Schizophrenia and depression overlap in many ways. First of all, the negative symptoms and dysfunctional attitudes are essentially the same in both conditions. Factor II of the negative symptoms is also similar in the two conditions: flat affect, alogia, and reduction of social gestures. The main difference seems to be apathy, which is characteristic of schizophrenia but not of depression (although it can occur in depression). Disorganization is more common in schizophrenia but can occur in depression. I believe that some studies showed that there are similar heritability and possibly genetic anomalies in both conditions.
The evidence of the overlap is provided by the frequent use of the term schizoaffective or schizophrenic spectrum disorder. I don’t know if this relationship between depression and schizophrenia has been explored before, but it is rather compelling that the two conditions are merged in schizoaffective disorder.
Conclusion: Is it possible that depression is the unitary condition and the addition of unique schizophrenic features, such as the positive symptoms lead to the diagnosis of schizophrenia or schizoaffective disorder? It should be noted in this regard that the diagnosis of depression generally precedes the diagnosis of schizophrenia, and the same types of stressors that predispose to schizophrenia also predispose to depression. Final word: Except for the genetic anomaly leading to psychosis, the two conditions are basically the same. Please send me your thoughts and comments on this.