Explain how twin and adoption studies attempt to distinguish genetic and environmental factors underlying the onset of schizophrenia within families. Review the studies and discuss two limitations of this.

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Psychology Essay

Explain how twin and adoption studies attempt to distinguish genetic and environmental factors underlying the onset of schizophrenia within families. Review the studies and discuss two limitations of this.

   Schizophrenia is the label applied to a group of disorders characterised by severe personality disorganisation, distortion of reality, and an inability to function in daily life. Symptoms are mainly disturbances of thought processes, but also extend to disturbances of emotion and behaviour. There are two symptom categories - acute schizophrenia characterised by positive symptoms, such as  hallucinations and delusions; and chronic schizophrenia, characterised by negative symptoms such as apathy and withdrawal. However, DSM-IV has now moved away from these definitions and classified schizophrenia into three main sub-types: paranoid, disorganised and catatonic.

Understanding Schizophrenia

   More research has, probably, been devoted to trying to understand the nature of schizophrenia than any other mental disorder. Somatogenic approaches have focused on the role of genetic mechanisms influencing the propensity to develop schizophrenia, while psychogenic approaches emphasise the effect of adverse childhood experiences, particularly abnormalities in family interaction in the aetiology of the disorder.

 

Twin studies

   Twin studies offer a powerful method of disentangling the effects of genetic and environmental factors underlying the onset of schizophrenia within families. Monozygotic (MZ) twins have almost identical genomes, while Dizygotic (DZ) twins share only approximately half their genes. However, both usually share the same rearing environment. Thus, if environmental factors entirely explain  familial clustering there should be no differences in the concordances between MZ and DZ twins. Conversely, if genetic factors are important, the concordance should be higher among MZ than DZ twins. If genetic factors alone were sufficient determinants, there should be a 100% concordance among MZ twins.

   In a Finnish study, the heritability of schizophrenia was 83% if first-degree relatives had been diagnosed with the disorder, the remaining 17% of the variance in liability was due to unique environmental factors; while common environmental factors seemed to have no influence on the liability (Cannon et al,1998). The concordance was 46% among MZ twins and 9% among DZ twins. The results from a British study were almost identical. The concordance was 41% among MZ and 5% among DZ twins (Cannon et al, 1999).

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   The importance of genetic factors also emerged in two studies that investigated the risk of schizophrenia among offspring of identical twins discordant for schizophrenia (Gottesman et al, 1982, Kringlen and Cramer, 1989). In the Danish twin study there was no significant difference in the risk of schizophrenia between offspring of affected twins (10%) and unaffected co-twins (17%) (Gottesman et al, 1982). However, sample sizes were quite small: three schizophrenic twins had a total of 14 offspring, 1 of whom was affected; while 6 unaffected co-twins had 24 offspring, 4 of whom were affected.

   The sample size in the ...

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In this well-researched account of the contribution made by genes and the environment to the development of schizophrenia, the writer shows clearly how MZ and DZ twin studies have aided our understanding of the role played by these underlying factors. This is an eloquent piece of scientific writing which wastes few words and relies on a wide range of studies carried out in the latter part of the 20th century. If there is any weakness in the essay, it is that the contribution made to the subject by 21st century DNA sequencing technology has been omitted (e.g. Singh and O'Reilly), but overall, this is an impressive piece of writing given the very small word limit.