Studies have found that individual for schizophrenia is 2x higher in fraternal twins than in siblings. Fraternal twins are genetically the same as siblings in that they are two separate eggs and two separate sperms that just happen to fertilize at the same time. So the fact that risk of schizophrenia is so much higher in fraternal twins than siblings suggests that the shared environment of the womb while they are fetuses is a key contributor to the development of schizophrenia. 

University of Michigan - Ann Arbor

- Every major review for the past half-century, including those authored by members 
of our group, has described schizophrenia as a disease of “unknown etiology,” one 
“suspected” of having developmental origins. While this is a valid characterization 
of an enigmatic disease, its consistent application belies the advances made in the 
field of schizophrenia research.
- For example, multiple risk factors for the disease have now been elucidated. 
Some meet the standard of reproducibility, and several findings have been replicated in population-based studies. Additionally, risk and vulnerability at key points 
throughout the lifespan have been investigated, and outcomes in this area have 
strengthened the plausibility and consistency of possible developmental origins. 


- Preconceptual
- Prenatal
- Perinatal

Environmental Risk Factors of Schizophrenia

Starr, Dr. Barry. “What Role Do Genes Play in Schizophrenia, Depression, and Mental Illness in General?” The Tech Interactive, The Tech Interactive, 13 May 2005, https://www.thetech.org/ask-a-geneticist/ask112#:~:text=In%20twin%20studies%20looking%20at,will%20have%20it%20too1. 

What Role Do Genes Play in Schizophrenia

We have classified “preconception” environmental risk factors as those involving demographic or other parental features potentially identifiable prior to conception.

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