Comorbidity Among Mental Disorders: A New Approach

Mental disorders have traditionally been viewed as distinct categorical entities, but about 50% of people who meet the criteria for one disorder also meet the criteria for a second disorder. The large number of people with comorbid disorders suggests there may be a simpler underlying structure to psychopathology than the one implied by the current classification system.

APS Fellow Avshalom Caspi (Duke University, Kings College London) and colleagues examined the structure of psychopathology using data from the Dunedin Multidisciplinary Health and Development Study — a prospective longitudinal study of health and behavior. Participants in the study were repeatedly assessed for mental health disorders between ages 18 and 38.

The authors tested several models using confirmatory factor analysis and found that a three-factor model comprised of an internalizing vulnerability to depression and anxiety, an externalizing vulnerability to antisocial and substance-use disorders, and a thought disorder vulnerability to symptoms of psychosis fit the data. The data was also well explained by a single general psychopathology dimension, which the researchers named the “p factor.”

According to the authors, a useful way to think about this general psychopathology factor is to relate it to the general factor of intelligence — the g factor. Under the umbrella of cognitive abilities are many dissociable and separate abilities such as verbal skills, working memory, and processing speed. Despite this, researchers find that the g factor accounts for the positive correlation between these separate abilities, explaining why people who do well on one measure of cognitive ability tend to do well on another.

The authors hypothesize that, similar to the g factor of cognitive ability, the p factor of psychopathology accounts for the presence or absence of psychiatric symptoms and summarizes people’s propensity to develop all types of psychopathology. In support of this proposal, the researchers found that the higher a person scores on the p factor, the worse they score on indicators of psychopathic severity, duration, comorbidity, and impairment.

A single-factor view of psychopathology may help explain why it has been so difficult to find causes, biomarkers, and treatments for individual mental disorders, and could help spur new research into the relationships between, and the structures of, common mental disorders.

Reference

Caspi, A., Houts, R. M., Belsky, D. W., Goldman-Mellor, S. J., Israel, S., Meier, M. H., … Moffitt, T. E. (2014).  Clinical Psychological Science, 2(2), 119–137. doi: 10.1177/2167702613497473