Gender diversity signals inclusivity, but meta-analyses suggest that it does not boost individual or group performance. This research examined whether a social-psychological intervention can unlock the benefits of gender diversity on college physics students' social and academic outcomes. Analyses of 124 introductory physics classrooms at a large research institution in the eastern United States (N = 3,605) indicated that in classrooms doing "business as usual," cross-gender collaboration was infrequent, there was a substantial gender gap in physics classroom belonging, and classroom gender diversity had no effect on performance. The ecological-belonging intervention aimed to establish classroom norms that adversity in the course is normal and surmountable. In classrooms receiving the intervention, cross-gender interaction increased 51%, the gender gap in belonging was reduced by 47%, and higher classroom diversity was associated with higher course grades and 1-year grade point average for both men and women. Addressing contextual belongingness norms may help to unlock the benefits of diversity.