The Muscularity Bias Internalization Scale: Validation in a U.S. cisgender adult sample and measurement invariance across U.S. and Chinese samples.

The Muscularity Bias Internalization Scale (MBIS) assesses individuals' endorsement of stereotypical beliefs about muscularity and the extent to which they apply self-stigmatizing evaluations about their muscularity to themselves. Prior validations have supported its psychometric properties, although its structural representation has varied across samples. This cross-sectional study examined the factor structure, psychometric properties, and measurement invariance by gender of the MBIS among U.S. cisgender men and women and additionally tested cross-country measurement invariance with Chinese samples. Participants included individuals from the United States (men: n = 270; women: n = 287) and China (men: n = 299; women: n = 299). Exploratory factor analyses identified a two-factor structure, and confirmatory analyses supported an acceptable fit for the second-order two-factor model. The bifactor model showed better global fit and indicated a dominant general factor, supporting prioritization of the total score. The MBIS also demonstrated a consistent factor structure and adequate convergent, concurrent, discriminant, and incremental validity in U.S. cisgender men and women. Additionally, strong measurement invariance was supported across gender in the U.S. and across countries (U.S. vs. China) within each gender. The findings provide initial support for using the MBIS in research with U.S. cisgender adults and for comparisons across the U.S. and Chinese samples examined. Further evidence is needed regarding temporal stability and generalizability to more diverse U.S. populations.