Twin Studies

Twin studies involve comparing MZ and DZ twins to see what differences there are in the incidence of a certain characteristic. MZ twins are identical in genetic make-up as they come from one egg. DZ twins share 50% of their genes like any other brother/sister pair as they come from two eggs. The idea in studying twins is that if a characteristic is completely genetically given, MZ twins would both show the characteristic. If a characteristic is not genetically given but comes from environmental influences and factors, then MZ twins will not share the characteristic any more than DZ twins. In practices, it is not expected that a characteristic is totally shared between MZ twins, so a higher sharing of that characteristic for MZ twins than for DZ twin is thought to indicate a genetic component for that characteristic.
With regard to schizophrenia, if one twin has schizophrenia and the condition is inherited it would be expected that with MZ twins the other twin is more likely to have schizophrenia too, but with DZ twins this is less likely.

Evaluation of twin studies

Study using twin studies: Gottesman and Shields (1966) (SmartALevels)

Aims: To see if whether schizophrenia had a genetic basis. Using a twin study methodology, Gottesman and Shields planned to test different twin pairs to investigate the concordance rates of schizophrenia in twins. They also wanted to replicate other studies that had indicated a genetic basis to the disorder, to find if their findings would be similar (to test for reliability).
Procedure: Researchers looked at patient records from a short-stay psychiatric hospital with a large outpatients department. The records covered a sixteen year history from 1948 to 1964. Over that time, 392 patients were seen who said that they had a twin of the same sex, and of those 68 had a diagnoses of schizophrenia or a related psychosis – although six patients were discounted either because their twin counterparts were overseas or it was unclear whether they were monozygotic (MZ) or dizygotic (DZ) twins. This left 62 patients: and 5 twin pairs were both on the register, leaving 57 study pairs. The other 52 patients’ twin counterparts were tracked down to participate in the study.

Of the 62 patients in the participant sample, exactly half were male, and half were female. Their ages ranged from 19 to 64 when they were last assessed, with the average patient age being 32. Zygocity was determined using three methods: fingerprint testing, blood testing and resemblance assessments (this was done before DNA testing was around). Multiple data collection methods were used, involving both primary and secondary resources.

These included:
· Hospital notes
· Case histories based on self-report questionnaires and interviews with the twins and their parents · tape-recorded 30 minute samples of verbal behaviour from semi-structured interviews
· Personality testing
· Testing to measure disordered thinking, conducted both on the twin sets and their respective parents

The researchers wanted to look at concordance rates.  If the one twin had been diagnosed with schizophrenia, they wanted to know how often their counterpart twin would also be diagnosed with schizophrenia, or a related psychosis. The patient who had been admitted to the psychiatric hospital initially first coming to attention was called the proband. The data was analysed between each proband and their twin, with twin pairs being categorised in four ways.

Findings: The tables below display the main results from Gottesman and Shields’ study. The four categories were:

• Category 1 – both the proband and co-twin had been hospitalised and diagnosed with schizophrenia
• Category 2 – both had hospitalisation, but the co-twin had been given another diagnosis related to schizophrenia
• Category 3 – the co-twin had some psychiatric abnormality, but nothing related to schizophrenia
• Category 4 – proband had schizophrenia, but the co-twin was diagnosed as clinically normal

It is recommended that you learn those figures highlighted, as learning them all is not realistic.

Note: Severe means having longer than two years’ hospitalisation, mild means less than two years.

In summary, the results showed:
· 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
· 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

Conclusions: The findings suggested that the closer the genetic relationship between two people, the greater the likelihood that if one of them is diagnosed with schizophrenia, the co-twin is also going to develop schizophrenia or a related psychotic disorder. The study also suggested that this relationship was more prevalent among females than males.
However, as twins in the monozygotic twin pairs did not have 100% concordance rates, the implication is that schizophrenia is not caused entirely by genes. Instead, the results led Gottesman and Shields to believe that genetic factors do predispose someone to schizophrenia, by lowering their threshold for coping with stress, but environmental triggers may also be needed to actually start the development of the disorder. They said the diathesis-stress model best fit this explanation, taking on board a biological and environmental explanation.
Gottesman and Shields also looked at 11 other studies, and concluded that their results did support each other, although they thought there were some methodological issues, such as with the sampling techniques. This is how they concluded their results were reliable, and that there is a genetic element to schizophrenia.

Evaluation: