Autism, attention deficit-hyperactivity disorder, bipolar disorder, major depressive disorder and schizophrenia all share several genetic risk factors, according to a major study.
Versions of four genes increased the odds of all five disorders.
Researchers hope to move the psychiatry away from describing symptoms towards fundamentally understanding what is going wrong in the brain.
The findings were reported in the Lancet medical journal.
The international study compared the genetic codes of 33,000 people with a psychiatric disorder with 28,000 people without a psychiatric disorder.
Four genetic variants appeared to increase the risk of all five disorders studied. Two genes were involved in the balance of calcium in the brain.
Hundreds of genes and the environment are likely to affect the odds of developing such conditions.
However, the rapidly advancing field of psychiatric genetics is trying to describe these disorders on the basis of what is causing them, rather simply by symptoms.
One of the researchers Nick Craddock, a professor of psychiatry at Cardiff University, said: "It signals the opening of a potential new era for psychiatry and mental illness.
"This is a scientific method that helps understand what is going wrong in the brain, the chemicals, the brains systems, that are important in illness."
He said that ultimately it could help devise treatments and better ways of diagnosing patients.
Dr Gerome Breen, from the Institute of Psychiatry at King's College London, said: "It points out fairly clearly that there is a common genetic effect between these disorders.
"These studies give a window into the biology of these disorders, that's really valuable."
Marjorie Wallace, chief executive of mental health charity Sane, said the findings "highlight the need to understand the genetic and biological factors of these life-changing conditions, in order that more effective treatments and therapies may be found".
She added: "While it may take a decade for research studies like this to translate into new drugs and other treatments, we may yet be working towards a breakthrough which has so long eluded scientists working in this field."