We report here progress on a research program at MIT to increase the sense of “belonging” (Walton & Cohen, 2007) for women and under-represented/minoritized undergraduates in the MIT Physics Department, as a means to support recruitment and retention in the physics major and onwards into graduate education. This work builds on social-psychological research to develop social-belonging interventions for the first-year physics courses required of all undergraduates at MIT. The interventions consist of having students, at an early or critical juncture in their education, read and write reflections on short testimonials in which peers describe how they faced and overcame challenges in similar situations. Previous randomized experimental results have shown that these seemingly small, isolated interventions are capable of producing substantial, lasting effects, in particular for under-represented students (Walton & Wilson, 2018). This work takes place in the context of larger departmental efforts to improve the environment for all members of the physics community, and these interventions complement, but do not replace, other resources, peer-groups, and support programs for students. In this contribution, we will share what we have learned from preliminary mid-course and post-course survey results, grades, and first-year major declaration and final graduation statistics.