In this study, we explore the determinants of cosponsorship activity within state legislatures. Utilizing a social dynamic framework, we develop and test a model of the interplay of the activities of sponsorship and cosponsorship that includes both individual‐level and social network characteristics as determinants of agenda‐setting behavior; the latter demonstrating how collaboration and mutual interests shape the agenda‐setting process. We find several consistent factors that influence the frequency of cosponsorship activity: (1) ideological distance, (2) proximity of legislators' districts, (3) homophily (similar characteristics such as race, gender, and ethnicity), and (4) transitivity (the idea that friends of my friends are also my friends).