A large-scale genetic study has revealed that many mental disorders are closely linked at the biological level, and having multiple diagnoses in one person is more the rule than the exception.

A New Understanding of the Nature of Mental Disorders

An international team of scientists has shed light on a longstanding question in mental health: why are so many people diagnosed with more than one psychiatric disorder over the course of their lives? The study, published on December 10 in the journal Nature, presents the most comprehensive and detailed analysis to date of the shared genetic foundations of 14 psychiatric conditions.

The research was conducted by the Cross-Disorder Group of the Psychiatric Genomics Consortium. For most people with a psychiatric diagnosis, that diagnosis is rarely the only one—over time, many encounter a second or even third disorder, complicating both classification and treatment. While life experience and environment undoubtedly influence the risk of developing disorders, genetics also plays a significant role in why these conditions often overlap.

Study Scale and Key Findings

To better understand genetic influences, scientists analyzed data from more than 6 million people. The results showed that the 14 psychiatric disorders studied are not genetically isolated. Instead, they cluster into five major categories with significant genetic similarities. A deeper understanding of these overlaps could help doctors more effectively tailor treatments for patients with complex diagnoses in the future.

Psychiatry is the only medical specialty without clear laboratory tests: you can't do a blood test to determine if someone has depression, so clinicians must rely on symptoms and signs. Genetics is becoming a promising tool for understanding the connections between disorders. The results of this study represent the most complex analysis of psychiatric genomic data to date and help explain why people with one psychiatric disorder often have a second or third.

How the Study Was Conducted

The study examined the genetic material of more than 1 million people with psychiatric diagnoses, both childhood- and adult-onset, as well as data from 5 million people without diagnosed conditions. By identifying genetic markers more common in people with certain disorders, researchers can more precisely determine the biological factors that contribute to the development of mental illnesses.

Using various analytical methods, the research team studied the genetic structure of all 14 psychiatric disorders. This approach allowed them to identify 428 genetic variants associated with more than one condition, as well as pinpoint 101 regions on chromosomes where these shared genetic variants were especially concentrated.

The Five Genetic Groups of Mental Disorders

Statistical modeling enabled the grouping of disorders by genetic similarity. Five groups were identified:

• Compulsive Disorders: obsessive-compulsive disorder, anorexia nervosa, and to a lesser extent Tourette syndrome and anxiety disorders.
• Internalizing Disorders: major depression, anxiety disorders, and post-traumatic stress disorder.
• Neurodevelopmental Disorders: autism spectrum disorders, attention deficit hyperactivity disorder, and partially Tourette syndrome.
• Schizophrenia and Bipolar Disorder.
• Substance Use Disorders: dependence on opioids, cannabis, alcohol, and nicotine.

Some conditions showed particularly strong genetic links. For example, major depression, anxiety disorders, and post-traumatic stress disorder shared about 90% of their genetic risk factors. Schizophrenia and bipolar disorder also demonstrated significant overlap, sharing roughly 66% of genetic markers.

Biological Patterns and Future Perspectives

Researchers found that disorders with shared genetic risks often follow similar biological patterns. This includes periods of activity of common genes during human development and the types of brain cells they affect. For instance, genes active in oligodendrocytes (important cells in the central nervous system) were more closely linked to internalizing disorders, while genes expressed in excitatory neurons were associated with schizophrenia and bipolar disorder.

Implications for Future Medicine

According to scientists, these findings provide a solid scientific foundation for defining psychiatric disorders. The data obtained could aid in developing new treatment methods or adapting existing therapies for conditions that frequently occur together.

This study clearly demonstrates that combining efforts leads to greater achievements, both for science and for people living with mental illnesses.