However, limited attention has been paid to how firms mobilize an initial population of complementors (Dou & Wu, 2018). In nascent ecosystems, firms must either produce complements internally (Hannah & Eisenhardt, 2018; Kaul, 2013) or actively cultivate relationships with external complementors (Dattée, Alexy, & Autio, 2018; Hannah & Eisenhardt, 2018). These initial relationships can jumpstart the virtuous cycle of network effects, i.e., solve the chicken-or-egg problem that platform entrepreneurs face (Caillaud & Jullien, 2003). Although practitioners and scholars recognize the importance to nascent platforms of fostering relationships with complementors, empirical research on how to achieve this has been largely anecdotal (e.g., Iansiti & Levien, 2004; Parker, Van Alstyne, & Choudary, 2016).In this study, we examine a specific strategy that firms use to cultivate relationships with potential complementors: the sponsorship of software development hackathons. Hackathons bring developers together to create new software applications in a short time frame (Lifshitz-Assaf, 2018). Software developers come to these events to learn new skills, create new applications (with teammates), and compete for prizes. As sponsors of the hackathon, platform owners provide financial, in-kind, and in-person logistical support to attending developers. These events are an opportunity for platform owners to evangelize their platform to third-party software developers (Parker, Van Alstyne, & Jiang, 2017).To understand how hackathons mobilize complementors, we build on economic and organizational theories of technology adoption (Rogers, 2003). Sponsorship of a hackathon constitutes an economic subsidy that incentivizes a complementor to join a platform (Economides & Katsamakas, 2006). Beyond this well-recognized incentive effect, we propose that hackathons also act as forums for social influence (Abrahamson & Rosenkopf, 1997; Wade, 1995). We conceptualize a hackathon as a social focus that orients third-party developers towards the sponsoring platform(s) and towards one another (Feld, 1981; Lomi et al., 2014).