Why Are There No Biological Tests in Psychiatry? - Scientific American Blog NetworkThe problem of teasing out heterogeneous clinical presentations in psychiatry is compounded by the fact that they also have heterogeneous underlying mechanisms. There will not be one pathway to schizophrenia; there may be dozens, perhaps hundreds. Biological tests that appear to be associated with schizophrenia are never useful for making the diagnosis because they always show more variability within the category than between categories. And seemingly intriguing findings usually don't replicate.