Association between loneliness and suicidality among general populations and persons with depressive and bipolar disorders: A systematic review

Background: Loneliness and suicidality are interrelated phenomena. Several studies suggest that they often co-exist, but the magnitude is unclear. This systematic review aims to analyze this association in general population samples of varying age groups and persons with depressive and bipolar disorders. Methods: Search terms in this review included words related to loneliness and suicidality among general populations and depressive and bipolar disorders. Databases included PubMed, PsychINFO, and Web of Science. The search occurred up until May 27, 2024. Screening and data extraction were performed independently. Studies were categorized by age group or clinical diagnosis. Quality assessments were conducted using NIH tools. Results: Fifty-six studies met eligibility criteria: 52 involved general population samples and 4 involved patients with depressive or bipolar disorders. In healthy adults aged 18 to 64, loneliness mainly showed a moderate positive correlation with suicidality (r = 0.26 to 0.59), while correlations in older adults (aged 65+) (r = 0.498) and in adolescents aged 13-17 were weaker. In depressive and bipolar disorders, correlations ranged from weak to moderate (r = −0.06 to 0.40), with associations stronger in unadjusted models (UOR = 2.8 to 7.07). Furthermore, some studies suggested that depressive symptoms mediate the association between loneliness and suicidality. Conclusion: A moderate and positive association was observed between loneliness and suicidality in healthy adults and patients with depressive disorders. However, the role of bipolar disorders in the association remains unclear. Practitioners should routinely evaluate persons living with depressive or bipolar disorders for loneliness as part of a comprehensive assessment.