The unequal cognitive consequences of social disconnection: mechanisms, modifiers and opportunities for dementia...

Social isolation and loneliness are increasingly recognized as important determinants of cognitive decline and dementia risk in older adults. However, the strength and nature of these associations vary substantially across populations, cognitive outcomes, and stages of ageing. This review examines the sources of such heterogeneity by distinguishing objective social isolation from subjective loneliness and exploring their different psychological, behavioural, and biological pathways. Using a systematic multi-database search and thematic synthesis, we integrated evidence from longitudinal studies, neuroimaging research, and intervention trials to identify factors that increase vulnerability and promote resilience. The review highlights how psychosocial, behavioural, neuroendocrine, and neurostructural mechanisms contribute to differences in cognitive outcomes and discusses implications for targeted and context-sensitive interventions. Remaining challenges include measurement heterogeneity, limited causal evidence, and underrepresentation of low- and middle-income countries. Future research should prioritize harmonized assessments, stratified intervention trials, and culturally adapted implementation approaches. Early identification and prevention of social disconnection may provide greater benefits than attempts to reverse established isolation.