As a part of their normative theory of expertise, Harry Collins and Robert Evans proposed that interactional expertise forms the third kind of knowledge, located between formal propositional knowledge and embodied skills. Interactional expertise refers to the capability to grasp the conceptual structure of another’s social world, and it is expressed as the ability to speak fluently the language spoken in that social world. According to their theory, it is a key concept of sociology, because it refers to the understanding and coordination of joint actions between members of different social groups. Collins and Evans have further claimed that minority social group members tend to outpace majority social group members in terms of interactional expertise. Drawing on ethnomethodology, we detail the ways in which interactional expertise is displayed and revealed in experiments. This allowed us to specify the underlying reasons for the distribution of interactional expertise between social groups. Our results indicate that the difference between the groups depends on whether a group is either actively maintained or a passive latent category, because interactional expertise provides for not only the crossing of social boundaries but also their maintenance. The minority social group members’ greater interactional expertise or competence is therefore proven to be illusory.