Twin Studies and Adoption Studies

A way of finding out whether a disorder has a genetic component is to see whether it runs in families. If relatives of sufferers have a higher than average risk of getting the disorder themselves, then it may be that the disorder has a genetic component.

However, family members typically share similar environments. Consequently, increased risk amongst close relative may simply indicate that that are exposed to the same set of environmental risks.

An alternative approach is to do a twin study. This looks at the concordance rate (degree of similarity) of twins with respect to the disorder being considered. Concordance rates means the probability of one twin having the disorder if the other already has it expressed as a percentage.

In a twin study, Monozygotic (MZ - identical) and Dizygotic (DZ - non-identical) twins are compared. Whilst MZ twins have a greater degree of genetic similarity, both types of twin pair grow up in identical environments. So if we discover that MZ twins have a higher concordance, this cannot be because their environments are more similar than those of DZ twins; it must therefore be because their genes are more similar. 

• Twins provide a perfect way of controlling for genetic inheritance as MZs always share 100% and DZ share 50%, a naturally occurring manipulation of an independent variable, yet both have the same environmental experience (control of confounding variables) meaning that the effect of nature over nurture can be studied effectively.
• With increasing numbers of multiple births, it is possible to replicate the findings of twin studies with large samples in many different world cultures, increasing the reliability and generalisability of the findings; records of multiple births means that researchers can easily find large samples with which to test their hypotheses.

One of the grounding assumptions of the twin study methodology concerns the degree of similarity between the environments of MZ and DZ twins. Because both types of twin pair are born at the same time into the same environment it is assumed that each member of a twin pair is exposed to exactly the same set of environmental influences, regardless of zygosity. However, this not strictly true as ...

• MZ twins can experience differences in terms of environmental experiences, even in the womb
• MZ twins are typically closer than DZ twins, their parents are more likely to dress them similarly and they are always the same sex; all these factors mean that people will treat them more similarly and therefore it may not be right to assume that both MZ and DZ twin pair share equally similar environments; MZ environments may be more similar than DZs
• Even though genetically identical, MZ twins are not exactly the same;
• o   their fingerprints are different.
• o   One twin is typically larger and more robust than the other; this is first observable during pre-natal development

• Genes turn on and off at different point in life and in interaction with differing environmental experiences (epigenetic modification); therefore MZ twins may both share a gene or cluster of genes which predispose them to schizophrenia however, only one twin may be exposed to the environmental circumstances which trigger that gene to start affecting the person’s thinking and behaviour (cross reference to nature-nurture debate).
• In studies of separated twins, whereby similarity in developmental outcomes must be due to genes and not to similar environments are problematic as often the environments that they are placed in are actually more similar than the researches have credited.

Aim: To investigate whether schizophrenia has a genetic basis and try to replicate other twin studies into schizophrenia. Their procedure was:

• The researcher used secondary data from the Maudsley and Bethlem, Royal Joint Hospital studying both MZ and DZ twins, where at least one of the twins had been diagnosed with schizophrenia. (Out of the 392 patients, they found 68 patients were one of twins and had been diagnosed with schizophrenia.)
• 3 patients were omitted because they couldn’t tell if they were MZs or DZs and another 3 were omitted because they were overseas, leaving 62 patients. Of these, 5 cases showed both MZ twins had been diagnosed with schizophrenia, leaving 57 pairs with at least one who had a diagnosis. G&S tracked down the other twin in each case.
• There were 31 males and 31 females in the patients diagnosed, with ages from 19-64.
• They used blood tests and visual tests to check whether they were MZs or DZs.
• Data was collected using hospital records, case histories for the twins, tape recordings gained through semi-structured interviews, personality testing and a thought disorders test.
• They looked at the concordance rate within MZ twins to see when one twin had been diagnosed with schizophrenia the likelihood of the other twin also developing the disorder.

Results:

They found a 42% concordance rate for MZ twins meaning around 42% of the time when one twin has schizophrenia, the other one has it too.

For DZ twins, the concordance rate was 9%.

Conclusion:

The research concluded that there appears to be a genetic component in the cause of schizophrenia. Findings can be explained through the diathesis-stress model where one may have a genetic predisposition to schizophrenia which is triggered through environmental factors.

The diathesis-stress model suggests that there is some kind of environmental trigger that causes an already present genetic mutations or genetic disorders to be realised.

Strengths:

The study replicates other studies and the results are backed up by them, which mean they are likely to be reliable. For example, Inouye (1961) in Japan found a 74% concordance rate for twins with progressive chronic schizophrenia and 39% where twins had a mild transient schizophrenia. 

