Robinson’s Cognition Hypothesis and Skehan’s Limited Attentional Capacity Model provide the major impetus for this study. The present article reports the findings of a between-subject factorial experimental research study which explored 1) the effects of increased cognitive task complexity, manipulated through the intentional reasoning demands and number of elements on the lexical and syntactic complexity, accuracy, and fluency (CAF) of EFL writers’ productions; and 2) the joint effects of cognitive task complexity factor and learners’ language learning aptitude (Low vs. High) on the written output. Firstly, we gave Carroll and Sapon’s Modern Language Aptitude Test (MLAT) to 226 participants and then did a random stratification of the low- and high- aptitude learners into three groups. The participants received letter writing tasks with different cognitive complexity levels (low, medium, and high). The findings indicated that increasing cognitive task complexity resulted in significantly higher lexical and syntactic complexity and lower fluency, whereas no significant effect was found on writing accuracy. Moreover, the statistical results revealed no significant interaction effect between task complexity factors and learners’ language aptitude. With regard to the first objective of the study, the findings supported the predictions of Cognition Hypothesis while it is not the case in relation to the second objective of the study.
