Why So Many Mental Health Diagnoses Overlap

At a glance

Psychiatric disorders share genetic roots: A large international study found that many mental health conditions overlap genetically, helping explain why people are often diagnosed with multiple disorders.
Five broad genetic clusters identified: Analysis of data from over six million people showed that 14 psychiatric conditions group into five categories with shared genetic risk and brain-related processes.
Implications for diagnosis and treatment: These findings challenge traditional diagnostic boundaries and could lead to more biologically informed approaches to treating complex mental health conditions.

A major international study has provided new insight into why many people are diagnosed with more than one psychiatric disorder over their lifetime. Published in Nature, the research offers the most comprehensive analysis to date of the shared genetic foundations of 14 psychiatric conditions, helping to explain why mental health diagnoses so often overlap.

Led by the Psychiatric Genomics Consortium’s Cross-Disorder Working Group, the study analysed genetic data from more than six million people worldwide. The findings show that psychiatric disorders are not genetically distinct, but instead cluster into five broad groups that share substantial genetic risk. This overlap may help clarify long-standing challenges in how mental illnesses are classified and treated.

Kenneth Kendler, professor of psychiatry at Virginia Commonwealth University and one of the study’s co-chairs, explained the significance of the work.

“Psychiatry is the only medical specialty with no definitive laboratory tests. We can't give a blood test to tell whether someone has depression — we have to rely on symptoms and signs. And that's true for almost every psychiatric disorder. Genetics is a developing tool that allows us to understand the relationships between disorders. The findings from this study reflect the most comprehensive analysis of psychiatric genomic data to date and shed new light on why individuals with one psychiatric disorder often have a second or third.”

The researchers examined genetic information from more than one million people diagnosed with psychiatric conditions and compared it with data from around five million people without diagnoses. Their analysis identified hundreds of genetic variants linked to multiple disorders, as well as key regions of the genome where shared risk factors are concentrated.

Using advanced statistical methods, the team grouped the disorders into five genetically related categories, revealing particularly strong overlap between conditions such as depression, anxiety and post-traumatic stress disorder, as well as between schizophrenia and bipolar disorder. These shared genetic patterns were also associated with similar biological processes in the brain, including the timing of gene activity during development and the types of brain cells involved.

The findings provide a stronger biological basis for understanding psychiatric diagnoses and may support future efforts to develop treatments that better reflect the underlying causes of mental illness, particularly for people with complex or multiple diagnoses.

Reflecting on the collaborative nature of the research, Kendler said, “I feel very proud to be a part of this effort. This work really shows that we gain more for our field and for those suffering from mental illness when we come together to tackle these scientific challenges.”