After exploring different theories of teaching and learning ESL/EFL writing, along with successful models of writing courses and instructional methods, this study investigates the internal complexity of ESL/EFL writing at the tertiary level, as well as the contributing factors in students’ writing performance. The characteristics of different types of language learners are identified in conjunction with the plasticity and lateralization of the bilingual brain and the critical period hypothesis. The study also delves into certain biases and heuristics while it analyzes Kahneman’s System 1 and 2 of thinking and various personality theories in order to determine students’ innate abilities to excel in ESL/EFL writing tasks. Furthermore, the paper elucidates the importance of psychological equivalence in the writing process with the help of Pym’s translation theories, and explains students’ interconnectedness through Berne’s transactional analysis and the ego states of each personality. By integrating neuropsychological perspectives with established pedagogical approaches, such as the scaffolding, product, process, genre and grammar-translation techniques, the article offers a holistic framework for educators and policymakers to enhance ESL/EFL writing instruction, and ultimately proposes future research directions for a better understanding of ESL/EFL students’ needs.