Second language (L2) researchers, L2 teachers, and textbook designers have shown great interest in the relationship between task characteristics and interlanguage development. Although the literature is inundated with research on the effects of task complexity on speech, less attention has been paid to its effects on writing. To this end, the present study investigated how increasing task complexity led to changes in cognitive load, and in turn, changes in L2 written performance. It also explored whether limiting the number of acceptable solutions to a task, i.e., task closure, had an effect on writing. Finally, the roles of working memory capacity (WMC) and aptitudes for implicit and explicit learning in task performance were investigated as well.

 Eighty-three Korean learners of English and seven L2 teachers deemed as experts were recruited for the study. The L2 learners were randomly assigned to one of two conditions: the Open condition, in which participants carried out open task versions, and the Closed condition, in which they carried out closed versions. Participants carried out two tasks, each with a simple and complex version. Learner self-ratings, expert judgments, and time-on-task were used to obtain independent evidence that increasing task complexity led to changes in cognitive load. An Ospan task and the LLAMA D and F were used to measure WMC and aptitudes for implicit and explicit learning, respectively. 

 A series of mixed effects models revealed that tasks intended to be more complex were perceived as such by both learners and experts. While significant task complexity effects were found on lexical diversity and one measure of accuracy, its effects on syntactic complexity measures were not significant. Task closure effects were only found for lexical diversity, such that open versions elicited more diverse vocabulary than complex versions. Cognitive individual differences also played a role, such that higher WMC was related to greater lexical diversity, and higher aptitude for implicit learning was associated with greater accuracy in writing.