Evaluating Associations Between Friendship Characteristics and Mental, Physical, and Cognitive Health.

Friendships are cited as important contributors to physical, mental, and cognitive health, but many studies rely on single-time-point measures and models that conflate within- and between-person variation. Using data from 23,248 middle-aged and older adults in the Health and Retirement Study, we applied advanced longitudinal models-specifically STARTS variants of ARTS and RI-CLPM-to examine reciprocal associations between friendship characteristics (number, contact frequency, quality) and health indicators (self-rated health, depression, cognition) over a 16-year period. Cross-lagged effects were consistently small, and more robust evidence emerged for health predicting friendship outcomes than the reverse. The best-fitting models accounted for occasion-specific variance, suggesting traditional lagged models may overestimate these effects. Our results raise doubts about strong causal claims linking friendships to health and underscore the need for more careful modeling of temporal dynamics. Findings highlight the importance of decomposing trait-like and state-like variation to clarify the role of friendships in healthy aging.