Evaluation of twin and adoption studies

Strengths of Twin Studies

There is no other ways to effectively study the influence of genetics and MZ twins share 100% of their DNA
Because MZ twins look the same, and are the same sex, they tend to be treated as twins rather than individuals. This gives natural control over the effects of the environment
Although unusual, twin studies have generated large volumes of data.This makes the findings which point to genetic and environmental factors more reliable.

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MZ have 100% identical DNA, but epigenetic modification needs to be taken into account. They may grow and develop differently because of environmental differences
MZ twins may be treated more similarly than DZ twins because they look the same and are the same sex. The environment may therefore be not as controlled as we thought.
The usefullness of data collected from twin studies depends on the instruments used to gather data, such as personality and IQ tests being valid and reliable. Such psychological variables are, however, notoriously difficult to measure.

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They are longitudinal so that they can study developmental factors
It is the only type of study that can find a direct comparison between nature and nurture because it isolates the effects of the environment as the participant does not share the same genetics as the parents
They investigate a range of variables and used varied samples and methods. These have included trans-racial studies and meta-analysis. These have all produced similar results which suggest that findings are reliable.

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Adoption agencies tend to try to find parents similar to the biological parents, so effectively the environment will play a part
Participants who take part in adoption studies are usually selected by a volunteer sample. This introduces an element of bias into the sample, making it harder to generalise.

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