The molecular pathology showed significant overlap between distinct disorders, such as autism and schizophrenia.
Scientists have found that autism, schizophrenia and bipolar disorder share some physical characteristics at the molecular level, specifically, patterns of gene expression in the brain.
Gene expression is the process by which instructions in DNA are converted into a product, such as a protein, according to a study published in the journal Science.
"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.
Every cell in the body contains the same DNA.
RNA molecules, on the other hand, play a role in gene expression in different parts of the body, by "reading" the instructions contained within DNA.
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.