Collaborative software development is a hallmark of agile methodologies such as Extreme Programming (XP). These methodologies have practices like pair programming, where two programmers collaboratively work on all aspects of software development. There is however a dearth of empirical research in this area. Studies with sound theoretical underpinnings and strong empirical rigor are called for to inform the software practice of the effectiveness of this important practice. While such collaborative working is relatively new to software community, small group research has grappled with it over the years looking for the task and other contingencies impacting the effectiveness of collaborative working versus individual working. In this paper, we provide a brief overview of some research streams in small group research and social psychology that could potentially inform IS research on collaborative software development. These small group research topics include group task typologies, individual versus group performance, social facilitation, social loafing, and group motivational gains. We then discuss implications for research on collaborative programming and provide some illustrative research questions.