Co-twin Control Analyses Reveal Genetic Contributions to SES Influences in Mean Level and Longitudinal Change in Physical Aging

Socioeconomic status (SES) predicts age-related changes in health status; however, the source of SES-health associations is heavily debated. Twin studies allow tests of causal hypotheses by modeling within and between twin pair differences in longitudinal latent growth curve models (LGCM) of physical aging, examining level of functioning and rate of change with age. Three longitudinal twin studies of aging (mean age at baseline = 71.36, SD = 10.7) from the Swedish Twin Registry (N = 1369) included up to 27 years of follow-up on a Functional Aging Index (FAI) consisting of lung function, grip strength, walking speed, and self-report sensory functioning. SES indicators included education, financial strain, and occupation-based socioeconomic position. Pair means (between family effect) and within pair differences (within family effect) for SES were included as covariates of both intercept and slopes in a two-slope LGCM (intercept at age 75); models were corrected for sex and parental SES. LGCM results were compared across the full sample, then separately for both monozygotic and dizygotic twin pairs. Results indicated genetic confounding in the associations between multiple SES indicators and the FAI intercept. Additionally, the relationship between education longitudinal change in physical aging up to age 75 was subject to genetic confounding. These patterns were replicated among men. In contrast, findings for women pointed to shared environmental influences rather than genetic confounding, although statistical power was reduced in sex-stratified analyses. Results highlight the importance of considering the timing of socioeconomic exposures and gendered life-course trajectories when examining health inequalities in aging.