Highlighting scientists from historically excluded groups in educational materials increases student engagement in STEM. However, which specific elements of these educational materials maximize their impact remains untested, leaving educators guessing how to best highlight counter-stereotypical scientists in their classrooms. We tested the effects of including visual and humanizing descriptions of scientists featured in quantitative biology activities on over 3,700 students across 36 undergraduate institutions. We found that including humanizing information about counter-stereotypical scientists increased the extent to which students related to those scientists, which in turn translated to higher student engagement. Students who shared one or more excluded identity(s) with the featured scientists related most strongly. Our findings demonstrate the importance of humanizing counter-stereotypical scientists in classrooms, beyond simply adding a photo to increase representation.