The Hierarchical Taxonomy of Psychopathology (HiTOP)

The Hierarchical Taxonomy of Psychopathology (HiTOP) is an advanced classification system designed to address the shortcomings of traditional diagnostic systems for mental disorders, such as the DSM and ICD.

Instead of relying on discrete categories of mental illnesses, HiTOP organizes symptoms into a hierarchy, starting from fine-grained symptoms and moving to broader syndromes and factors.

This structure reflects the reality that many mental health symptoms co-occur and can be grouped into larger dimensions of psychopathology.

Kotov, R., Krueger, R. F., Watson, D., Achenbach, T. M., Althoff, R. R., Bagby, R. M.,…Zimmerman, M. (2017). The Hierarchical Taxonomy of Psychopathology (HiTOP): A dimensional alternative to traditional nosologies. Journal of Abnormal Psychology, 126(4), 454–477. http://dx.doi.org/10.1037/abn0000258

Key Points

• HiTOP aims to address limitations of traditional diagnostic systems like the DSM by using a dimensional approach based on empirical research on the structure of psychopathology.
• HiTOP is organized hierarchically, with general dimensions of psychopathology at the highest levels and increasingly specific components at lower levels.
• The main levels of HiTOP are spectra (broad dimensions like internalizing and externalizing), subfactors (clusters within spectra), syndromes (related symptom groups), and components (specific symptoms).
• Six main spectra have been identified: internalizing, thought disorder, disinhibited externalizing, antagonistic externalizing, detachment, and somatoform.
• HiTOP dimensions show relationships with genetic risk factors, biomarkers, neural substrates, illness course, impairment, and treatment response.
• Measures have been developed to assess many HiTOP dimensions, making clinical implementation feasible.
• While still a work in progress, HiTOP shows promise to improve research and clinical care by providing a more valid classification system.

Rationale

Traditional psychiatric classification systems like the DSM have faced criticism for lacking a strong empirical foundation and having significant limitations in reliability, heterogeneity within diagnoses, high comorbidity between disorders, instability of categorical diagnoses over time, poor coverage of subthreshold conditions, and arbitrary boundaries between normality and pathology (Clark, Watson, & Reynolds, 1995; Widiger & Samuel, 2005).

Quantitative research on the structure of psychopathology has sought to develop an empirical classification system to address these weaknesses (Kotov, 2016).

Factor analysis and related techniques have identified dimensional spectra like internalizing and externalizing that explain patterns of comorbidity among disorders (Krueger, 1999).

Mental disorders often have a lot of overlap in their symptoms. The dimensional approach tries to group similar disorders together based on their shared symptoms.

This dimensional approach addresses weaknesses of traditional diagnostic systems that view disorders as distinct categories. It provides a more valid framework for research and treatment.

Recent efforts have built on this work to elaborate a hierarchical taxonomy encompassing both broad spectra and narrow components related to specific symptoms (Kotov et al., 2011).

The current paper introduces the HiTOP model as a synthesis of existing structural research aimed at providing an alternative nosology with greater validity and clinical utility.

Method

The HiTOP model was developed through an extensive review of quantitative research on the structure of psychopathology.

This has included factor analytic studies of relationships between categorical diagnoses and dimensional measures of symptoms and maladaptive traits.

Both exploratory and confirmatory factor analysis methods have been used. Complementary cluster analytic approaches have also provided support for dimensional representations.

Studies have examined child, adolescent, adult, and elderly populations using self-report, informant-report, and interview measures.

Samples have included both general community participants and psychiatric patients. The structural findings have been supplemented by research on risk factors, biomarkers, neural substrates, longitudinal course, disability, and treatment response relating to the identified dimensions.

