According to Basil Bernstein theory the result of this research is about the effects of stratification of the social class on the development of lexicons and structures of written language in Mashhad girl’s primary educational schools. Bernstein theory (1975) who surveys the effects of social class differences on the use of grammatical categories is implied about: numbered used in lexicons, nouns, nominal groups, personal pronouns, adjectives, adverbs, and prepositions in order to answer the research questions. Based on statistical conclusions of the frequency of occurrence in these grammatical categories within the writing of the upper middle class relative to lower class are more visible. This finding shows that upper class students are more knowledgeable and skillful, compared to lower class students. Reasons for such skillfulness are due to reading more books, magazines and news papers. The main outcome of this research is the structural differences of the social class in the society can be an important factor in causing two different speech codes. At the end social class differences can effect the proficiency of the language which primary students use.
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