronger tendency to align with 'computer' than with 'human' partners, and with computers that were presented as less capable than with computers that were presented as more capable. The tendency to align therefore appears to be mediated by beliefs, with the relevant beliefs relating to an interlocutor's perceived communicative capacity. Copyright © 2011 Elsevier B.V. All rights reserved.