Many factors may be involved in determining why some students are more and some are less proficient in language. This study tried to explore the factors affecting Iranian EFL learners' proficiency. 221 students at second, third and fourth year of university, including 50 male and 171 female were selected randomly to participate in the study. A researcher-made questionnaire and a proficiency test comprised the data collection instruments. The researchers were able to identify three factors

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... ing language proficiency. They were classified as social factors, cultural factors and linguistic factors, Then a model was developed to represent the relationship among these factors and language proficiency. In line with Bourdieu's (1986), the results of data analysis indicated that social factors are among the most dominant factors affecting the learners' language proficincy. Introduction There has been a growing interest in theories and models of SLA that focus on social context, though they do not address the social factor of the language and its relation to linguistic and culture explicitly. Language, according to socio-cultural theorist Vygotsky (1962), comes out from cultural and social activity and only later becomes reconstructed as an individual, psychological phenomenon. In this way of thinking, SLL theory should be centered not so much on the process of learning new structures and sounds and then using them to communicate, but rather on the learner's participation in social activities such as having out-of-class conversations or talking to classmates and teachers. Lantolf (2002) believed that one of the primary concepts of sociocultural theory is that the human mind is mediated. Lantolf states that Vygotsky finds an important role for what he calls "tools" in humans' realization of the world and of themselves. He maintains, Vygotsky believes that human beings do not act upon the physical world directly and without the using of mediating tools. Whether symbolic or signs, Vygotsky considers tools as artifacts produced by human beings under certain cultural and historical conditions, and they carry with them the characteristics of the culture. They are utilized as aids in solving problems that cannot be solved in the same way if they are not present. In turn, they also have an impact on the individuals who make use of them since they increase the previously unknown activities and previously unknown manners of conceptualizing phenomena in the world. So, they are continually modified while they are passed from one generation to the next, and each generation modifies them with the aim of meeting the needs and aspirations of its individuals and communities. Vygotsky states that the role of a psychologist should be to recognize how human social and mental activities are organized through culturally created artifacts.