can be applied to study within-person changes over time in both the short term and across the lifespan, and we discuss their potential clinical applications. We conclude with future directions for the classification of mental disorders at the intersection of the three approaches.
Transdiagnostic dimensional approachesTransdiagnostic dimensional approaches apply continuous (versus categorical or dichotomous) dimensions to psychopathology data, which represent unbroken spectra (also referred to as factors) that range from very low to very high levels (and all levels in between). Further, these spectra are transdiagnostic: these dimensions are not simply continuous reflections of official dichotomous diagnoses but instead cut across the diagnostic boundaries separating disorders 23,24 . In doing so, these dimensions are interpreted as reflecting core 'building blocks' of variation that characterize multiple disorders. Thus, a single transdiagnostic dimension, such as 'internalizing', can include psychiatric phenomena from different diagnoses of the same type (in this case, major depressive disorder and dysthymic disorder, both of which are mood disorders) as well as from different groupings of disorders (in this case, mood disorders and anxiety disorders) 24 .Transdiagnostic dimensions overcome many of the problems with official nosologies. First, their dimensionality (versus a present/ absent dichotomy) captures the structure of real-world data, where samples of individuals report levels of psychopathology that generally range widely -above and below DSM-5 diagnostic thresholds -and have no clear points of discontinuity across severity levels 25 . Further empirical support for dimensionality comes from taxometric research that finds little evidence for discrete groups within a spectrum 26 , and genetic evidence suggests that liability to mental illness is continuously distributed 27 .Second, although diagnostic comorbidity is viewed as a problem in traditional classification frameworks, transdiagnostic dimensional models explicitly embrace comorbidity by modelling these relationships among mental health variables. These models explicitly allow for greater-than-chance correlations among different forms of psychopathology and for overlap among them.Third, dimensionality overcomes the need for largely arbitrary diagnostic criterion thresholds. As an example, using the official nosologies' threshold of at least five of nine criteria being present to support the diagnosis of a given mental disorder, individuals with similar levels of psychopathology (such as a person meeting four criteria and another meeting five criteria) would be described as being totally different (in this case, diagnosis absent and diagnosis present, respectively), whereas individuals with notably different levels of psychopathology (such as a person meeting five criteria and another meeting nine criteria) would be described as exactly the same (both receiving a diagnosis). Thus, thresholds obscure important similarities and differences within diag...
