Objective: The objective of this study was threefold: (a) to create a rubric for response complexity (RC), defined as an admixture of response length, grammatical diversity, categorisation, and sophistication; (b) to measure behavioural and cognitive engagement through students' written responses on a school-based written activity, and (c) to study if RC in writing can lead to mindset alteration compared to self-report engagement. Design: The study used a randomised control-group pre-test-post-test design with college students. The experimental group participated in mindset activities, while the control group read an article. After each activity, both groups answered open-ended questions. Data were analysed using two models: one with RC and one with self-report engagement as a mediator between group assignment and mindset post-test scores. Results: Using RC as a mediator and controlling for the mindset pre-test, participants in the experimental condition demonstrated a change in mindset at post-test. This did not occur for self-report engagement. Conclusion: Our findings suggest that RC can be used as a measure of engagement through writing that could then lead to learning and knowledge changes. Implications for RC is that teachers may find it as a useful and easily implemented measure of writing and student engagement.