Science educators are placing increasing emphasis on the development of students' interdisciplinary competence. This study examined the degree to which disciplinary, cognitive and affective factors could explain students' (N = 385) individual differences in interdisciplinary competence. Multiple linear regression indicated that students' disciplinary knowledge (DK), attitudes toward interdisciplinary approach and interdisciplinary learning opportunities were significant when added to the prediction model. Notably, the results of a semistructured interview and think-aloud session suggested that students' engineering design knowledge was not robust in their responses. Students' feelings toward interdisciplinary learning affected how they mobilized DK to create comprehensive insight into all aspects relevant to the possible solution. Additionally, students' feelings about the interdisciplinary approach influenced their exposure to interdisciplinary issues outside school and engagement in interdisciplinary learning tasks at school, which affected their integration of scientific ideas across disciplines. The results of this study can inform both the movement toward incorporating an interdisciplinary approach into pedagogy and educators and instructors about the factors and possible mechanisms that may shape students' interdisciplinary competence.