It addresses criticisms of previous studies by detailing the sampling carefully so that it was understood which twins were included and why. This is great detail about the different diagnoses – for example, whether the diagnosis was schizophrenia, some other psychosis or some abnormality.

Weaknesses:

The researchers felt that a concordance rate simply notes if one twin has an abnormality, the other one has it too. It would have been useful to have information about the degree of the abnormality, such as scale showing ‘schizophrenia’ though ‘other psychiatric diagnoses’ to ‘some abnormality’ to ‘normal’.

The researchers suggest there might be different forms of schizophrenia. They also suggest that some of the disorders diagnosed might come from life experiences (such as being a prisoner of war) rather than genes, and the study did not easily distinguish between reasons for schizophrenia.

Adoption studies are carried out because the environment of adopted children is not the same as that of their biological family, yet they have genes in common.

For example, if the effects of genes on schizophrenia is being studied, a researcher might want to find out whether children of a schizophrenic parent are more likely to develop the disorder than those with no family history.

By choosing children adopted children who do not share the environment of their biological families then the issue of environmental influences which would exist if they lived with their biological families is controlled.

Kety et al (1994) carried out an adoptive family study in Finland and found evidence for a genetic cause of schizophrenia. Out of a sample of 155 children, schizophrenia was diagnosed in nine children. Eight of them had biological mothers who had been diagnosed with schizophrenia.

Strengths

• Adoption studies are a way of separating genes from the environment which is very difficult to do otherwise. This gives them good ecological validity.
• Developmental trends can be studied because they can be longitudinal following the same group of children over a number of years. This again increases validity.

Weaknesses

• It is likely that only certain types of family are accepted as adopters of children so this means adopting families may be similar to each other. This means the environment of an adopted child may not be a true reflection of a broad range of family environments.
• Children tend to be placed in families like their own so their birth family and adopted family could have similarities in terms of environmental influences meaning there is not a guarantee that environmental influences can be discounted completely.

Looked at data from the Early Growth and Development Study  (EGDS) in the USA and followed 360 linked sets of birth and adoptive parents and the adopted children involved. The focus was on genes and the environment.

Aim 1: To look at specific environmental issues related to parenting and family processes that mediate (interact with) genetic factors on a child

Aim 2: same as aim 1 but focusing on how the environment might moderate genetic influences

Aim 3: to look at adopted child and adoptive parent to see, how, when and why genes and environment interactions occur between them.

Procedure:

Study used “adoptive triad” which is birth parents, adoptive parents and adopted child.

360 linked sets of adoptive triads as children entered school. Ppts were recruited between 2003 and 2006 via adoption agencies. The baby had to have been adopted within 3 months of birth and have no major medical conditions.

The study measured executive function (a measure that can foretell externalising problems), early literacy skills (a measure that can foretell school performance) and cortisol reactivity (linked to internalising problems).

Assessment was by means of a questionnaire and interviews for birth and adoptive parents, observation for adoptive families, standardised testing for both sets of parents and children, salivary cortisol for birth parents and adoptive children and salivary DNA for all ppts.

Results:

Found that externalising behaviour of birth mothers could show a genetic risk for externalising problems and adoptive parents’ level of anxiety or depression could be an environmental issue that was associated with the child being unable to shift attention from a frustrating task or event.

This shows that a genetic issue around externalising behaviour and attention remaining on frustrated tasks is affected by the environment .

Conclusion:

The main conclusion is that genetic factors in an adopted child interact with environmental influences within the adoptive family to produce behaviour. If such things are known about there can be a possible intervention to help alleviate the issue.

For example, an adopted child whose focus is on frustrating events rather than moving on (and so likely to show more aggression) in the presence of an adoptive mother who is depressed or anxious is more likely to have ongoing issues compared to a child whose adoptive parent doesn’t have these conditions. 

Strengths: 

• Leve at al ensured that sample was generalizable in terms of demographics(ethnicity , background etc) across the USA
• Data was gathered using lots of different methods –triangulation – this allows for reliability of findings to be checked.

Weaknesses:

• Difficult to show cause and effect relationships between variables e.g., it is hard to measure attention and frustration and then link this to later aggressive behaviour.
• Inferring genetic influence from looking at birth mother and child similarities is not the same as using DNA to show genotype.
• Although researchers intended to collect DNA samples this was not done at this point in research so this wasn’t available at time.