This study investigates whether differences in candidate selection methods and/or the changes in the Israeli political system affect cooperation between parliament members and whether such cooperation explains legislative effectiveness. To examine these issues, we discuss different types of cooperative strategies using a scale we devised that defines three options for cooperation: 1) uncooperative, 2) cooperation within the party and 3) cooperation between parties. Then, we categorize the various methods that Israeli political parties have used to select their candidates since the establishment of the state, creating four categories of a variable we call the effect of the primaries. Additionally, we consider differences in four periods of changes in the Israeli party system. Finally, we assess the results of cooperation in light of our dependent variable, legislative effectiveness, using data from 1949 to 2015. Our findings indicate that the majority of bills passed without cooperation, but when cooperative strategies were used, they usually involved inter-party rather than intra-party support. Furthermore, the adoption of primaries reduced the probability of passing bills. In addition, when one party was dominant, 68% of the representatives initiated legislation alone, while during the multi-polar fragmented period 41.9% cooperated with legislators from other parties.