Longitudinal Patterns of Within- and Cross-Domain Multimorbidity Across Physical, Psychological, and Cognitive Conditions in China and the United States: The Role of Socioeconomic and Healthcare Inequalities

Background: Multimorbidity is a growing global health challenge in aging populations, yet its progression across physical, psychological, and cognitive domains and its contribution to health inequalities remain unclear. We compared longitudinal patterns of multimorbidity in China and the United States (US) and examined the socioeconomic and healthcare-related factors associated with these patterns and their functional consequences. Methods: This longitudinal cohort study included adults aged ≥45 years from China (2011–2020) and the US (2012–2020), matched 1:1 by baseline age and sex. Multimorbidity was classified into eight domains spanning physical, psychological, and cognitive conditions and their combinations. Longitudinal changes across five survey waves were assessed. Multinomial logistic mixed models examined socioeconomic and healthcare-related correlations, and Cox models estimated associations with subsequent limitations in activities of daily living (ADL) limitations and instrumental activities of daily living (IADL) limitations. Results: A total of 7064 participants from China (mean age 60.15 ± 7.93 years; 45.4% male) and 7064 matched participants from the US were included. Multimorbidity patterns were more complex in the US at baseline, but progression toward cross-domain multimorbidity occurred in both countries and was more pronounced in China. Psychological conditions occupied a central position in the development of more complex multimorbidity patterns. Higher educational and household wealth were consistently associated with lower odds of cognitive-related and cross-domain multimorbidity in both countries, whereas associations with healthcare-related factors varied across settings. Cross-domain multimorbidity was more strongly associated with functional limitations than within-domain multimorbidity. In particular, physical–psychological–cognitive multimorbidity was associated with substantially higher risks of ADL limitations (hazard ratio (HR)  =  5.70, 95% confidence interval (CI)  =  4.87–6.67 in China; HR  =  7.72, 95% CI  =  5.90–10.11 in the US) and IADL limitations (HR  =  4.14, 95% CI  =  3.64–4.71; HR  =  5.84, 95% CI  =  4.50–7.59, respectively). Conclusions: Multimorbidity increasingly spans physical, psychological, and cognitive domains in both China and the US. Psychological conditions appeared to bridge physical and cognitive conditions in more complex multimorbidity patterns. Integrated care models incorporating psychological health and strategies addressing socioeconomic inequalities may help reduce the burden of multimorbidity and related functional decline.