Copyleft prevents the source code of open source software (OSS) from being privately appropriated. The ethos of the OSS movement suggests that volunteer developers may particularly value and contribute to copylefted projects. Based on social movement theory, we hypothesized that copylefted OSS projects are more likely than non-copylefted OSS projects to succeed in the development process, in terms of two key indicators: developer membership and developer productivity. We performed an exploratory study using data from 62 relevant OSS projects spanning an average of three years of development time. We found that copylefted projects were associated with higher developer membership and productivity. This is the first study to empirically test the relationship between copylefted licenses and OSS project success. Implications for OSS project initiators as well as future research directions are discussed.