Five major psychiatric disorders share common genetic risk factors, research has shown.
Scientists looked at genetic links to autism, attention deficit hyperactivity disorder (ADHD), bipolar disorder, major depressive disorder and schizophrenia.
Four genetic variants were found to be associated with all five.
In particular, the research highlighted mutations in two genes that help govern the balance of calcium in brain cells.
The discovery could pave the way to new treatments, say the researchers writing in the latest online edition of The Lancet medical journal.
Scientists scanned the genome, or genetic code, of more than 33,000 patients with mental disorders as well as 27,888 healthy individuals.
The study was the largest investigation of genetic links to psychiatric illness ever conducted.
Dr Jordan Smoller, a member of the team from Massachusetts General Hospital in Boston, US, said: "This analysis provides the first genome-wide evidence that individual and aggregate molecular genetic risk factors are shared between five childhood-onset or adult-onset psychiatric disorders that are treated as distinct categories in clinical practice.
"Significant progress has been made in understanding the genetic risk factors underlying psychiatric disorders. Our results provide new evidence that may inform a move beyond descriptive syndromes in psychiatry and towards classification based on underlying causes."