Genetic Factors

Genetic:

• Being genetically related to someone with schizophrenia can significantly increase a person’s chances of developing it. Family and twin studies have looked at concordance rates

• Gottesman (1991) reviewed about 40 twin studies and found that with identical (MZ) twins there was about a 48% concordance rate (ie. there is a 48% chance that one MZ twin will have schizophrenia if the other does). With non-identical (DZ) twins the concordance rate was about 17%. This is significant because if it’s assumed that MZ and DZ twins share the same amount of environmental influences, it means the reason for increased concordance is the genetic difference- MZ twins share 100% of their genes, compared to DZ who share 50%

Evidence for…

1. Shields (1962) found that MZ twins raised in different families showed around 50% concordance

2. Adoption studies found that if children are adopted because one or both of their biological parents have schizophrenia, the chance of them developing it stays the same. It suggests that genetics are more significant than the environment

Kety 1994:

• Found high rates of schizophrenia in individuals whose parents had the disorder but had been adopted.

• Tienari 1991; in the Finnish adoption study, identified 155 adopted children whose biological mothers had schizophrenia and compared them with a matched group of adopted children who had no family history of schizophrenia

  • He found that about 10% of the adopted group whose mother had schizophrenia developed schizophrenia compared to only 1% of the second group

  • Studies such as these provide strong evidence for a genetic component however, this is a complex issue

Evidence against…

1. No study has found a 100% concordance rate between MZ twins, so schizophrenia can’t just be caused by genes. A shared environment may cause higher concordance rates in family studies because children imitate ‘schizophrenic’ behaviours

2. This means other factors need to be considered, e.g. biochemical or psychological factors

Gottesman and Shields: Twin Study of Schizophrenia:

Aim:

• To investigate to what extent schizophrenia is genetic. To investigate the relative importance of genetic and environmental influences on schizophrenia by comparing MZ and DZ twins.

Method:

• Records of twins from the Maudsley and Bethlehem Royal Joint Hospital provided a sample of 57 twin pairs - 24 MZ, 33 DZ, age range 19 - 64.

• Twins were tested for zygosity using blood tests, fingerprints, and visual appearance.

• In addition to the hospital diagnosis, the following information was obtained:

  • Case histories based on a self-report questionnaire and interview with the twins and their parents to provide a record of verbal behaviour and a personality test

  • A test used to measure disordered thinking was conducted on twins and parents

• Analysis of the data has looked for similarities between each client and their twin. Concordance was assessed in three different ways:

  • Grade 1: both client and co-twin have been hospitalised and diagnosed with schizophrenia.

  • Grade 2: both client and co-twin have had psychiatric hospitalisation but the co-twin has a different diagnosis.

  • Grade 3: The co-twin has some psychiatric abnormality (e.g. out-patient care, GP care, neurotic or psychotic personality profile or being abnormal on interview)

Results:

• There was a significant difference found between MZ and DZ twins in all measures

• MZ twins were always more similar in diagnosis than DZ twins in each case where the co-twin had some diagnosis

• The similarity was greater between female twins compared to male twins (for both MZ and DZ twins)

• Concordance rates were higher for both MZ and DZ twins for severe schizophrenia compared to mild schizophrenia

  • Grade 1 - MZ = 42%, DZ = 9%

  • Grade 2 - MZ = 12%, DZ = 9%

  • Grade 3 - MZ = 25%, DZ = 27%

Conclusion:

• Genes appear to play an important role in schizophrenia because the concordance rate is higher in MZ twins than in DZ twins. However environmental factors must also be important as the concordance is not 100%.

Evaluations:

• Generalisability: A large sample of male and female patients was used, including different ages, meaning the findings could be generalised. However, as the sample only focused on twins, the findings may only be generalised to twins. Also, only one hospital was used.

• Reliability: Sampling was carefully controlled using multiple measures to make sure that twins were correctly allocated either MZ or DZ twin status, and a lot of data was gathered using multiple research methods to check the diagnoses were correct. However, interviews and self-report data may be less reliable.

• Application: If there is a genetic component for Schizophrenia this is useful because it can help lead to identifying specific genes.

• Other studies support findings of MZ sharing traits - Bailey (2000) concordance rates for homosexuality - MZs 24% females, 20% males these figures were much lower for DZ twins

• If schizophrenia was entirely a product of genes the concordance rates for MZs would be 100% - this is not the case so environmental factors must play a role

• Twins may be treated the same or may copy each other, therefore higher concordance due to environmental rather than biological factors

• The study did not look at reasons for schizophrenia or forms of schizophrenia therefore it is limited in its use - it simply identifies a concordance between schizophrenia and genes

Weakness:

• It would have been useful to know more about what related psychosis meant when describing the schizophrenia scale to take into account the related disorders, as they failed to make this clear

• Gottesman and Shields suggested that there are different types of schizophrenia and some might be caused by life events (so environmental stimuli), such as being a prisoner of war, and it was hard to distinguish between such types in the results of the study

Strengths:

• The results are supported by other studies which have produced similar findings, such as a 74% concordance rate for twins with a progressive chronic schizophrenia disorder, and a 39% concordance rate for mild schizophrenia.

• One of the findings of their analysis of the other studies was that the sampling techniques were questionable, but a strength of Gottesman and Shields' study is that the sampling was carefully controlled using multiple measures to make sure that twins were correctly allocated either MZ or DZ twin status, and a lot of data was gathered using multiple research methods to check the diagnoses were correct

Epigenetic modification:

• Changes in DNA through environmental influences without changing the DNA sequence

• This can have an effect on gene expression through gene silencing, and other processes

Co-Mobiditity between family members (Kallmann 1938):

• It is clear from these figures that the closer the genetic link, the higher the concordance rate.

• However, Reed et al showed that the relationship is not that simple.

• Using parents with psychosis rather than schizophrenia they found that if a mother had psychosis then the risk to her child was 20% whereas if the father had psychosis, the risk to the child was only 8%

Evolutionary Explanation:

• Some evolutionary explanations of schizophrenia suggest that there must have been an advantage to having schizophrenia for it to remain in the population

• One evolutionary idea is that people diagnosed with schizophrenia today share similar characteristics to shamans of the past

• They were likely to split off from a group when it got too big, starting new cultures

Evidence for:

• There is such a strong genetic link to schizophrenia, that there must be some form of evolutionary explanation

Evidence against:

• There is little evidence and the theory is difficult to prove

*Generally discredited