Genetic hypothesis of Schizophrenia

Question 1

Why are twin studies used?

Answer 1

The assumption underlying twin studies is that MZ twins will show a greater concordance rate than DZ twins, as MZ twins are genetically identically whereas DZ twins share approximately 50% of the same genes as normal siblings.

Question 2

What did Cardno et Al (2002) twin study find?

Answer 2

Reported a study based on Maudsley’s Twin Register and found that it showed a concordance rate of 26.5% for MZ twins and 0% for DZ twins

But 26.5% only suggests a correlational factor not a causal factor

And MZ twins are relatively rare so the sample size is small, as only 1% would be expected to have schizophrenia and doesn’t provide much support for the theory

Difficult to assess reliability - different diagnostic criterion will produce different concordance rates, so comparisons can’t be made.

Difficult to separate the effects of heredity from the effects of the environment as twins are usually raised together

Question 3

Why are adoption studies used?

Answer 3

Allows researchers to look at people that have been born to schizophrenic mothers but brought up by adoptive parents with no history of the disorder. Allows to identify whether schizophrenia is something one is socialised into or if their biology expresses the disorder.

Question 4

What did Tienari (1991) find?

Answer 4

Identified 155 adopted children who had biological links to schizophrenia and compared them with a matched group of adopted children who had no family history of schizophrenia.

Found that about 10% of the group with biological links developed schizophrenia compared to 1% of the second group.

Suggests that biology determines schizophrenia more so than socialisation.

However, classification systems are regularly updated means that the criteria of the diagnosis may differ between mother and child, the different definitions may affect the validity.

Question 5

What does the genetic hypothesis suggest?

Answer 5

It suggests that the risk for a particular individual developing schizophrenia is proportional to the amount of genes they share.

MZ twins: 48% risk
Children with affected parents: 46% risk
DZ twins with one affected parent: 17% risk
Grandchildren: 5% risk
General population: 1% risk 

Even with relatively identical genes, i.e. MZ twins, the risk is still well below 100% which suggests that there are other factors involved which contribute to its developing.

However, there is no specific gene or genetic pattern linked to schizophrenia and so fails to establish cause and effect.