Many businesses and private households rely on Free Libre Open Source Software (FLOSS). Due to a lack of sustained contributors, however, most FLOSS projects do not survive. The early identification of developers who are likely to remain is thus an eminent challenge for the management of FLOSS initiatives. Previous research has shown that individuals' subjective assessment is often inaccurate emphasizing the need to objectively evaluate retention behavior. Consistent with the concepts Person-Job (P-J) and Person-Team (P-T) fit from the traditional recruitment literature, we derive objective measures to predict developer retention in FLOSS projects. In an analysis of the contribution behavior of former Google Summer of Code (GSoC) students we reveal that the level of development experience and conversational knowledge is strongly associated with retention. Surprisingly, our analysis reveals that students with abilities that are underrepresented in the project and students with a higher academic education do not remain considerably longer.