Is morbidity compressing or expanding? Cross-national trends in midlife multimorbidity across cohorts.

While life expectancy has increased globally, it remains uncertain whether recent generations are entering midlife with a greater or lower burden of chronic disease and whether these cohort patterns differ by gender. Moreover, cross-national evidence comparing cohort differences in morbidity remains limited, constraining our understanding of how health trajectories vary across national contexts. Using harmonized longitudinal data from the Health and Retirement Study and 3 international partner studies (13 countries in total), we compared two midlife birth cohorts-adults aged 50-58 surveyed in 2006 and 2014-each followed for 4 years. Multimorbidity was measured as the count of eight self-reported chronic conditions. Over half of the study countries experienced increases in multimorbidity between 2006 and 2014, including the USA (0.22; 95% CI = [0.12, 0.32]), the UK (0.09; 95% CI = [0.00, 0.17]), and Switzerland (0.36; 95% CI = [0.22, 0.49]), consistent with an expansion of morbidity. In contrast, South Korea showed a decline (-0.12; 95% CI = [-0.17, -0.07]), suggesting potential evidence of morbidity compression. Gender-stratified analyses showed heterogeneity, with larger increases among women in the USA and a decline concentrated among women in South Korea. These findings underscore the importance of national context in shaping cohort health trajectories and suggest that policy environments may play a critical role in mitigating morbidity expansion.