Results

• Researchers identified major dimensions that describe different types of mental health symptoms. These include internalizing (anxiety, depression), externalizing (substance use, aggression), thought disorder (psychosis), and a few others.
• Analysis of patterns of comorbidity between disorders consistently revealed higher-order internalizing, externalizing, and thought disorder spectra (Kotov et al. 2011). Internalizing encompasses depressive, anxiety, trauma, obsessive-compulsive, and related disorders. Externalizing includes substance use, antisocial behavior, ADHD, and more.
• Thought disorder involves psychotic disorders and related personality pathology. In addition, detachment and somatoform dimensions emerged in some studies. Within internalizing, subfactors of distress, fear, eating pathology, and sexual problems have been identified. Externalizing comprises antisocial behavior and substance abuse subfactors.
• At lower levels, analyses of multidimensional symptom and trait measures have revealed components related to depression, various anxiety disorders, mania, psychosis, detachment, disinhibition, antagonism, and somatic distress. For example, depression comprises dysphoria, lassitude, insomnia, suicidality, and other specific symptom dimensions (Watson et al. 2007).
• HiTOP dimensions demonstrated expected patterns of relationships with putative validators like genetic risk factors, brain imaging findings, temperament, and treatment response (Andrews et al., 2009; Caspi et al., 2014; Nelson et al., 2015). Measures have been developed to assess components of HiTOP, like the Inventory of Depression and Anxiety Symptoms (Watson et al., 2007)

Implications

• The HiTOP system has significant potential to improve both research and clinical care in multiple ways. In research, HiTOP dimensions may provide more clearly defined phenotypes for studying genetic and neurobiological factors.
• In clinical practice, transdiagnostic HiTOP profiles can summarize key patient characteristics more succinctly.
• Tracking quantitative severity on relevant dimensions over time better captures clinical progress. Matching patients to interventions by considering both broadband and specific HiTOP dimensions may improve outcomes.
• If limitations can be adequately addressed, HiTOP offers a scientifically informed framework to advance the validity and utility of psychiatric diagnosis.

Future Research

• Longitudinal studies may benefit from the stability of dimensional measures compared to categorical diagnoses.
• Treatment research can target underlying spectra like internalizing more efficiently than isolated categorical diagnoses.

Strengths & Limitations

The study had many methodological strengths, including:

• Synthesizes a broad range of structural research on psychopathology using diverse samples and measurement methods
• Provides a hierarchical taxonomy encompassing multiple levels from specific components to broad spectra
• Operationalizes dimensions using psychometrically-sound instruments
• Shows expected relationships of dimensions with external validators
• Offers an alternative nosology with potential to address reliability, heterogeneity, comorbidity, instability, and boundary problems with traditional categorical diagnoses

However, this study was limited in a few ways:

• Still a work in progress requiring further elaboration and validation
• Optimal lower-order symptom dimensions are not yet fully defined
• Syndromes are the least studied level of the model so far
• Applicability to youth and elderly populations needs further study
• Lacks integration of longitudinal factors like onset and course
• Based largely on self-report data; integration of informant reports needed
• Thresholds for clinical use will need development with further norming

Insights

A key insight from the HiTOP model is that diagnostic comorbidity often results from imposing arbitrary boundaries on dimensions of psychopathology that exist on a continuum in nature. For example, dividing internalizing pathology into categorical diagnoses obscures the common distress dimension underlying mood and anxiety disorders.

Representing internalizing as a spectrum circumvents this problem. HiTOP goes beyond previous dimensional models like internalizing/externalizing by elaborating the full hierarchical structure from specific symptoms up through higher-order spectra.

This multilevel organization can provide a more complete clinical picture, with the optimal level of analysis depending on the clinical or research context.

For instance, the broad internalizing dimension may predict recurrence risk, while the fear subfactor is more relevant to phobia treatment selection.

While dimensional systems face challenges for clinical implementation, HiTOP shows promise as a scientifically valid framework that addresses many limitations of traditional nosologies.

Conclusion

In sum, the HiTOP model provides a promising step toward an empirically-based quantitative classification of mental disorders.

By organizing psychopathology into a hierarchy of dimensions derived through structural research, HiTOP aims to rectify important limitations of conventional categorical systems.

