Another biological explanation for the development of schizophrenia is the role of genes and so far research has revealed as many as 700 possible culprits! Chromosomes 13 and 6 are of particular interest although genes on as many as 11 other chromosomes have also been investigated. The genes of greatest interest have been those that code for dopamine and GABA (e.g. COMT and DISC1) and for the enzymes which break down these neurochemicals.
One strength of the genetic explanation is that it is supported by familial research which demonstrates that the risk of developing schizophrenia increases in line with the amount of shared DNA.
For example, first degree relatives share 50% of DNA and the concordance rate for full siblings, for example, where one has schizophrenia is 8%, however for third degree relatives who share only 12.5% DNA, e.g. first cousins, the concordance rate is as low as 2%.
This is important as it clearly shows that the more DNA you share the greater the risk, suggesting that schizophrenia may indeed have a genetic component.
This said, familial research is not without its problems, as family members do not just share DNA they also often share very similar environments as well, meaning it is not possible to disentangle nature from nurture.
However, twin studies which compare MZ and DZ twins help with this as studies such as Gottesman and Shields (1966) demonstrate that for MZ twins, who share 100% of their DNA, the concordance rate is 42%, whereas for DZ twins, who share only 50%, the concordance rate when one twin has schizophrenia is 9%. These studies are important as both twins share the same environment, however MZs and DZs differ with regard to the amount of shared DNA which becomes a naturally occurring IV. The only possible reason for a difference in the concordance rate could be argued to be that schizophrenia has a genetic component.
The evidence to support this argument is particularly strong and a meta-analysis conducted by Sullivan et al (2003) suggested that the average concordance rate was as high as 81%. This said, some psychologist argue that you need to take care when interpreting results from MZ/DZ twin studies however, as they say it is not just the amount of genetic material that differs between MZ and DZ twin pairs, they argue that MZs tend to be treated more similarly by other people including their parents as they look so similar and this means that their shared experiences are greater as well as their shared DNA, meaning that the conclusions of MZ/DZ twin studies cannot be as easily interpreted as first thought, i.e. differences in the concordance rates cannot be said to be caused by biology alone.
Furthermore, if schizophrenia was a truly heritable condition one would expect the concordance rate for MZ twins to be 100% and even in Sullivan’s analysis, this is far from the case indicating that environmental factors clearly have an important role to play as highlighted in the many research findings which support the environmental breeder hypothesis explanation, such as research which demonstrates that second generation immigrants seem to be more at risk than first generation immigrants and also that immigrants are only more at risk when they are part of a minority and indeed when they face prejudice and discrimination. These factors point towards the role of social stress as the trigger which may be necessary for a person to begin to exhibit symptoms of schizophrenia, supporting the diatheses –stress model suggested by Zubin and Spring.
In conclusion, it would appear from twin study research that schizophrenia is very likely to have a genetic underpinning which creates a predisposition for developing the disorder however, it also apparent that this is only part of the answer and thus further research into the epigenetic factors of schizophrenia is clearly necessary. Sir Robin Murray has recently noted that for too many years the research community have focused on biological and genetic research, failing to thoroughly research the role of environmental factors in determining the development and prognosis of this condition. Research such as Luhrmann’s cross cultural study on hearing voices clearly demonstrate the need to recognise that schizophrenia does not occur in a socio-cultural vacuum and the beliefs, values and attitudes of the person experiencing the condition and those around them are pivotal in how the conditions manifests itself. These factors are completely ignored in genetic research, arguably to the detriment of the field and to the affected families.