Project Summary (Parent Grant) Categorization is a fundamental aspect of human cognition that allows for efficient learning, generalization, and communication. Throughout development, humans categorize the objects, people, and events around them. The proposed work examines social categorization with a focus on gender. Gender categories are early emerging, yet influential throughout the lifespan. Broader societal ideas about gender appear to be changing, however, and this grant asks how those changes may or may not be reflected in how adolescents are categorizing gender, their beliefs about gender, and their self-categorization by gender. Using a pair of longitudinal studies, including a 5-year annual study (ongoing from the previous grant period) and a month-long daily experience sampling study, this grant will document how these constructs are or are not changing amongst adolescents and the relation between these constructs and their social environments. Finally, in this work, we examine the links between adolescents’ beliefs about gender, self-categorization with gender, and their health and well-being. Participants include adolescents and adults who are gender diverse and cisgender. Together this work will advance theory and practice about topics ranging from categorization and adolescent gender development to reducing mental health disparities in transgender youth.