Safe to Stay: Psychological Safety Sustains Participation in Pull-based Open Source Projects

Background: Psychological safety refers to the belief that team members can speak up or make mistakes without fear of negative consequences. While it is recognized as important in traditional software teams, its role in open-source software development remains understudied. Open-source contributors often collaborate without formal roles or structures, where interpersonal relationships can significantly influence participation. Code review, a central and collaborative activity in modern software development, offers a valuable context for observing such team interactions. Aims: This study investigates whether team-level psychological safety, inferred from code review activities, is associated with contributors' sustained participation in open-source projects. Method: Using data from 60,684 pull requests across multiple repositories, we developed a psychological safety index based on observable cues such as merge decisions, comment activity, interaction diversity, and mentions. We analyzed the relationship between this index and contributors' short-term (within 1 year) and long-term (over 4--5 years) sustained participation using three logistic regression models. Results: Contributors are more likely to remain active in repositories with higher levels of psychological safety. Psychological safety is positively associated with both short-term and long-term sustained participation. However, prior participation emerges as a stronger predictor of future engagement, reducing the effect of psychological safety when accounted for. Conclusions: This study introduces a scalable, data-driven approach to measuring psychological safety through pull request data and provides new empirical evidence of its relevance in sustaining participation within open-source development.