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 )].

Keywords: Development; Diagnosis And Classification; Etiology; Transdiagnostic; p Factor.

Figure 1. Psychiatric comorbidity is ubiquitous

Panel A shows the high rates of lifetime comorbidity in the Dunedin Longitudinal Study. The data include information from repeated diagnostic interviews carried out over 25 years, from when the research participants were 11, 13, 15, 18, 21, 26, 32, and 38 years old, and include information about the following 7 diagnostic groups: anxiety, depression, ADHD, conduct disorder, substance dependence, bipolar disorder, schizophrenia (24). Of individuals meeting criteria for one disorder in their lifetime, 66% met criteria for a second; of those meeting criteria for 2, 53% met criteria for a third; of those meeting criteria for a third, 41% met criteria for a fourth. The heat map of correlations in Panel B shows that comorbidity/covariation is not just limited to disorders in the same domains (e.g., Internalizing, Externalizing, Psychotic Experiences), but transcends diagnostic domains. Data are from symptom scales of disorders assessed at the last wave of the Study, age 38 years.

Figure 1. Psychiatric comorbidity is ubiquitous

Panel A shows the high rates of lifetime comorbidity in the Dunedin Longitudinal Study. The data include information from repeated diagnostic interviews carried out over 25 years, from when the research participants were 11, 13, 15, 18, 21, 26, 32, and 38 years old, and include information about the following 7 diagnostic groups: anxiety, depression, ADHD, conduct disorder, substance dependence, bipolar disorder, schizophrenia (24). Of individuals meeting criteria for one disorder in their lifetime, 66% met criteria for a second; of those meeting criteria for 2, 53% met criteria for a third; of those meeting criteria for a third, 41% met criteria for a fourth. The heat map of correlations in Panel B shows that comorbidity/covariation is not just limited to disorders in the same domains (e.g., Internalizing, Externalizing, Psychotic Experiences), but transcends diagnostic domains. Data are from symptom scales of disorders assessed at the last wave of the Study, age 38 years.

Figure 2. Schematic illustrations for thinking about the positive statistical correlations among psychiatric disorders

Panel A shows a Correlated-Factors Model, which is the original structural model used in psychopathology research. In this model, the latent variables represent variance shared (or comorbidity) among the disorders within each of three spectra: Internalizing, Externalizing, Psychotic Experiences. The high correlations between these latent traits suggested the possibility that they could be accounted for by a general factor of psychopathology, labelled “p”. Panels B1 and B2 show two ways to conceptualize p, respectively. The Higher-Order Factor Model (B1) shows that there is a second-order factor arising from the Internalizing, Externalizing, and Psychotic Experiences first-order latent variables; p represents the variance shared among the three spectra. The Bi-Factor Model (B2) shows that there is one common liability to these three forms of psychopathology (p) as well as a set of independent factors which influence a smaller subset of symptoms and disorders. The use of the term “bi-factor model” is an unwieldy historical and statistical legacy; it harkens back to the early days of intelligence research which first proposed a general factor that is common to all items on a test (g) and more specific factors that are common to a smaller subset of related items and are thought to represent independent cognitive modules. Panel C shows an alternative conceptualization of positive correlations among disorders. Here there is no common cause, but instead there is a causal network in which disorders influence each other (straight arrows) and themselves over time (looped arrows) (131). Panel D shows that rather than a cause of disorders, p is constructed from the disorders, reflecting a common manifestation that is shared by the different disorders.

Figure 3. Childhood cognitive deficits forecast adult p which is associated with cognitive difficulties in everyday life

Panel A shows the link between IQ assessed via the Stanford-Binet test administered at age 5 and p at ages 18–38 in the Dunedin Longitudinal Study (24). The p factor is a latent variable that reflects common variance among all of the multiple psychiatric disorders assessed. Attesting to the ecological validity of the cognitive deficits associated with pPanel B shows that adults with high levels of p also experience cognitive problems in their everyday life. Dunedin Study participants nominated people who knew them well, and we asked these people to rate the Study participants (at age 38) on a list of cognitive problems in everyday life.

Figure 3. Childhood cognitive deficits forecast adult p which is associated with cognitive difficulties in everyday life

Panel A shows the link between IQ assessed via the Stanford-Binet test administered at age 5 and p at ages 18–38 in the Dunedin Longitudinal Study (24). The p factor is a latent variable that reflects common variance among all of the multiple psychiatric disorders assessed. Attesting to the ecological validity of the cognitive deficits associated with pPanel B shows that adults with high levels of p also experience cognitive problems in their everyday life. Dunedin Study participants nominated people who knew them well, and we asked these people to rate the Study participants (at age 38) on a list of cognitive problems in everyday life.