Childhood Loneliness and Cognitive Decline and Dementia Risk in Middle-Aged and Older Adults.

Importance: Adult loneliness has been linked to cognitive decline and dementia risk, but the long-term implications of childhood loneliness remain underexplored. Objectives: To investigate whether childhood loneliness is associated with cognitive decline and dementia risk and whether adult loneliness mediates or modifies these associations. Design, Setting, and Participants: This cohort study used data from the China Health and Retirement Longitudinal Study, a nationally representative cohort study conducted between June 1, 2011, and December 31, 2018, with a maximum follow-up of 7 years. Data analyses were performed from October 1, 2024, to January 15, 2025. Exposure: Childhood loneliness was defined as self-reported frequent feelings of loneliness and the absence of close friendships before age 17 years. Adult loneliness was measured by a single item from the Centre for Epidemiological Studies Depression Scale. Main Outcomes and Measures: Cognitive function was assessed through measures of episodic memory and executive function. Dementia was identified as the coexistence of cognitive and functional impairments or a self- or caregiver-reported physician diagnosis of dementia. Associations between childhood loneliness and cognitive decline were evaluated using linear mixed-effects models, and dementia risk was analyzed using Cox proportional hazards regression models. Results: Among 13 592 participants (mean [SD] age, 58.34 [9.39] years; 7175 [52.8%] female), 6525 (48.0%) experienced possible childhood loneliness, and 565 (4.2%) reported childhood loneliness. Compared with no childhood loneliness, childhood loneliness was associated with significantly faster cognitive decline (β, -0.03 [95% CI, -0.05 to -0.02] SD per year), as was possible childhood loneliness (β, -0.02 [95% CI, -0.02 to -0.01] SD per year). Moreover, childhood loneliness was associated with an increased risk of dementia (hazard ratio, 1.41 [95% CI, 1.03 to 1.93]). These associations remained significant when adjusted for adult loneliness and restricted to participants without adult loneliness. Adult loneliness mediated 8.5% (95% CI, 2.9% to 14.1%) of the association of childhood loneliness with cognitive decline and 17.2% (95% CI, 4.9%-29.5%) of the association with dementia risk but did not significantly modify these associations. Conclusions and Relevance: In this cohort study, childhood loneliness was associated with cognitive decline and dementia risk in middle and later adulthood, even in the absence of adult loneliness. Early interventions aimed at reducing childhood loneliness may help promote lifelong cognitive health and reducing dementia risk.