"These findings provide a molecular, pathological signature of these disorders, which is a large step forward," said Daniel Geschwind from the University of California, Los Angeles in the US.
"The major challenge now is to understand how these changes arose," Geschwind said.
The researchers know that certain variations in genetic material put people at risk for psychiatric disorders, but DNA alone does not tell the whole story.
The researchers reasoned that taking a close look at the RNA in human brain tissue would provide a molecular profile of these psychiatric disorders.
They analysed the RNA in 700 tissue samples from the brains of deceased subjects who had autism, schizophrenia, bipolar disorder, major depressive disorder or alcohol abuse disorder, comparing them to samples from brains without psychiatric disorders.
The molecular pathology showed significant overlap between distinct disorders, such as autism and schizophrenia, but also specificity, with major depression showing molecular changes not seen in the other disorders.
"These molecular changes in the brain are connected to underlying genetic causes, but we don't yet understand the mechanisms by which these genetic factors would lead to these changes," Geschwind said.