The extent of co-authorship in IS research is on the rise. Why has collaboration between IS academics increased? While prior research on the incidence of co-authorship provides several reasons for why academics collaborate, little is known about whether these rationales are equally adept at explaining the growing extent of co-authorship. To answer this question, we delve into extant research on collaboration and delineate four rationales for why papers have more co-authors. These include information processing, access to social resources, convenience, and the opportunity cost of time. We formulate several variables and propose several hypotheses based on these rationales. We collected data by coding 641 papers from six major U.S. and European journals. The results generally support the proposed hypotheses. We discuss the implications of the results in terms of how they inform the field and policy makers.