Genetic causes of schizophrenia 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.
TWIN RESEARCH Concordance rates are the chances that someone will develop a disorder if they related to someone who has it.
TWIN RESEARCH For example Gottesman (1991) reviewed about 40 studies and found that with identical (MZ) twins there was a 48% chance of both being schizophrenic. With non-identical (DZ) twins there was a 17% chance of developing schizophrenia.
Evidence for... Shields (1962) found that MZ twins who were raised in different in families still showed around a 50% concordance rate for the development of schizophrenia.
Evidence for... Adoption studies have found that when children are adopted because one or both of their biological parents has schizophrenia, the chance of them developing it is still the same. This suggests that genetics are more significant than the environment.
Evidence against.... No study has found 100% concordance rate, so schizophrenia cannot just be caused by genes. Shared environment may be cause higher concordance rates in family studies because children imitate ‘schizophrenic’ behaviours from relatives.
Evidence against.... This means other factors need to be considered e.g. biochemical or psychological factors.
The genetic causes of schizophrenia Genes-attempts to identify specific genes responsible for this illness have not been successful. Claims were made about genetic markers on chromosomes 5 & 22 but these have been retracted.
The genetic causes of schizophrenia Schizophrenia has a tendency to run in families. A child of a schizophrenic mother has a 6% chance of developing schizophrenia, compared to a 1% chance if unrelated. How can we investigate genetics?
Family studies: Children with two schizophrenic biological parents are 46% more likely to develop the disorder. These studies do not rule out environmental learning though.
The Copenhagen high risk study (Kety et al, 1962) The Copenhagen high risk study (Kety et al, 1962) identified 207 children whose mothers had schizophrenia (high risk) along with 104 children with healthy mothers (low risk). They were aged between 10 and 18 years at the start of the study and were matched on age, gender, parent socio-economic status and urban/rural residence.
The genetic causes of schizophrenia A follow up of these children was conducted in 1974 and 1989, and the results were published in 1993 by Parnas et al. These results strongly suggested a genetic link, as schizophrenia was diagnosed in 16.2% of the ‘high-risk’ group, and only 1.9% of the ‘low-risk’ group. Schizotypal personality disorder was diagnosed in 18.8 % of the ‘high-risk’ group, and in 5% of the ‘low-risk’ group.
What does this research tell us about schizophrenia?
Cross-cultural research - Twin studies: ...studies from many countries have produced different estimates. (narrow/broad) but Gottesman (1991) suggests that monozygotic (identical twins) who have the same genes have slightly higher concordance rates (48%) for schizophrenia than dizygotic (non-identical) twins (17%) Concordance rates refer to whether both twins develop this disorder. However identical twins also share the same environment.
The genetic causes of schizophrenia Gottesman and Shields (1972) investigated 45,000 individuals treated at a London hospital and identified 57 sets of twins who agreed to take part in the study. They identified a 42% concordance rate for MZ twins and only 9% for DZ twins.
What does this tell us about schizophrenia? Klaning et al (1996) cite that it may be due to twins' greater exposure to perinatal complications that they develop later disorders.
Adoption studies... where the adopted subjects environment are matched, the rates of schizophrenia are higher for adoptive children with schizophrenic biological parents compared to adoptive children with non- schizophrenic parents (Kety et al1975) ideally identical twins with schizophrenia raised in different adoptive environments would be the best evidence for genetic causes but obviously these cases are extremely rare.
The genetic causes of schizophrenia Tienari (1991) examined the rate of schizophrenia in Finnish people who had been adopted and whose biological mothers were schizophrenic. As predicted a biological mother with schizophrenia increased the rate of schizophrenic in the adoptees, even if they were adopted by non-schizophrenic families.
Tienari (1991) continued But the schizophrenic genetics revealed itself only if the adopted family was psychologically disturbed in some way. So, vulnerable individual could be protected from schizophrenia if their family of rearing were healthy.
Family studies continued: Heston (1966) compared 47 children of schizophrenic mothers adopted before the age of one month, with 47 children raised in the home of their biological and non-schizophrenic mothers.
Family studies continued: Heston found that many of the children with schizophrenic mothers developed schizophrenia, whereas none of the children from non- schizophrenic mothers developed schizophrenia. This is the same as in the normal population.
Evaluation It seems that heritability is an important component to any medical condition such as CHD, hypertension or diabetes (Lilienfeld 1995). The precise mode of inheritance remains controversial.
Evaluation continued The most popular view is the multifactorial (polygenic) model: a number of genes are involved that determine a predisposition which requires environmental factors such as stress to trigger the symptoms. According to Claridge and Davis (2003), the contribution of genetic influences is one of the few certainties about schizophrenia