Twitter users change word usage according to conversation-partner social identity

•We study social identity on communities found on the Twitter website.•We find users adjust word usage according to the community of the interlocutor.•More isolated communities change word usage to a greater extent.•Large scale studies of Twitter can test hypotheses on social behaviour.

This paper investigates how people express social identity at a large scale on a social network. We looked at communities of users on the Twitter website, and tested two established social-psychology theories that are usually performed at local scale. We found evidence of Communication Accommodation Theory, where community members vary their language characteristics depending on which community they are communicating with. We also found the level of linguistic variation correlated with how isolated a community was: evidence that there is Convergence between linked members. This demonstrates the power of methods which analyse subtle human behaviour on social networks.