Individuals feel lonely when they perceive a discrepancy between their aspired and their actually experienced amount of closeness and intimacy in social relationships. In the present study, we disentangled developmental constancy factors, time-varying factors such as person-environment transactions, and stochastic mechanisms as sources of interindividual differences in loneliness by applying STARTS models in nationally representative samples from Germany (n = 13,397), Switzerland (n = 6,599), Australia (n = 30,496), and The Netherlands (n = 12,810). Across the 4 studies, we found trait-like sources reflecting developmental constancy factors and the influence of time-varying factors to account for 19–43% and 23–30% of interindividual differences in loneliness, respectively. Depending on the type of measurement instrument, state-like stochastic mechanisms accounted for 30% to 55% of interindividual differences, whereby multi-item measures were less affected by stochastic mechanisms. We found gradual sex differences but considerable differences between age groups regarding the amount of interindividual differences in loneliness accounted for by the three sources. Taken together, the results demonstrate that interindividual differences in loneliness are to a large degree trait-like. Based on these findings, we discuss implications for theory and measurement.

Individuals feel lonely when they perceive a discrepancy between the desired and actually experienced quantity and quality of social relationships. Prior research has demonstrated the importance of loneliness for various health-related aspects. In the present article, we extend the existing literature on loneliness by investigating its role for predicting personality traits and their development from late adolescence to early midlife. Using data from a representative German sample (N = 12,402) sampling individuals from three different birth cohorts, we found loneliness to predict the levels of all Big Five traits but Openness five years later. The effects of loneliness on the development of neuroticism and extraversion reached statistical significance but were only marginal in terms of effect size. Furthermore, we found that a self-regulatory focus geared to the prevention of negative events mediated the effects of loneliness on later levels of the Big Five.