The fundamental problem is that although we know a lot about the biomedical etiology of neurological disorders, we still fathom embarrassingly little about the biology of the major mental illnesses. And even the burgeoning understanding of genetic and metabolic risk factors has not translated into clinically useful tests or cures. Modern technology has not produced a single blood test or brain scan that can be routinely used to diagnose depression, bipolar disease, schizophrenia or other common mental illnesses.
American Scientist Vol. 90. p. 569