Project Summary/Abstract Transgender youth experience higher levels of mental illness than their cisgender (i.e., non-transgender) peers. One proposed contributor to these high rates of mental illness is the discordance transgender youth experience between their bodies and appearances, on one hand, and their gender identities, on the other. Medical organizations have suggested that endocrine interventions—specifically puberty suppression and hormone therapy—can improve mental health in transgender youth by halting the development of gender- incongruent secondary sex characteristics and producing gender-congruent secondary sex characteristics, respectively. Yet legislators throughout the US have recently introduced bills that would ban transgender youth from accessing puberty suppression and hormone therapy, asserting that these interventions are not psychologically beneficial. Five studies to date have longitudinally examined the relationship between one or both of these interventions and mental health in transgender youth. However, these studies have had relatively small samples, none have been able to isolate the effects of endocrine interventions, none have included a cisgender comparison group, and none have examined the mechanisms by which endocrine interventions might improve mental health. The goal of the proposed research is twofold: (1) To examine the relationship between gender-affirming endocrine interventions and mental health in transgender youth and compare the mental health trajectories of transgender youth undergoing gender-affirming endocrine interventions to those of their cisgender peers; and (2) To explore the mechanisms underlying the relationship between endocrine interventions and mental health in transgender youth. To achieve these goals, I will measure mental health (anxiety and depression, as well as self-esteem) and body image (dissatisfaction, surveillance, depersonalization, and identity-appearance congruence) at three timepoints (before puberty suppression, after puberty suppression, and after hormone therapy) in a large sample (n=204 for puberty suppression, n=188 for hormone therapy) of transgender youth, as well as in two comparison groups (siblings, n=122, and unrelated cisgender youth, n=204). This research will allow me to determine whether endocrine interventions might help to reduce or eliminate mental health disparities between transgender and cisgender youth. I will conduct this research within the Human Diversity Lab at Princeton University, with mentorship from the lab’s PI, Dr. Kristina Olson (Sponsor), who runs the TransYouth Project, the world’s largest study of transgender youth who socially transitioned at an early age. I will also receive training from Dr. Katie McLaughlin (Collaborator), a developmental psychopathology expert at Harvard University, Dr. Robin Gomila (Consultant), a statistics expert at Princeton, and Dr. Ian Marshall (Consultant), a pediatric endocrinologist at Rutgers University. T...