Collaborative learning is a common teaching strategy in classrooms across age groups and content areas. It is important to measure and understand the cognitive process involved during collaboration to improve teaching methods involving interactive activities. This research attempted to answer the question: why do students learn more in collaborative settings? Using three measurement tools, 142 participants from seven different biology courses at a community college and at a university were tested before and after collaborating about the biological process of natural selection. Three factors were analyzed to measure their effect on learning at the individual level and the group level. The three factors were: difference in prior knowledge, sex and religious beliefs. Gender and religious beliefs both had a significant effect on post-test scores.