Social Function as a Potentially Modifiable Late-Life Correlate of Cognitive Reserve Across Mild Cognitive Impairment Phenotypes

Objective Cognitive decline is common in neurodegenerative diseases and affects quality of life. Cognitive reserve (CR) may mitigate cognitive deterioration, but traditional proxies, such as education and occupation, may not fully capture late-life sources of reserve. This study examined whether social function explains cognition beyond traditional CR proxies and whether these associations differ across neurodegenerative conditions. Methods A total of 617 participants were enrolled: 315 normal controls, 84 individuals with neuropsychologically defined AD-like mild cognitive impairment (AD-like MCI), 122 patients with Parkinson’s Disease (PD) with normal cognition (PD-NC), and 96 PD patients with MCI (PD-MCI). Social function was assessed using the Geriatric Social Function Scale (GSFS), including Daily Function, Interpersonal Function, and Social Participation. Global and domain-specific cognition were evaluated with a neuropsychological battery. Group differences, hierarchical regression, and moderation analyses were conducted. Results Social function was positively associated with global cognition and explained additional variance beyond traditional CR proxies ( ΔR² = 0.04, p  < 0.001). Among the three dimensions, only Social Participation significantly predicted MoCA performance ( β  = 0.18, p  < 0.001). After FDR correction, diagnosis-by-social function interactions showed group- and domain-specific patterns, primarily involving memory in AD-like MCI and executive function, language, and attention/working memory in PD-MCI. Conclusions Social function, particularly social participation, was modestly associated with global cognitive function beyond conventional CR proxies. Associations between social function dimensions and cognitive domains differed across clinical groups. Longitudinal replication is needed before social function can be interpreted as contributing to cognitive resilience or serving as an intervention target.