) Cognition Hypothesis and Skehan"s (1998, 2003) Trade-Off Hypothesis -intends to explore how second language writing can improve language learning through manipulating cognitive task complexity dimensions. Second, it attempts to examine if task complexity factors and cognitive learner-related variables have interaction effects on linguistic production. It focuses on planning time and intentional reasoning demands, language learning aptitude, and measures of lexical complexity, syntactic complexity, and accuracy. The participants were 226 intermediate Iranian EFL learners. In a between-subjects design, the participants were first assigned to three experimental groups randomly, each group receiving a task with different level of reasoning demands. Then, within each group, we did a stratified random sampling and assigned an equal number of learners with higher and lower aptitude levels to planned and unplanned groups. The results revealed that (a) planning time availability led to significantly more syntactic complexity, (b) increasing task complexity with regard to reasoning demands resulted in higher gains of lexical and syntactic complexity, whereas no significant effect was detected on accuracy, (c) a significant interaction effect between planning and reasoning demands was found only on accuracy, and (d) a three-way interaction among planning, reasoning demands, and language learning aptitude was observed on accuracy. Generally, the findings are discussed with regard to the predictions of the Cognition Hypothesis and the Trade-Off Hypothesis.Keywords: task complexity factors, planning time, reasoning demands, language aptitude
IntroductionTask utilization has been an appealing issue in the field of second language teaching and research in the past two decades. Although the relationship between different task designs and spoken production has been extensively studied, task-based research with writing is quite emerging in the literature. According to Kuiken and Vedder (2008), "in the literature on both L1 and L2 writing, it has been suggested that some task types result in lower test scores than others, but the relationship between task type or task complexity and writing performance is by no means clear" (p. 49). The influence of task complexity on language production is a principal issue in task-based language learning and teaching. In order to manipulate task complexity, researchers have utilized various complexity features, such as pre-task planning time, reasoning demands, here-and-now variables, and removal of narrative context (e.g., Ishikawa, 2007; Simin Sattarpour et al.The Journal of Asia TEFL Vol. 14, No. 4, Winter 2017, 736-754 737 Kormos, 2011;Kuiken & Vedder, 2007, 2008Ong, 2014;Ong & Zhang, 2010, 2013.Due to high diversity in manipulated task features, linguistic measures, and consequently inconclusive results, more similar research studies are needed to reach rigorous conclusions. Johnson (2017), who did a comprehensive meta-analysis of the task complexity research in L2 writing, contend...