Is There a Single Dimension of Mental Illness?
Many different types of psychiatric disorders may share a common thread
Posted January 15, 2015
People who study the classification of psychiatric disorders are often grouped into lumpers and splitters. The lumpers tend to see just a few broad categories of mental illness while the splitters try to subdivide psychopathology into smaller and more specific entities. Both sides have some research evidence to support their view. The fact that the number of official psychiatric disorders has risen to about 300 suggests that the splitters have had the advantage. At the same time, trying to find just about anything (genes, brain regions, causes, treatments, etc.) that are specific to just one disorder or even type of disorder has proven to be extremely challenging, becoming easy fodder for the folks who still deny that many disorders even exist.
A new study, however, scores big for the lumpers. Using data from the well-known Dunedin Multidisciplinary Health and Development Study, the authors argue for the existence of a single general dimension of psychiatric illness. They arrived at this conclusion by interviewing adults ages 18 to 38 at multiple time points about many types of emotional-behavioral symptoms and then testing various classification structures using statistical models called confirmatory factor analyses to see which model actually fit the data best. They also examined how different classification structures performed in terms of other factors such as personality, life impairment, developmental history, and brain integrity.
The bottom line from all of these rather sophisticated analyses is that the study found evidence for three broad areas of difficulties. namely: Internalizing Problems (e.g. anxiety, depression), Externalizing Problems (e.g aggression, defiance) and Thought Problems. That part really isn’t new. What was novel was that in addition to these dimensions there was a very strong signal for an overall general psychopathology dimension that they called the p factor. This factor united a wide range of different disorders and had strong and independent associations with things such increased life impairment, higher mental health problems in the family, and more compromised early developmental history and brain function.
In explaining their results, the authors make a comparison to cognitive abilities. Despite all the fanfare about the different types of intelligence, there is now strong evidence for the presence also of a single general intelligence factor that has been called the g factor. The study basically argues for the same thing in mental health.
What is this factor really about? It is hard to say exactly at this point. The study did look at it with regard to personality dimensions and found that people high in the p factor tended to have a personality profile that contained high levels of Neuroticism, low levels of Agreeableness, and low levels of Conscientiousness. Another idea is that it is linked closely to one’s ability to self-regulate.
This study is important because it points to an explanation for why so much controversy exists in delineating the boundaries between diagnosis A and B and suggests that some of our research and clinical efforts might be better targeted at this more global dimension that unites multiple different psychiatric disorders. It also sheds light on the phenomenon of comorbidity which was originally meant to convey the presence of multiple psychiatric diagnoses present in the same person. Individuals who suffer from multiple disorders are well known to be some of the people who struggle the most, yet comorbidity is much more common than would be expected under the model that each disorder is truly a separate diagnosis that is occurring independently. This study suggests that what we have been calling comorbidity might be better understood as higher levels of p and hopefully will spark additional research into how to address p more directly.
@copyright by David Rettew, MD
David Rettew is author of Child Temperament: New Thinking About the Boundary Between Traits and Illness and a child psychiatrist in the psychiatry and pediatrics departments at the University of Vermont College of Medicine.
Follow him at @PediPsych and like PediPsych on Facebook.