About 80% of schizophrenia cases are estimated to be hereditary. The other 20% of cases are sporadic, not having an apparent familial cause.
A new study suggests that at least half of sporadic cases are attributable to genetic mutations that arise for the first time in them and are not present in their parents, something that tends to have an elevated frequency in children with older fathers (the risk factor is called advanced paternal age).
Thus, at least 90% of schizophrenia cases are genetic, even though just 80% are hereditary. Evidence has also pointed to seasonal tick infections around the start of a pregnancy, low vitamin D levels during pregnancy, and rare recessive genes of inbred individuals as possible causes of sporadic schizophrenia.
This combined pattern of causes make schizophrenia makes its causes better understood than almost any other mental health condition, which is appropriate because schizophrenia is quite common, with about 2.2 million Americans affected.