This comparative, mixed-methods study illustrates the impact of weekly facilitated peer review ("Writing Circles") in STEM courses across time: 1) in a lower-division course, Circles improve all learning outcomes for writing and critical thinking, and most significantly, writing; 2) in an upper-division course, Circles are most effective at improving learning outcomes for critical thinking; 3) when comparing scores in the lower- and upper-division courses, we see that critical thinking improves significantly from second to fourth year; 4) finally, we see that upper-division students grant their peers more disciplinary authority during the Circles peer review.