A study led by a psychologist from Trinity College Dublin raises important questions on how mental illness is currently diagnosed and whether these diagnoses accurately reflect the underlying neurobiology of mental illness.

The findings, just published in the leading peer-reviewed medical journal, JAMA Psychiatry, are significant in highlighting the need for more individualised approaches to defining mental illness.

In this study the researchers showed that a compulsive dimension of mental health maps onto various aspects of 'cognitive flexibility' better than an expert-assigned diagnosis. Cognitive flexibility reflects a set of brain processes that are thought to be essential for controlling our habits.

Prior research shows that habits play a role in a range of mental health conditions characterised by compulsive, repetitive behaviours. These include obsessive-compulsive disorder, the focus of the present study, but also binge-eating, excessive shopping and forms of addiction.

Mental disorders are currently defined in terms of diagnostic and statistical manual (DSM) diagnoses. They are labelled in categorical terms; patients either meet criteria or they don't. This is extremely important for making clinical decisions ? to treat, or not ? but may not reflect the true nature of mental health and illness in the population.

There is now a substantial body of research suggesting that our existing categorical frameworks for mental illness need revision. This is in part because there is substantial overlap across disorders, with most patients meeting criteria for multiple disorders and the fact that many disorders share commonalities, such as compulsiveness.

In the current study, patients met an average of 3.7 concurrent diagnoses. Disorders are also highly heterogeneous - which means that two patients might have the same diagnosis, but have little to no overlapping symptoms and might respond in entirely different ways to the same treatment.

Focusing on obsessive-compulsive disorder (OCD), the findings of this study suggest that self-reported levels of compulsive behaviour are a better predictor of alterations in cognitive flexibility than whether someone has a diagnosis of OCD.

Commenting on the findings, lead author, Dr Claire Gillan said: "By defining mental health and illness in a way that is true to the biology of the brain and respects the reality that most mental illness varies in the population, it is hoped that we are charting a path towards a future where treatments can be prescribed on a more individualised basis, based on well-defined brain systems and circuits and crucially, with a higher rate of success."

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This study involved a collaboration between a team of researchers and clinicians at Trinity College Dublin, the Department of Psychology at New York University, New York State Psychiatric Institute, Harvard Medical School, the Warren Alpert Medical School of Brown University and the Hofstra Northwell School of Medicine.