Generalized Anxiety Disorder (GAD) is classified as an anxiety disorder. High co-morbidity with other anxiety and depressive disorders blurs boundaries between these disorders, clinically as in research. This is particularly relevant for genetic research into causes of these disorders.

We attempt to clarify where GAD belongs in the anxiety/depression spectrum disorders.

The cohort is based on a population-wide screening for anxiety and depression in Iceland as part of a genetic research project. Following the screening participants underwent the Composite International Diagnostic Interview (CIDI) for possible ICD-10 diagnoses. Odds ratios (OR) were calculated by logistic regression analysis for GAD and the other disorders. The phobias (simple, social and agoraphobia) were pooled together in the analysis.

A total of 3.150 participants underwent the CIDI. The OR between GAD and dysthymia was 2.99 (2.37-3,78), Panic disorder, PD, 2.03 (1.59-2.59); any phobia 1.15 (0.92-1.42) and Major Depressive Disorder, MDD, 1.07 (0.84-1.37). The OR between dysthymia, MDD and GAD is very high, . The OR, with co-morbidity accounted for by logistic regression analysis, is slightly lowered for all except dysthymia.

Our results show that GAD is significantly associated with dysthymia, followed by PD, but non-significant with the phobias and MDD. Dysthymia, on the other hand, has a robust relationship both to GAD and MDD, 2.97 and 2.91 respectively. Logistic regression confirms the strong link between GAD and dysthymia and gives these disorders the possible role of a genetic bridge between anxiety and depressive disorders.