The age or size of your body doesn’t matter: Measurement invariance of the State Functionality Appreciation Scale (S-FAS) and evaluation of the Expand Your Horizon intervention across adult age and body size groups in a UK sample

While still nascent, research has yet to examine the factor structure and measurement invariance of the State Functionality Appreciation Scale (S-FAS; Longhurst et al., 2025) across demographic groups, beyond gender identity. Additionally, there is a need to explicitly examine the utility of Expand Your Horizon (EYH), a programme designed to target functionality appreciation, among underrepresented groups, including older adults and those who live in larger bodies. To this end, using secondary data from Longhurst et al. (2025), the aims of this study were two-fold: First, among a UK-based community sample (N = 861), we sought to examine the factorial stability and measurement invariance of the S-FAS across adult age groups and body size groups. Among a second UK-based community subsample (N = 293), the secondary aim was to assess the preliminary efficacy of EYH across adult age groups and body size groups. Confirmatory factor analysis supported a unidimensional structure of S-FAS baseline scores and measurement invariance (up to latent means) across adult age groups and (up to partial scalar) body size groups. Additional results demonstrated that baseline S-FAS scores were highest among older adults, while changes in state functionality appreciation following intervention were highest among emerging adults. EYH improved state functionality appreciation across all body size groups, particularly among those who live in larger bodies. Future research should assess the longitudinal measurement invariance of the S-FAS and qualitatively explore body functionality across marginalised populations.