Recent advances in the conceptualization and evidence supporting the HiTOP approach

The Hierarchical Taxonomy of Psychopathology (HiTOP) endeavor has been underway for less than a decade, yet is showing signs of having a major impact on numerous areas of research and practice in mental health.

Broadly speaking, HiTOP is a consortium of hundreds of investigators interested in articulating an empirical alternative to traditional psychiatric nosologies. These nosologies are framed by assumptions that may or may not comport with empirical observations. Primarily, they articulate categories of mental disorder, regardless of the evidence (or lack thereof) that mental disorders are categorical in nature.

The perspective taken by the HiTOP consortium draws on two robust systematic observations about the nature of human variation in psychopathological tendencies ¹ , ² . First, these tendencies are generally better modeled as dimensions as opposed to categories. An extensive literature shows that, when formal models of psychopathological variation are compared, dimensional models tend to fit better than categorical ones. Second, continuous dimensions of psychopathology delineate a hierarchy of constructs in a reasonably reliable manner. Based on evidence, psychopathology is not well modeled as hundreds of categories. Rather, dimensions of psychopathological experiences are organized into constructs that vary in their breadth (e.g., a broad internalizing spectrum, encompassing diverse forms of emotional dysregulation) vs. specificity (e.g., anhedonia).

Here I emphasize two areas in which the consortium has been particularly active recently: a) clinical utility and interfacing with medicine, and b) neuroscientific utility. In selecting these areas, I do not mean to omit or slight the numerous other areas in which the HiTOP consortium has been active in the professional literature. Rather, I emphasize these areas because they strike me as having specific relevance to the scope of World Psychiatry.

Clinical utility is an area of research and scholarship that arguably has great potential to impact the lives of our patients most immediately. Many front‐line practitioners are understandably frustrated by the limitations of traditional nosologies, because their patients do not fit neatly into ordinary categorical rubrics. Real life patients tend to present with a mix of symptoms from putatively distinct psychiatric categories, frustrating efforts at effective case conceptualization and development of corresponding treatment plans. Transitioning to more effective approaches to modeling psychopathology will be easier if practitioners can readily perceive the value of dimensional alternatives to traditional nosologies.

Along these lines, Balling et al ⁴ conducted a study involving 143 active clinicians, with the aim of gauging the clinical utility of the HiTOP and DSM approaches to making diagnostic ratings of clinical vignettes. Each clinician in the study made diagnostic ratings using both systems and was asked about the clinical utility of the systems after making the ratings. Participants favored HiTOP (compared with DSM) for overall clinical utility, as well as for specific areas of utility (e.g., formulating an effective approach to intervention). These findings suggest that clinicians readily perceive the utility of the HiTOP approach to diagnostic formulation. The HiTOP consortium is very active in cultivating a network of clinicians and providing specific clinical tools and workshops (see www.hitop‐system.org/the‐clinical‐network).

Looking toward the future, many psychiatric researchers and practitioners are heavily investing in the prospect of neuroscientific technologies to place mental disorder research and intervention on surer scientific footing. Remarkable technologies exist for imaging the human brain non‐invasively, and the hope is that neuroscience can provide a mechanistic understanding of mental disorder that transcends the epistemic limitations of patients’ verbal reports of their experiences.

Various ways forward can be articulated at the interface of neuroscience and psychiatric disorder. For example, it might be possible to delineate neural “circuitry” in infrahuman species with a high degree of experimental precision and draw on that knowledge to articulate ways that homologous circuitry may go awry in humans. Nevertheless, bringing that “bench level” understanding to the “bedside” requires a model‐based view of human phenotypes. For example, a specific “circuit” in rodents and corresponding behavioral observations may be somewhat distant from the experience of delusions of religious persecution in humans.

Along these lines, neuroscientists active in the HiTOP consortium have emphasized the importance of behavioral phenotyping in humans as a key tether in the search for neurobiological substrates of human psychopathology ⁵ , ⁶ . For example, a specific neurobiological observation (e.g., individual differences in a specific event‐related potential, or connectivity in a specific circuit delineated via resting state functional magnetic resonance imaging) could be associated with a highly specific psychopathological phenotype (e.g., somatic delusions) and/or a broad range of phenomena (e.g., diverse forms of thought problems, ranging from delusional beliefs to eccentric interests to hebephrenic behaviors) ⁷ .

The dimensional‐hierarchical approach delineated in HiTOP provides a phenotypic tether for neuroscientific inquiry of this type. Instead of being limited to cataloguing piecemeal bivariate associations between putative categories and specific neurobiological paradigms, the HiTOP approach emphasizes the importance of multivariate research that ties together breadth of phenotypic assessment with diverse neurobiological approaches. Relatedly, many areas of clinical medicine are fundamentally dimensional in their approach to diagnostic formulation, yet are stymied by reliance on psychiatric categories (e.g., oncology) ⁸ . There is therefore much promise in linking the HiTOP approach with clinical medicine.

To summarize, the HiTOP consortium has grown from a modest cadre of investigators interested in modeling psychopathology empirically, to a group of hundreds of clinicians and investigators working together to articulate a coherent vision that ties together basic research and clinical application. The consortium is eager to grow, and new members are always welcome (see www.hitop‐system.org). Moreover, the future is in the hands of younger generations of clinicians and researchers; hence, the HiTOP consortium places a strong emphasis on generativity (see www.hitop‐system.org/trainees). Importantly, consortium members have also carefully articulated a principled approach to updating the HiTOP model as evidence accumulates ⁹ , hoping that such guardrails can help to moderate the stressors associated with the continuous evolution of knowledge in mental health research and practice.

Of course, HiTOP is but one of several promising alternatives to traditional psychiatric categories. Ultimately, openness and unbounded conversations among diverse groups of scholars in mental health is the key to progress. Hopefully, this brief update can serve to promote constructive and wide‐ranging discourse that improves the lives of our patients worldwide.