All for One and One for All: Mental Disorders in One Dimension

Abstract

In both child and adult psychiatry, empirical evidence has now accrued to suggest that a single dimension is able to measure a person's liability to mental disorder, comorbidity among disorders, persistence of disorders over time, and severity of symptoms. This single dimension of general psychopathology has been termed "p," because it conceptually parallels a dimension already familiar to behavioral scientists and clinicians: the "g" factor of general intelligence. As the g dimension reflects low to high mental ability, the p dimension represents low to high psychopathology severity, with thought disorder at the extreme. The dimension of p unites all disorders. It influences present/absent status on hundreds of psychiatric symptoms, which modern nosological systems typically aggregate into dozens of distinct diagnoses, which in turn aggregate into three overarching domains, namely, the externalizing, internalizing, and psychotic experience domains, which finally aggregate into one dimension of psychopathology from low to high: p. Studies show that the higher a person scores on p, the worse that person fares on measures of family history of psychiatric illness, brain function, childhood developmental history, and adult life impairment. A dimension of p may help account for ubiquitous nonspecificity in psychiatry: multiple disorders share the same risk factors and biomarkers and often respond to the same therapies. Here, the authors summarize the history of the unidimensional idea, review modern research into p, demystify statistical models, articulate some implications of p for prevention and clinical practice, and outline a transdiagnostic research agenda. [AJP AT 175: Remembering Our Past As We Envision Our Future October 1910: A Study of Association in Insanity Grace Helen Kent and A.J. Rosanoff: "No sharp distinction can be drawn between mental health and mental disease; a large collection of material shows a gradual and not an abrupt transition from the normal state to pathological states."(Am J Psychiatry 1910; 67(2):317-390 )].

Heterotypic trajectories of dimensional psychopathology across the lifespan: the case of youth‐onset attention deficit/hyperactivity disorder

“…Therefore, the late‐onset ADHD phenomena might represent another example of problems assigning caseness status for dimensional manifestations of psychopathology over the lifespan. This is consistent with previous evidence showing the prominence of a general factor of psychopathology – the p‐factor ‐ which may manifest in distinct ways over development and wax and wane over time (Caspi & Moffitt, ; Caspi et al., ; Lahey et al., ; Martel et al., ).…”

The p factor: genetic analyses support a general dimension of psychopathology in childhood and adolescence

“…For example, suggestive evidence of links between p and personality has begun to emerge (Rosenström et al., ). In addition, instead of testing competing factor structures, we focused on the common pathway model, since the present study aimed to investigate the most parsimonious highest order part of the hierarchy that we call p. This is further justified by evidence for correlations and heterotypic sequential comorbidity across the internalising and externalising domains (Caspi & Moffitt, ).…”

“…Notably, some interesting results also emerge about the environment. There are some known general ‘environmental’ risks for psychopathology such as birthweight, birth complications and childhood maltreatment that are associated with diverse neurodevelopmental outcomes (Caspi & Moffitt, ; Lim et al., ). However, we find that nonshared environmental effects contribute less than genetic effects to the general psychopathology factor and its temporal stability.…”

A polygenic p factor for major psychiatric disorders

“…Assuming a hierarchical model with p at the highest level 6,7 , broader psychiatric dimensions at a middle level, and specific psychopathologies at the lowest level, the question is how much genetic variance is accounted for by the three levels. In the realm of cognitive abilities, there continues to be debates about the nature of the middle level 38 .…”

“…However, these higher-order dimensions also correlate with each other 3 , which suggests the possible existence of a general factor of psychopathology 4 . This general factor has been called the p factor 5 as it captures the shared variance across psychiatric symptoms, and predicts a multitude of poor outcomes and general life impairment 6,7 .…”