According to the genetic hypothesis, the more closely related the family member to the schizophrenic the greater their chance of developing the disorder. Evidence from twin, family and adoption studies has tended to indicate schizophrenia to have a genetic component although a major problem is separating out environmental influences.
Recently gene mapping studies have been performed that compare genetic material from families with a high and low incidence of the disorder with results indicating several genes rather than a single one to be at work to produce increased vulnerability to developing schizophrenia.
Support for the genetic argument was provided by Gottesman and Shields 1976 who reviewed 5 twin studies to find a concordance rate of up to 91% between MZ identical twins with severe forms of schizophrenia implying genetics to have a bigger influence on chronic schizophrenia. This was further supported by Kety et al 1992 who identified schizophrenics who had been adopted and compared the rate of schizophrenia to be 10 times higher among biological relatives, implying genes to play a much larger role than environmental factors.
However Leo 2006 argues Kety et al’s evidence to be suspect as sample sizes were small making generalisation difficult and many of the biological relatives were rather distant ones such as half siblings and therefore low biological similarity.
In addition twin studies have not gone without criticism either; they may suggest a genetic influence but often do not consider the influence of social class and socio-psychological factors between twins. Furthermore Hedgecoe 2001 argues that scientists have tried to construct schizophrenia as a genetic disease by using evidence from twin and adoption studies in a biased way to create a narrative about schizophrenia that subtly prioritises genetic