This study aimed at examining the main and interaction effects of increased intentional reasoning demands, planning time, and also language learning aptitude on syntactic complexity, accuracy, lexical complexity, and fluency (CALF) of 226 EFL learners’ performance on letter writing tasks. The participants were first randomly assigned to three experimental groups to be given a task with differing degrees of reasoning demand (low, medium, and high) to each group. Then, within each reasoning group, we reassigned an equal number of high- and low- aptitude learners to Planning and No-planning groups by random stratified sampling. The results revealed that (a) increasing task complexity with regard to the amount of intentional reasoning demands resulted in greater lexical and syntactic complexity and less fluency while no significant effect was observed on accuracy; (b) increasing task complexity through planning time led to significantly lower syntactic complexity and fluency; (c) reasoning demands and planning time had a significant interaction effect on accuracy; and (d) the interaction effect of language aptitude was significant with neither planning nor reasoning factor, but a three-way interaction effect was found on accuracy. The findings are discussed in relation to cognitive task complexity (CTC) models which were the main impetus for this study.
