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Biological Explanations of schizophrenia- Family S

  • Family studies find individuals who have schizophrenia and determine whether their biological relatives are similarily affected more often than non-biological relatives.
  • Family studies have established that schizophrenia is more common among biological relatives of a person with schizophrenia, and that the closer the degree of genetic relatedness, the greater the risk.
  • For example, children with two schizophrenic parents have a concordance rate of 46%, children of one schizophrenic parent 13% and siblings 9%.
  • however, many researchers now accept that the fact that schizophrenia appears to run in families may be more to do with common rearing patterns or other factors that have nothing to do with heredity.
  • this evidence shows that there is clear genetic influence
  • there has to be something aswell as biology as the concordance rates are not 100%
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Biological Explanations of schizophrenia- Twin Stu

  • Twin studies offer a unique opportunity for researchers to investigate the relative contributions of genetic and environmental influences.
  • If monozygotic twins who share 100% of genes are more concordant in terms of a trait like schizophrenia than are dizygotic twins who only share 50% of genes, then this suggests that the greater similarity is due to genetic factors.
  • Joseph 2004 calculated that the pools data for all schizophrenic twin studies carried out prior to 2001 shows a concordance rate for monozygotic twins of 40.4% and for dizygotic twins of 7.4%
  • More recent, methologically sound studies have tended to report a lower concordance rate for monozygotic twins
  • however, despite this, twin researchers still argue that even these findings support the genetic position, because they provide a monozygotic concordance rate that is many times higher than the dizygotic concordance ratee.
  • A crucial assumption underlying all twin studies is that the environments of monozygotic twins and dizygotic twins are equivalent.
  • it is assumed therefore that the greater concordance for schizophrenia between MZ twins is a product of greater genetic similarity rather than greater environmental similarity.
  • however, as Joseph 2004 points out it is widely accepted that MZ twins are treated more similarily, encounter more similar environments and experience more identity confusion than DZ twins,.
  • as a result, argues joseph, there is a reason to believe that the differences in concordance rates between MZ and DZ twins reflect nothing more than the environmental differences that distingusih the two types of twins.
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Biological Explanations of schizophrenia- Adoption

  • because of the difficulties of disentagling genetic and environmental influecnes for individuals who share genes and environment, studies of genetically related individuals who have been reared apart are used.
  • probably the most methodologically sound study of this type was carried out by Tienari et al in finland.
  • of the 164 adoptees whose biological mothers has been diagnosed with schizophrenia, 11 also recieved a diagnosis of schizophrenia compare to just four of the 197 control adoptees.
  • The investigators concluded that these findings showed that the genetic liability to schizophrenia had been decisevely confirmed.


  • A central assumption of adoption studies is that adoptees are not selectively placed, i.e. adoptive parents who adopt schizophrenic biological parent are no different from adoptive parents who adopt children whose background is normal.
  • Joseph claims that this is unlikely to have been the case, particularily in the early studies.
  • In countries like Denmark and the US, potential adoptive parents would have been informed of the genetic background of children prior to selection for adoption.
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Dopamine Hypothesis


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