UC Davis

Loneliness and social isolation are associated with numerous adverse physical and psychological health outcomes in older adulthood, including cognitive impairment and mortality risk. Yet, how the individual and joint effects of loneliness and social isolation contribute to these outcomes remains unclear, particularly given the interplay between individual differences (loneliness) and environmental factors (social isolation) in shaping these important health outcomes in older adulthood (i.e., person-environment interactions). We used Cox regression, logistic regression, and multistate survival models to systematically investigate the individual and adjusted associations among loneliness, social isolation, cognitive aging outcomes, and mortality risk. Further, extensive between-study variability in operational definitions, modeling approaches, and covariate adjustments may be contributing to mixed results in the existing literature. In this registered report, we applied a multistudy approach (i.e., coordinated data analysis) in which independent but identical models were fit across 11 longitudinal studies representing participants from 18 countries (N_total = 175,070). Random effects meta-analyses synthesized results across studies, showing that loneliness was consistently associated with an elevated risk of cognitive impairment and mortality across statistical approaches, even after adjusting for social isolation. Conversely, social isolation was not consistently associated with cognitive impairment and showed weaker associations with mortality risk. Together, our findings suggest that loneliness is a robust, proximal predictor of major aging outcomes and highlight avenues for theory development, strategies to strengthen cognitive resilience and independence in older adulthood, and more efficient resource allocation of public health resources. (PsycInfo Database Record (c) 2026 APA, all rights reserved).