Schizophrenia is the most common serious mental illness with abnormalities in perception, thought, emotion, and behavior. It mostly starts in young adults and costs a staggering amount of money in medical expenses each year. The causes of schizophrenia are complex and still not well understood. Schizophrenia is multifactorial. Genetic factors are important. It is now believed that schizophrenia is a complex, polygenic genetic disorder. In children of identical twins or both parents with schizophrenia, the incidence is more than 40 times higher than in children without genetic factors. Schizophrenia is not a monogenic disorder, but may be a combination of multiple micro- or meso-effective genes, and is largely influenced by environmental factors. So there is a clear genetic predisposition to schizophrenia, but it is not just genetics that play a role in the onset. It is generally impossible to tell at birth whether a child will develop schizophrenia, and the onset of symptoms is mostly between 16-35 years of age, which is the high prevalence age. In short, schizophrenia has a huge genetic risk, but it is not a given that children of schizophrenic patients will develop schizophrenia because schizophrenia is a multifactorial mental illness, and genetics is only one of the important factors.