My understanding is that historically, schizophrenia has been presumed to have a partly genetic cause since around 1910, out of which grew an intermittent research program of family and twin studies to probe schizophrenia genetics. An opposing camp that emphasized environmental effects emerged in the wake of the Nazi eugenics program and the realization that complex psychological traits needn't follow trivial Mendelian patterns of inheritance. Both research traditions continue to the present day.

Edit to add - Franz Josef Kallman, whose bibliography in schizophrenia genetics I somewhat glibly linked to in the grandparent comment, is one of the scientists who was most firmly in the genetic camp. His work (so far as I know) dominated the study of schizophrenia's causes between the World Wars, and for some time afterwards.

seem that some Freudians believed schizophenia was a mental rather than physical disorder

The last point in the abstract at cupholder's link seems strikingly defensive to me:

8. The genetic theory of schizophrenia does not invalidate any psychological theories of a descriptive or analytical nature. It is equally compatible with the psychiatric concept that schizophrenia can be prevented as well as cured.

Now I'm trying to work out what weird sexual thing involving one's mother could possibly be construed to cause schizophrenia.

• http://en.wikipedia.org/wiki/Schizophrenia#Genetic
• "A family history of schizophrenia is the most significant risk factor (Table 12).3"