In 2018, psychiatrist Oleguer Plana-Ripoll was wrestling with a puzzling fact about mental disorders. He knew that many individuals have multiple conditions — anxiety and depression, say, or schizophrenia and bipolar disorder. He wanted to know how common it was to have more than one diagnosis, so he got his hands on a database containing the medical details of around 5.9 million Danish citizens.

He was taken aback by what he found. Every single mental disorder predisposed the patient to every other mental disorder — no matter how distinct the symptoms. “We knew that comorbidity was important, but we didn’t expect to find associations for all pairs,” says Plana-Ripoll, who is based at Aarhus University in Denmark.

…The idea that mental illness can be classified into distinct, discrete categories such as ‘anxiety’ or ‘psychosis’ has been disproved to a large extent. Instead, disorders shade into each other, and there are no hard dividing lines — as Plana-Ripoll’s study so clearly demonstrated.

…The most immediate challenge is working out how to diagnose people. Since the 1950s, psychiatrists have used an exhaustive volume called the Diagnostic and Statistical Manual of Mental Disorders, currently in its fifth edition. It lists all the recognized disorders, from autism and obsessive–compulsive disorder to depression, anxiety and schizophrenia. Each is defined by symptoms. The inherent assumption is that each disorder is distinct, and arises for different reasons.

…Few patients fit into each neat set of criteria. Instead, people often have a mix of symptoms from different disorders.

…This implies that the way clinicians have partitioned mental disorders is wrong. Psychiatrists have tried to solve this by splitting disorders into ever-finer subtypes. …But the problem persists — the subtypes are still a poor reflection of the clusters of symptoms that many patients have.

…Clinically, the evidence that symptoms cut across disorders — or that people frequently have more than one disorder — has only grown stronger. For this reason, although individual symptoms such as mood alterations or impairments in reasoning can be diagnosed reliably, assigning patients to an overall diagnosis such as ‘bipolar disorder’ is difficult.

Even seemingly separate disorders are linked.

…A 2018 study examined people who had been diagnosed with either major depression, panic disorder or post-traumatic stress disorder (PTSD). The volunteers were assessed on the basis of their symptoms, cognitive performance and brain activity. The researchers found that the participants fell into six groups, characterized by distinct moods such as ‘tension’ and ‘melancholia’. The groups cut across the three diagnostic categories as if they were not there.

…In the early 2010s, there was a push to eliminate disorder categories from the DSM-5 in [favor] of a ‘dimensional’ approach based on individual symptoms. However, this attempt failed — partly because health-care funding and patient care has been built up around the DSM’s categories.

…Most scientists agree that what is needed is more data, and many remain unconvinced by such simple explanations. “I’m a little less certain that that’s how it’s going to play out,” says Neale. At the genetic level at least, he says, there are many disorders, such as PTSD and generalized anxiety disorder, that remain poorly understood.

All such sweeping hypotheses are premature, says Hyman. “I think it’s a time for much more empirical research rather than grand theorization.”

The hidden links between mental disorders

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