Systematic research of instruction-based conceptual change in Mathematics and Science is characterized by examining the effectiveness of a particular instructional principle in isolation. It is suggested that the field could gain from studying how different instructional principles interact when they are combined. The goal of this research was to systematically study the combined effects of collaborative learning and hypothesis testing on cognitive growth. In a randomized experiment, 496 9th graders solved challenging tasks that required fully developed proportional reasoning. Half of them were given the opportunity to test their solutions. Based on individual pretests, each student was assigned to one of three competency levels (low, medium, high), and randomly assigned to either work alone or with a (low, medium, high) peer. The findings show that the effectiveness of hypothesis testing are conditioned by fine-grained differences in the contingencies between the target student’s level of competence, the peer partner’s level of competence and the feedback they receive from the objective testing device.