Studies of relatives show that scientists have found repeatedly that schizophrenia is more common among the relatives of the people with the disorder. The more closely related that relatives are to the person with schizophrenia, the greater the likelihood of developing the disorder, according to Zimbardo (Smith, 1993). Clearly, if Schizophrenia were entirely due to genetic factors, the concordance rates for identical and fraternal twin pairs would be 100 per cent and 50 per cent, respectively, the same percentage as governs the sharing of their genetic make up as a whole. If we pool their results, we discover concordance rates of approximately 53 per cent for identical twin pairs and 15 per cent for fraternal twin pairs (Hemmings, 1989). This can be taken as evidence of the presence of hereditary component in schizophrenia; however, the fact that the concordance rate for schizophrenia among identical twins is only 53 per cent suggests that some type of environmental factor or factors also must be involved. For it would be expected that if Schizophrenia were totally genetic than both identical twins would always get the disease if the other did. Therefore, in relation to the evidence, those who support the environmental (psychosocial) explanation are right to suggest that the disease is in part caused by genes and in part by upbringing.
A second major group of studies looked at adopted children to examine the effects of heredity and environment. In 1974, L.L. Heston examined forty-seven adopted children whose mothers were schizophrenic and compared them with fifty adopted children- "controls"- whose mothers were not schizophrenic (Smith, 1993). He found that the same percentage of children of schizophrenic mothers developed the disease as would have been expected to if they had been bought up in their own homes: 11 per cent, as opposed to 0 per cent of the controls (Smith, 1993).
