Professor Daniel Weinberger explains that while genes play an important role in susceptibility to schizophrenia, environmental interactions are also important.

So, 'how do people get schizophrenia?' really is the big question. We know from over a century of studies of families, studies of twins, and studies of individuals adopted at birth that there are two main factors that determine how you get schizophrenia. The most important factor is genes, and this has been shown repeatedly in studies of twins where you compare identical twins who have the same genes, and fraternal twins who are just like siblings, they only have 50% of the same genes. And you show over and over again in these studies that the chance that if one twin has schizophrenia the other will also have it is much greater in the identical twins than the fraternal twins, even though both sets of twins theoretically share the same environments. Studies of children adopted away at birth, where they’re adopted away and their real parent was schizophrenic, but their adoptive parents do not have schizophrenia, show the same likelihood of developing schizophrenia in their life as if they were raised by a schizophrenic parent. So this has confirmed the role of genes. It is also clear, however, that environment is very important because even in the identical twins, when one twin has schizophrenia, it’s only 50/50 that the other twin will have schizophrenia. So genes are not enough. There’s evidence that problems early in brain development, maybe even in the uterus, increase the probability that someone will develop schizophrenia in their lifetime, and there’s been evidence that’s accumulating about those factors. But the big news about what causes schizophrenia is the arrival of real genes that have been found that increase the risk for someone having schizophrenia. Genes for schizophrenia are like genes for many common medical conditions like high blood pressure, diabetes, obesity. They’re not genes that determine your fate. If you inherit a gene that relates to schizophrenia, it does not give you schizophrenia. These are genes that are not about fate. They are genes that are about risk and they affect the probability that if you accumulate other factors, you will manifest the illness. So they change your risk. This is the same story with things like obesity, or diabetes, or high blood pressure. They’re not genes that cause disease. They are genes that slightly change the chances that you, if you have other risk factors converging in you, will show this condition.

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