s. This is not for lack of effort. Literally hundreds of causes have been proposed (Davidson 1995). They include viruses, accidental injury before and after birth, as well as illicit substances. However, only inherited factors are generally accepted as significant causes. Evidence for such factors comes from everyday experience: It is common to find families with several affected members. While such clustering' could also be due to shared non-inherited environmental factors, more convincing evidence has come from twin studies, as well as studies of ill individuals who were adopted away before the onset of illness. These studies do not suggest inherited factors as the sole causes (Mitchell 1991). As stated by Nancy Andreasen (1999) Schizophrenia is probably caused by these multiple environmental stimuli and a combination of inherited genes. A more reasonable explanation is that there are individuals who have an inherited proneness to illness. Such individuals may fall ill, if in addition, they are exposed to obnoxious factors in the environment such as drugs. It has already been proven that in the causation of diseases like diabetes and raised blood pressure. It is important to stress that statistical analysis does not support the presences of a single genetic factor in this scheme. Rather, the inheritance is probably due to several factors that may be inherited from either side of one's family. It is assumed that a single genetic factor is unlikely to cause psychotic illnesses like schizophrenia. Therefore, there is no trace to the inheritance of genetic factors in families (Nuthers 1993). Instead, the attempt is to identify genetic factors that are present at higher rates among ill individuals compared with non-ill individuals. Initially compared ill individuals with unrelated unaffected individuals, it is difficult to say if the differences reflected proneness to illness or some other unrelated characteristics, which happened to...