Additional validation and elaboration of the model are needed, but HiTOP represents an alternative nosology that may yield substantial benefits for research and practice.

It will be important to continue efforts to build an empirically grounded taxonomy while recognizing the complexities of translating scientific models into clinical realities.

Integrating multiple perspectives and balancing conceptual precision with clinical utility will be essential to advancing diagnostic systems in psychiatry.

References

Primary Paper

Kotov, R., Krueger, R. F., Watson, D., Achenbach, T. M., Althoff, R. R., Bagby, R. M.,…Zimmerman, M. (2017). The Hierarchical Taxonomy of Psychopathology (HiTOP): A dimensional alternative to traditional nosologies. Journal of Abnormal Psychology, 126(4), 454–477.

Other References

Clark, L. A., Watson, D., & Reynolds, S. (1995). Diagnosis and classification of psychopathology: Challenges to the current system and future directions. Annual Review of Psychology, 46, 121-153. https://doi.org/10.1146/annurev.ps.46.020195.001005

Kotov, R., Ruggero, C. J., Krueger, R. F., Watson, D., Yuan, Q., & Zimmerman, M. (2011). New dimensions in the quantitative classification of mental illness. Archives of General Psychiatry, 68(10), 1003-1011. https://doi.org/10.1001/archgenpsychiatry.2011.107

Kotov, R. (2016). The quantitative classification of mental illness: Emerging solution to boundary problems. In E. Bromet (Ed.), Long-term outcomes in psychopathology research: Rethinking the scientific agenda (pp. 140-157). New York, NY: Oxford University Press.

Krueger, R. F. (1999). The structure of common mental disorders. Archives of General Psychiatry, 56(10), 921-926. https://doi.org/10.1001/archpsyc.56.10.921

Nelson, B. D., Perlman, G., Hajcak, G., Klein, D. N., & Kotov, R. (2015). Familial risk for distress and fear disorders and emotional reactivity in adolescence: An event-related potential investigation. Psychological Medicine, 45(12), 2545-2556. https://doi.org/10.1017/S0033291715000471

Watson, D., O’Hara, M. W., Simms, L. J., Kotov, R., Chmielewski, M., McDade-Montez, E. A., Gamez, W., & Stuart, S. (2007). Development and validation of the Inventory of Depression and Anxiety Symptoms (IDAS). Psychological Assessment, 19(3), 253–268. https://doi.org/10.1037/1040-3590.19.3.253

Widiger, T. A., & Samuel, D. B. (2005). Diagnostic categories or dimensions? A question for the Diagnostic and Statistical Manual of Mental Disorders-Fifth Edition. Journal of Abnormal Psychology, 114(4), 494-504. https://doi.org/10.1037/0021-843X.114.4.494

Further Reading

• Conway, C. C., Forbes, M. K., Forbush, K. T., Fried, E. I., Hallquist, M. N., Kotov, R., … & Eaton, N. R. (2019). A hierarchical taxonomy of psychopathology can transform mental health research. Perspectives on psychological science, 14(3), 419-436.
• Kotov, R., Krueger, R. F., Watson, D., Cicero, D. C., Conway, C. C., DeYoung, C. G., … & Wright, A. G. (2021). The Hierarchical Taxonomy of Psychopathology (HiTOP): A quantitative nosology based on consensus of evidence. Annual review of clinical psychology, 17, 83-108.
• Ringwald, W. R., Forbes, M. K., & Wright, A. G. (2023). Meta-analysis of structural evidence for the Hierarchical Taxonomy of Psychopathology (HiTOP) model. Psychological medicine, 53(2), 533-546.
• Ruggero, C. J., Kotov, R., Hopwood, C. J., First, M., Clark, L. A., Skodol, A. E., … & Zimmermann, J. (2019). Integrating the Hierarchical Taxonomy of Psychopathology (HiTOP) into clinical practice. Journal of consulting and clinical psychology, 87(12), 1069.

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