The current review and synthesis serves to define and contextualize fear of the unknown relative to related constructs, such as intolerance of uncertainty, and contemporary models of emotion, attachment, and neuroticism. The contemporary models appear to share a common core in underscoring the importance of responses to unknowns. A recent surge in published research has explored the transdiagnostic impact of not knowing on anxiety and related pathologies; as such, there appears to be mounting evidence for fear of the unknown as an important core transdiagnostic construct. The result is a robust foundation for transdiagnostic theoretical and empirical explorations into fearing the unknown and intolerance of uncertainty.

The current review and synthesis was designed to provocatively develop and evaluate the proposition that "fear of the unknown may be a, or possibly the, fundamental fear" (Carleton, 2016) underlying anxiety and therein neuroticism. Identifying fundamental transdiagnostic elements is a priority for clinical theory and practice. Historical criteria for identifying fundamental components of anxiety are described and revised criteria are offered. The revised criteria are based on logical rhetorical arguments using a constituent reductionist postpositivist approach supported by the available empirical data. The revised criteria are then used to assess several fears posited as fundamental, including fear of the unknown. The review and synthesis concludes with brief recommendations for future theoretical discourse as well as clinical and non-clinical research.

Background: Canadian public safety personnel (PSP; e.g., correctional workers, dispatchers, firefighters, paramedics, police officers) are exposed to potentially traumatic events as a function of their work. Such exposures contribute to the risk of developing clinically significant symptoms related to mental disorders. The current study was designed to provide estimates of mental disorder symptom frequencies and severities for Canadian PSP. Methods: An online survey was made available in English or French from September 2016 to January 2017. The survey assessed current symptoms, and participation was solicited from national PSP agencies and advocacy groups. Estimates were derived using well-validated screening measures. Results: There were 5813 participants (32.5% women) who were grouped into 6 categories (i.e., call center operators/dispatchers, correctional workers, firefighters, municipal/provincial police, paramedics, Royal Canadian Mounted Police). Substantial proportions of participants reported current symptoms consistent with 1 (i.e., 15.1%) or more (i.e., 26.7%) mental disorders based on the screening measures. There were significant differences across PSP categories with respect to proportions screening positive based on each measure. Interpretation: The estimated proportion of PSP reporting current symptom clusters consistent with 1 or more mental disorders appears higher than previously published estimates for the general population; however, direct comparisons are impossible because of methodological differences. The available data suggest that Canadian PSP experience substantial and heterogeneous difficulties with mental health and underscore the need for a rigorous epidemiologic study and category-specific solutions.

Modern anxiety disorder models implicitly include intolerance of uncertainty (IU) as a critical component for the development and maintenance of these pervasive social and economic concerns. IU represents, at its core, fear of the unknown - a long-recognized, deep-seated fear identified in normative and pathological samples. Indeed, the intrinsic nature of IU can be argued as evolutionarily supported, a notion buttressed by initial biophysiological evidence from uncertainty-related research. Originally thought to be specific to generalized anxiety disorder, recent research has clearly demonstrated that IU is a broad transdiagnostic dispositional risk factor for the development and maintenance of clinically significant anxiety. The available evidence suggests that theorists, researchers and clinicians may benefit from explicitly incorporating IU into models, research designs, case conceptualizations and as a treatment target.