In the widest trawl yet of genetic mutations linked with mental disorders, US-led researchers looked through the DNA code of 33,332 people with autism, attention deficit-hyperactivity disorder (ADHD), bipolar disorder, major depressive disorder or schizophrenia.
Their genomes were matched against 27,888 "controls," or people who did not have these illnesses, in a bid to spot tiny changes in genes.
The five diseases have common risk factors in flaws on Chromosomes 2 and 10, and in two genes that help regulate the flow of calcium in brain cells, the investigators found.
One of the genes, called CACNA1C, has previously been fingered in bipolar disorder and schizophrenia.
The study, published in The Lancet, says the common genes are part of a much wider picture and do not by themselves explain the causes of these disorders or why their symptoms vary.
In addition, the gene samples examined by the Psychiatrics Genomics Consortium all came people from European ancestry, and the results may be different for people of different heritage, it said.
But, it added, the findings are a useful step towards better diagnosis of these illnesses.
Psychiatric disorders are difficult to categorise because symptoms can be hazy or contradictory, and little is known about their underlying cause. Environmental factors also play a part.
Previous genome comparisons have found intriguing common ground in an array of so-called auto-immune disorders, including arthritis, Crohn's disease and psoriasis.