Daily loneliness and suicidal ideation in borderline personality disorder.

Loneliness has been linked with suicidal ideation (SI) in people with a borderline personality disorder (BPD) diagnosis. However, the temporal dynamics of this association remain unclear: we do not know whether loneliness amplifies SI within clinically relevant short-term timeframes (e.g., day to day). To fill this gap, we used data from a 21-day ambulatory assessment study of individuals diagnosed with BPD (N = 152, 103 with a history of attempted suicide) and preregistered the hypotheses and code of our analyses. We tested the hypotheses that daily loneliness would be associated with same- and next-day SI, and that, among individuals with a BPD diagnosis, more severe self-reported BPD features would strengthen these associations. In line with our hypotheses, we found a significant contemporaneous and lagged association between loneliness and SI. Contrary to our hypotheses, these links were not significantly amplified by the level of BPD features. Exploratory analyses further suggested that loneliness did not account for the within-person link between daily social interactions and SI, nor did more general personality disorder features alter loneliness-SI links. The links between loneliness and SI highlight a significant vulnerability to suicide risk and our findings suggest that self-reported BPD symptom severity among individuals with a BPD diagnosis may impact the early stages of the suicidal process by predisposing to loneliness. (PsycInfo Database Record (c) 2025 APA, all rights reserved).