Age-standardized DALY rates for Alzheimer’s disease and other dementias and depressive disorders were extracted from the GBD Results Tool (1990–2021). CADI was defined as the decadal change in dementia ASDR minus the change in depression ASDR. Joinpoint regression identified temporal inflection points. Associations of CADI with the Socio-demographic Index (SDI) and life expectancy were assessed using linear regression.

Globally, dementia DALYs increased from 12.2 million (1990) to 34.5 million (2021), a 168% rise in ASDR (from 236 to 633 per 100,000). In contrast, depressive-disorder DALYs rose modestly from 37.4 million to 46.9 million, while age-standardized rates declined slightly (from 610 to 575 per 100,000). The mean global CADI = +12.3 DALYs/100,000 per year, positive in 83% of countries, confirming cognitive–affective decoupling. High-SDI regions (Western Europe, East Asia, and North America) exhibited the largest positive CADI, whereas low-SDI regions showed near-parallel slopes. CADI correlated inversely with SDI (β = –0.42, p < 0.001).

Between 1990 and 2021, dementia-related disability accelerated globally while depression-related disability remained stable or declined, indicating a pronounced cognitive–affective decoupling. This divergence likely reflects enhanced dementia detection alongside under-recognized late-life depression. Integrating affective screening within dementia care and strengthening geriatric mental health systems are urgent priorities to close this widening neuropsychiatric gap.