EAP students’ performance on reading comprehension tests is affected by English proficiency and discipline-specific background knowledge. Proponents of the Language Threshold Hypothesis (LTH) propose that the contributory effects of background knowledge are bound to two language thresholds: high and low proficiency levels; for high and low proficiency students, it does not make difference whether they take reading tests from inside or outside of their fields. On the other hand, it is suggested that the predictive power of the Hypothesis might fluctuate due to content specificity of reading texts. The study aims to, firstly, investigate the predictive potential of the LTH in the Iranian EAP context, and, secondly, to examine if the effects of language proficiency and background knowledge fluctuates with content specificity. Following convenience sampling, one-hundred sixty graduate and undergraduate students of biology and psychology took five tests, including an English proficiency test, two knowledge tests, and two cloze tests for psychology and biology, with each containing three paragraphs of varying levels of content specificity. A three-way factorial ANOVA indicated that the LTH was partially supported; no evidence was found for high proficiency threshold and partial evidence was observed for the lower threshold. It was also found that the contributory effect of English proficiency on Iranian students’ reading comprehension significantly fluctuates with changes in content specificity of reading tests; as content specificity increased, high and low proficiency students’ performances on field-related and field-unrelated tests became poorer and with a decrease in content specificity their performances improved. The implications of the study are discussed.
