This paper presents a stochastic simulation modelling tool used to study the implications of the mechanisms by which individual software developers’ efforts are allocated within large and complex open source software projects. It thus illuminates the role of different forms of “motivations-at-the-margin” in the micro-level resource allocation process of distributed and decentralized multi-agent engineering undertakings of this kind. We parameterize the model by isolating the parameter ranges in which it generates structures of code that share certain empirical regularities that are found to characterize actual projects. We find that, in this range, a variety of different motivations are necessarily represented within the community of developers. There is a correspondence between the indicated mixture of motivations and the distribution of avowed motivations for engaging in FLOSS development, found in the survey responses of developers who were participants in large projects.