Comorbidity of anxiety and depression
: an examination of nosology, measurement and overlap

Student thesis: Doctoral Thesis


Depression and Anxiety are two of the most common internalising disorders in the world. Depressive and anxiety disorders are associated with significant impairments and lead to considerable personal, economic, and societal burden. Although these disorders are characterised by high comorbidity putting their validity as separate clinical categories into question, this is not reflected in the current diagnostic manuals – the DSM-5 and ICD-11. While depression and anxiety share common genetic, biological and societal underpinnings, only inconclusive evidence as to the distinction and similarity of their symptoms is available. The current series of studies aimed to remedy that by examining the possible reasons for their comorbidity through different methodologies. First, thematic coverage of systematically obtained scales that measure the two constructs was examined. Results highlight that not only do measures that supposedly measure the same constructs differ widely, but there is also a non-negligible cross-construct overlap in symptoms measured. Second, using multiple nationally representative and community samples from the UK and ROI, bifactor analyses examined whether depression and anxiety are better represented as a single measurement construct. Results suggest that treating the two constructs as unidimensional provides greater parsimony and higher convergent and divergent validity when compared to using both measures separately. Third, factor mixture models examined how anxiety and depression manifest within the population using a nationally representative sample of UK adults. The results suggested that symptoms of depression and anxiety co-occur more often than they occur as separate disorders with implications for how these disorders are specified within diagnostic manuals. Fourth, network analysis examination of comorbid depression and anxiety revealed a symptom network suggestive of depression and anxiety as being highly interwoven. This suggested that comorbidity is not simply a phenomenon of the two disorders co-occurring, but rather that they are intimately connected. The results have implications for the entire field of depression and anxiety science-beginning with conceptualisation, measurement methods, diagnosis, treatment and treatment evaluation.
Date of AwardJan 2023
Original languageEnglish
SupervisorMark Shevlin (Supervisor)


  • Anxiety
  • Depression

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