In this study we combined understanding from the gender minority stress and resilience (GMSR) model (Testa, Habarth, Peta, Balsam, & Bockting, 2015) and acceptance and commitment therapy (ACT; Hayes, Strosahl, & Wilson, 1999, 2012) to test a theoretically integrated and expansive account of the development of psychological distress in transgender and gender nonconforming (TGNC) people. Specifically, we constructed a parallel multiple mediation model in which we examined the role of

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... processes deriving from the GMSR model (i.e., internalized transphobia and identity nondisclosure) and ACT (i.e., psychological inflexibility) in the relationship between gender-related discrimination and psychological distress (i.e., depression, anxiety, and stress). We based this model upon data from a 2-wave longitudinal panel design in which 358 TGNC people living in England responded to a battery of measures on 2 occasions, 12 months apart (herein, Time 1 and Time 2). Initial tests of model fit and temporal invariance indicated that our proposed measurement model offered an excellent fit to the data and demonstrated equivalence of measurement across the two study timepoints. Autoregressive cross-lagged manifest path analysis indicated that while our hypothesized full structural model offered an excellent fit to the data, psychological inflexibility alone mediated the relationships between gender-related discrimination and depression, anxiety, and stress. Model comparison analysis confirmed the redundancy of internalized transphobia and identity nondisclosure as mediators and ruled out alternative patterns of causality. We discuss theoretical, empirical, and practical implications for the field of TGNC mental health. (PsycINFO Database Record (c) 2019 APA, all rights reserved).