The question of whether and how bilingualism affects domain general cognition has been extensively debated. Less attention has been paid to the cognitive abilities of speakers of different variants of the same language, in linguistic situations such as bidialectalism and diglossia. Similarly to the bilingual situation, in bidialectalism and diglossia speakers need to use only one variant of the language in a given context. However, these situations provide fewer opportunities for mixing or switching between the variants, potentially leading to different domain general cognitive outcomes than those reported in bilingualism. Here we review the available evidence on the effects of bidialectalism and diglossia on cognition, and evaluate it in relation to theories of the effects of bilingualism on cognition. We conclude that investigations of bilingualism, bidialectalism and diglossia must take into account the conversational context and, in particular, the opportunities for language switching that this affords.