Loneliness as a Benchmark Social-connection Signal for Psychological Distress: Cross-national, Longitudinal and...

Loneliness is associated with psychological distress, but whether broader social measures add information is unclear.

To assess incremental information from broader social measures beyond loneliness.

This study combined cross-sectional data from European Social Survey Round 6 (ESS6; 48,377 respondents, 29 countries), prospective Health and Retirement Study (HRS) intervals, and exploratory Mendelian randomisation.

In ESS6, loneliness had the largest survey-weighted standardised coefficient (b = 0.221 per SD, 95% CI [0.195, 0.246]), 4.3 times the next-largest coefficient. Adding non-loneliness social variables to covariates plus loneliness yielded little cross-validation gain (delta AUC = 0.0058, corrected [0.0004, 0.0111]), whereas adding loneliness to covariates improved AUC by 0.0739 (0.0653, 0.0825). In HRS, baseline loneliness predicted next-wave depressive symptoms after adjustment for baseline symptoms and covariates (b = 0.108, [0.096, 0.120]); relationship strain was the clearest adjacent signal. Exploratory MR suggested that the loneliness-depressive-symptom association may not be unidirectional, but pleiotropy, relaxed reverse instruments, sample overlap and mixed marker findings precluded directional causal interpretation.

Loneliness may serve as a benchmark indicator for population research, while broader measures primarily contextualise relational strain and social safety.