The evidence for this is so strong and so generally accepted that the list of cites available would be extremely long. Just looking at my own bookshelf, I can provide you with this:
Schizophrenia and Manic-Depressive Disorder: The Biological Roots of Mental Illness as Revealed by the Landmark Study of Identical Twins, by E. Fuller Torrey et al. From Chapter 4, “Where Does Schizophrenia Originate?”:
“There is probably no researcher currently studying schizophrenia who does not believe that genes play some role in the causation of this disease.”
In the twin studies discussed in the book, 28% of identical twins were concordant for schizophrenia, whereas only 6% of fraternal twins were concordant for the same disorder. This result is statistically very significant.
It almost certainly is a result of both genes and environment in combination. Obviously, if it was all genes, there would be a 100% concordance rate with identical twins. If there was no genetic component, the concordance rate would be similar to that of fraternal twins. Therefore, environment is obviously involved.
There are theories out there about obstetric complications, diseases, being reared by schizophrenic parents, etc., but I don’t have information about that at my disposal right now.
In Surviving Schizophrenia, also by Torrey, there is a study discussed in which the following groups were followed: placebo, placebo plus therapy, drugs alone, and drugs plus therapy. The one year rehospitalization rates were as follows:
Placebo alone: 72%
Placebo plus therapy: 63%
Drugs alone: 33%
Drugs plus therapy: 26%
This, plus other studies discussed in the book, “suggest again that drugs are the single most important element in preventing rehospitalization but that a supportive relationship provides a measure of additional prevention.” The supportive relationship he refers to is with a therapist.
The efficacy of antipsychotic medication, from the same book:
70% clearly improve on antipsychotic medication
25% improve minimally or not at all
5% get worse
These effectiveness rates are similar to those of penicillin against pneumonia or streptomycin in tuberculosis.