Schizophrenia
There are many risk factors that may cause a person to develop schizophrenia. They include trauma (damage caused by stressful events) and genetics (schizophrenia running in families). Having a schizophrenic parent may be very stressful, and there may also be genes that influence the development of schizophrenia. It is very easy to show that trauma, such as sexual abuse (forced unwanted sexual behaviour towards somebody) increases the risk, but 40 years of searching for the right genes has not found anything that has been confirmed by independent research groups.