Psychology: Methodology: Twin Studies & Evaluation
Twin studies are used to assess the effect of genes on behaviour.
Identical twins are monozygotic, they came from one sperm and one egg, and are the product of one fertilisation, and they therefore share 100% of their genes. Their DNA is 100% the same, so if a characteristic is genetic, they will both display it. If a feature such as IQ is entirely genetic, it will be identical within the twins. In practice, no characteristic connected with behaviour is entirely shared by MZ twins, there is always some environmental influence.
It is important to note that MZ twins never share the exact same environment, even within the womb, so they tend to develop differently despite their DNA being the same. There are some small physical differences from the start, like fingerprints. Epigenetic modification is the term for how, over time, environmental influences affect which genes are turned on and off so identical twins have differences.
Non-identical, dizygotic, twins, come from two fertilised eggs. Since they develop from different eggs, their DNA is only as similar as that of regular siblings, it isn't 100% the same, they share about 50% of their DNA.
Therefore, if a characteristic is genetic, DZ twins would be expected to share it to an extent, but to a lesser extent than MZ twins.
Twin studies compare MZ & DZ twins on certain characteristics to see if there is a difference between them in terms of showing the characteristic. If there are quite strong differences, with it being present more within MZ twins, it is said to have a genetic basis to at least some extent. When both twins share a characteristic there is said to be a concordance rate.
Characteristics which have been studied in this way include schizophrenia, IQ, alcoholism, depression, personality & anorexia.
Gottesman & Shields [1966] is an example of a twin study, they used a twin study to look into schizophrenia, and found that there is a concordance rate for schizophrenia. 42% of MZ twins who have a twin with schizophrenia have schizophrenia or a similar illness; this figure is 17% for DZ twins. This suggests some genetic basis for schizophrenia.
Strengths:
Both MZ & DZ twins are born at the same time and share a similar environment, but MZ twins share 100% of DNA, whereas DZ twins share only 50%. This is the main way to study genetic influence on behaviour, there is no other way of having identical DNA.
Both MZ & DZ twins share the same environment and are treated as twins, so any results can be attributed to genes as shared environment applies to both types of twins. This offers a natural control over environmental factors!
They can be used to study many things.
Weaknesses:
MZ twins are of the same sex and look identical, so they will be treated more alike than DZ twins, which means that the environments are not the same- MZ are likely to share an environment to a greater extent than DZ twins!
Epigenetic modification must be taken into account, from the moment of conception for MZ twins, there are differences- some share the placenta, some don't. Many genes need an environmental switch which can differ as the environments of twins does.