HelenofReddit t1_isuo4tl wrote

It's both. There are loads of different factors that determine whether someone will develop schizophrenia. Many studies show that a genetic liability to the disease combined with being in a stressful environment (think neglect, severe trauma, drug abuse, etc.) can lead it to manifest. They call it a "gene by environment" interaction, or "epigenetic" effects. Here's an article from the NHS that breaks it down.

To be clear though, I don't think there's evidence that schizophrenia the disorder can develop if you don't have any genetic risk factors whatsoever (though there are so many likely working together that it's truly hard to know if you do or not). I also think it's possible to develop schizophrenia with the genes alone and without any obvious environmental factors. As someone pointed out below, there appears to be scientific consensus that non-genetic factors are necessary for schizophrenia to develop.