A psychiatric disorder rarely comes alone. More than half of all individuals who meet the diagnostic criteria for any psychiatric disorder are diagnosed with more than one condition. That high degree of comorbidity is often viewed as a consequence of the heterogeneity of psychiatric disorders – and as evidence that psychiatric diagnoses poorly reflect the underlying brain biology. Data published in Nature Human Behaviour on Jan. 12, 2023, has identified another likely contributor to the high degree of overlap between different psychiatric disorders.