A SINGLE DIMENSION BEST FITS THE DATA FOR MENTAL ILLNESS

Psychiatry has long been troubled by a lack of similarity to the other medical disciplines. Medical conditions usually exist as specific disorders with specific pathologies, diagnostic tests and treatments, whereas psychiatry is, in comparison, an overlapping mess. Some progress has been made through the DSM criteria and the establishment of three broad spectra of disorders. These three are the internalising (anxiety, depression), externalising (substance abuse, conduct disorder, antisocial personality disorder) and psychotic groups.

The majority of voices in the field have been asking for more categorisation, more definition of risk factors and specific neuroanatomical correlates to the various conditions. Call these the “splitters”. But in recent years it seems as if the real progress is being made in the opposite direction – by the “lumpers”. The important findings have mostly been coming from longitudinal studies – tracking cohorts of people from youth to middle age and beyond. And the findings are backing the idea that mental disorders are a unitary spectrum from mental health to mental illness, where connectedness between the different disorders is the norm and disorders in isolation are the exception.

In these studies, the various psychiatric disorders are being shown to have the same risk factors, the same neuro-imaging findings, and are all appearing as risk factors for each other. And in many cases, the same treatments are effective for the range of diagnoses. In an important open-access article “All for One and One for All – Mental Disorders in One Dimension“, (Am J Psychiatry, 2018) authors Avshalom Caspi and Terrie Moffitt put forward the case for a single dimension of psychopathology called “p“. P can indicate a person’s liability to develop mental disorders, along with their severity and persistence through time. It applies across all three major categories – internalising, externalising and psychotic.

This is all further evidence to a difference between the mind and the brain. What does it mean about the mind if all mental illnesses can be lumped together along a single dimension? Sounds very much like my own hypothesis of the mind as two mental energies – one constructive and one destructive. The “baseline” ratio between the two mental energies would then be the p value for that person. Unfortunately it doesn’t take us any closer to a measurable quantity for p.

All for One and One for All – Mental Disorders in One Dimension“, Caspi A & Moffitt TE: The American Journal of Psychiatry, 2018 175:9:831-844 //doi.org/10.1176/appi.ajp.2018.17121383