Influences over individual and collective behavior, as expressed by voting unity patterns in the legislative arena, constitute the main substantive topic for this dissertation. In this project we examine the main determinants of party unity, with an emphasis on explaining when and why party unity breaks down in legislative voting, in settings where party unity is generally high. Researchers have long focused on explaining party unity by relying on institutional factors, such as the structure and characteristics of the electoral system. By combining these dominant institutional approaches of party unity with a micro level study of the individual incentives legislators face to preserve or break the party line, we test and contrast the specific predictions from the literature regarding party and individual legislative voting behavior. The Mexican political system between 1998 and 2011 is taken as the case study to test the theoretical propositions set in the literature, as it combines availability of electronic legislative voting records with a mixed member electoral system that does not allow for consecutive reelection.