Background: Engineering problems are open-ended and complex, involving technical and social aspects, yet engineering education focuses on technical training and closed-ended problems. To prepare engineering students, curricula should foster sociotechnical thinking--the ability to consider the interplay of technical and social factors during problem solving. Although educational interventions in this area show promise, further insight into the benefits of interdisciplinary interventions is warranted, particularly with treatment and control group designs. Purpose: We examine the effect of a Humanities-Informed Engineering Projects (HIEP) course on students' sociotechnical thinking development. Method: We compared the sociotechnical thinking development of engineering students who took the HIEP course with those who did not, assessing performance at the beginning and end of the semester using a scenario-based assessment. We collected data over 2 years from 70 students (38 treatment, 32 control). We used a triangulated approach to assess students' sociotechnical thinking through quantitative scoring, thematic analysis of problem-related considerations, and coding of students' explanations. Results: Students in the course began the semester with enhanced sociotechnical thinking skills and experienced more development over the semester than those not in the course. They experienced increased development in their consideration of People and the Broader Context when solving problems and greater growth in an integrated approach to sociotechnical thinking. Conclusion: Humanities-informed engineering education leads to sociotechnical thinking development. We suggest curricular activities that may enhance sociotechnical thinking.