Why many mental health conditions may be more alike than we thought

Different mental health conditions may have far more in common at a biological level than scientists once believed, according to a new major genetic study.

Researchers from the University of Colorado Boulder and Mass General Brigham in the United States said the findings of this study could eventually improve how mental health conditions are diagnosed and treated, especially for people who live with more than one diagnosis.

The study analysed DNA from more than six million people, including over one million people diagnosed with at least one mental health condition. More than a billion people live with mental health conditions, according to the World Health Organization.

“Right now, we diagnose psychiatric disorders based on what we see in the room, and many people will be diagnosed with multiple disorders. That can be hard to treat and disheartening for patients,” said Andrew Grotzinger, assistant professor of psychology and neuroscience at the University of Colorado Boulder and corresponding author of the study.

The team studied 14 psychiatric disorders and found that most of the genetic differences between people with and without these conditions could be explained by just five broad genetic patterns. These patterns were linked to 238 genetic variants that affect how the brain develops and functions. Based on these shared genetic features, the researchers grouped the conditions into five categories.

One group included disorders with compulsive traits, such as anorexia nervosa, Tourette's disorder and obsessive compulsive disorder.

Another covered internalising conditions such as depression, anxiety and post-traumatic stress disorder. A third group focused on substance use disorders, while a fourth included neurodevelopmental conditions such as autism and attention-deficit hyperactivity disorder.

Bipolar disorder and schizophrenia formed a fifth group. The study found that around 70 per cent of the genetic signal linked to schizophrenia was also linked to bipolar disorder.

These two conditions have traditionally been viewed as very different, and clinicians rarely diagnose both in the same person.

“Genetically, we saw that they are more similar than they are unique,” said Grotzinger.

What this could mean for patients

The results, published in the journal Nature, challenge the long-held idea that mental health conditions are largely separate illnesses. Instead, the study suggests many are driven by shared biological processes.

However, researchers say it is too early to change how conditions are diagnosed.

The research team hopes the findings will inform future updates to the Diagnostic and Statistical Manual of Mental Disorders, the handbook used worldwide by mental health professionals.

“This work provides the best evidence yet that there may be things that we are currently giving different names to that are actually driven by the same biological processes,” said Grotzinger.

“By identifying what is shared across these disorders, we can hopefully come up with strategies to target them in a different way that doesn’t require four separate pills or four separate psychotherapy interventions,” he added.

The study also highlighted specific biological pathways linked to different groups of conditions. For example, genes affecting excitatory neurons, which help transmit signals in the brain, were more active in people with bipolar disorder and schizophrenia.

In conditions such as depression and anxiety, genetic variants linked to oligodendrocytes were more common. These specialised cells help maintain and protect the brain’s wiring.

Some shared genetic factors appear to influence brain development very early, even before birth, while others may play a bigger role later in adult life.

This could help explain why mental health conditions often overlap.

A 2018 review found that more than half of people diagnosed with one psychiatric disorder later receive at least one more diagnosis, and about 41 per cent meet the criteria for four or more during their lifetime.