It has been showed that schizophrenia has a tendency to run in different families. This implies that genes play a role. The closer the genetic relationship the more likely the people are to share the disorder. Gottesman (1991) showed that when both parents are schizophrenic, there is a 46% chance of the child also getting it, however, if only one parent had it, it dropped to 16% and dropped to a further 1% when the sibling of the child had schizophrenia. This suggests that a genetic factor is involved. Gottesman also looked at schizophrenics whose father had an identical twin. He found that there was a 17% of being schizophrenic when the father was but he also found that there was also 17% chance of developing the disease when the father’s twin had schizophrenia but the father didn’t.
MZ twins share 100% of their genes; DZ twins share 50% of their genes. If genes are a factor we would expect more identical twins to share the disorder than non-identical. Rosenthal took a case study which had a set of female quadruplets. They all developed schizophrenia although the onset and symptoms were very different. This could be because they had a troubled upbringing. This suggests a strong heritable component. Furthermore, Kety et al (1994) also provided support for a genetic explanation for schizophrenia. Their results suggested that there was a higher concordance rate in people with biological relatives with the disorder than a control group.
However, most first degree relatives and twins share the same or similar environments so it is difficult to separate genetic and environmental influences. Adoption studies compare people who have been raised in a different environment from their biological relatives. If they have similarities with their biological relatives this should be due to genes. There is evidence from adoption studies carried out by Tenari et all (2000). They conducted a longitudinal