This mixed methods study investigated high school students' evaluations of scientific arguments. Myside bias occurs when individuals evaluate belief-consistent information more favorably than belief-inconsistent information. In the quantitative phase, participants (n = 72 males) rated belief-consistent arguments more favorably than belief-inconsistent arguments; however, they also rated strong arguments more favorably than weak arguments, which indicated they did not evaluate the arguments exclusively on whether they were belief-consistent. In the follow-up qualitative phase, we conducted interviews with purposefully-sampled students who showed either higher or lower levels of myside bias. Results indicated that students in both groups applied normative evaluation criteria to the arguments. However, students who showed little or no myside bias applied the same evaluation criteria to arguments independent of whether they were belief-consistent, whereas students who showed high levels of myside bias applied different evaluation criteria to belief-inconsistent arguments. These findings suggest that procedural and conceptual metacognition may play a role in the extent to which individuals reason independent of their beliefs.