Linguistics also includes nonformal approaches to the study of other aspects of human language, such as social, cultural, historical and political factors.[13] The study of cultural discourses and dialects is the domain of sociolinguistics, which looks at the relation between linguistic variation and social structures, as well as that of discourse analysis, which examines the structure of texts and conversations.[14] Research on language through historical and evolutionary linguistics focuses on how languages change, and on the origin and growth of languages, particularly over an extended period of time.