Final answer:
The correct option is 4). Research shows that identical twins of schizophrenia patients do not always develop the disorder themselves. In a 2004 study, only 36.8% of those with high genetic risk in disturbed environments developed schizophrenia. This indicates that environmental factors are significant, with about half of identical twins not developing schizophrenia.
Step-by-step explanation:
The question addresses the genetic and environmental aspects related to the development of schizophrenia in twins. When considering identical twins, where one twin has schizophrenia, significant research indicates that the other twin does not always develop the condition. This is partly due to environmental factors, which can play a critical role in the expression of genetic predispositions. Notably, in a 2004 study by Tienari and others, findings show that adoptees with high genetic risks for schizophrenia, primarily those whose biological mothers had the condition, were more likely to develop schizophrenia if they were raised in disturbed family environments.
Of those adoptees with a high genetic risk and raised in disturbed environments, 36.8% developed schizophrenia. Conversely, if they were raised in healthy environments, only 5.8% developed the disorder. These statistics illustrate how environmental factors can influence the expression of genetic risks. Therefore, it is clear that identical twins share genes with a schizophrenia victim but due to various factors, including the environment, approximately half do not themselves develop schizophrenia.