Scientists reported on Wednesday that they had taken a significant step toward understanding the cause of schizophrenia, in a landmark study that provides the first rigorously tested insight into the biology behind any common psychiatric disorder.

More than two million Americans have a diagnosis of schizophrenia, which is characterized by delusional thinking and hallucinations. The drugs available to treat it blunt some of its symptoms but do not touch the underlying cause.

The finding, published in the journal Nature, will not lead to new treatments soon, experts said, nor to widely available testing for individual risk. But the results provide researchers with their first biological handle on an ancient disorder whose cause has confounded modern science for generations. The finding also helps explain some other mysteries, including why the disorder often begins in adolescence or young adulthood.

“They did a phenomenal job,” said David B. Goldstein, a professor of genetics at Columbia University who has been critical of previous large-scale projects focused on the genetics of psychiatric disorders. “This paper gives us a foothold, something we can work on, and that’s what we’ve been looking for now, for a long, long time.”