The latest edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM), the mental health field’s diagnostic standard, describes nearly 300 distinct mental disorders, each with its own characteristic symptoms. Yet increasing evidence suggests the lines between them are blurry at best. Individuals with mental disorders often have symptoms of many different conditions, either simultaneously or at different times in their lives. What’s more, as the family patterns suggest, the genes linked with these conditions overlap. “Everything is genetically correlated,” says Robert Plomin, a behavioral geneticist at King’s College London. “The same genes are affecting a lot of different disorders.”