Background: The aim of this study was to examine longitudinally in the general population (a) whether depressive symptoms co-occur with paranoid ideation from late adolescence to middle age (b) whether depressive subsymptoms are differently linked with paranoid ideation (c) whether depressive symptoms are associated with state-level or trait-level paranoid ideation.

Methods: Altogether 2109 subjects of the Young Finns study completed the Paranoid Ideation Scale of the Symptom Checklist-90 Revised and a modified version of the Beck Depression Inventory in 1992, 1997, 2001, 2007, and 2012, and the Beck Depression Inventory-II in 2007, 2011, and 2012.

Results: Higher self-rated depressive symptoms were associated with the course of more severe paranoid ideation over age, especially in late adolescence and early adulthood. Regarding depressive subsymptoms, the associations of negative attitude and performance difficulties with paranoid ideation were evident over age, whereas the influence of somatic symptoms (such as changes in sleep and appetite) was not significant until after early adulthood. Additionally, depressive symptoms were more evidently associated with the development of traitthan state-level paranoid ideation.Limitations: Our study mostly captured mild depressive and paranoid symptoms. The results cannot be directly generalized to clinical populations.

Conclusions: Depressive symptoms were associated with the course of paranoid ideation from late adolescence to middle age. Patients with paranoid ideation might merit from evaluation of potential depressive symptoms, especially in late adolescence and early adulthood. Among patients with co-occurring depressive symptoms and paranoid ideation, there may be a substantial need for neurocognitive rehabilitation and community-based treatments.