•Genetic research on schizophrenia has been disappointing.•The heritability of schizophrenia has been over-estimated.•What appears to be genetic may be infectious.•For patients this is good news since treatment of non-genetic causes is more feasible. In recent years schizophrenia has been assumed to be largely a genetic disease with heritability estimates, derived primarily from family and twin studies, of 80%–85%. However, the results of genetic research on schizophrenia have not yielded results consistent with that estimate of heritability. In particular, extensive genetic studies have not led to new methods for diagnosis and treatment. An examination of the twin studies on which heritability is based shows why such studies exaggerate the genetic component of schizophrenia. In addition, the effects of infectious agents such as Toxoplasma gondii and the composition of the microbiome can produce a clinical picture that would also appear to be largely genetic due to familial aggregation and a role for a partial genetic contribution to the immune system. It is concluded that the genetic component of schizophrenia may have been overestimated and an increased focus on gene-environmental interactions is likely to accelerate research progress on this disease.