Question

Discuss some of the advantages and disadvantages in using twins to analyze genetic heritability. Why might it be useful for future twin studies to separate out monozygotic (identical) from dizygotic (fraternal) twins?

Answer 1

• Twin studies are a mainstay of the nature v. nurture debate because they allow researchers to conduct “natural experiments” on human beings. When researchers try to determine the relative importance of genetics and environment with regard to a certain condition (for example, schizophrenia), they have a hard time because they cannot generally get people with the same genes. This is where twin studies come in. Identical twins have essentially identical DNA.  A major advantage of twin studies is that the twins share their DNA and so the effects of the environment can be established.
• The use of twins can improve the statistical power of a genetic study by reducing the amount of genetic and/or environmental variability; the extent to which different assumptions matter may depend on which trait is being studied.
• A major disadvantage is that twin studies are not representative of society as a whole and so the generalizability of the results may be limited. The environment that they are in when they are developing, the womb, is different for twins than it is for singular births as they have to share it and may be exposed to different levels of hormones. Also when they are born they may be treated differently as they are one half of a pair which could lead to certain traits being more define.
• Many twin registries depend on the voluntary participation of twins. This leads to volunteer bias or recruitment bias, a special type of selection bias, which may lead to overinclusion of identical and female twins, resulting in overestimation of the heritability of the trait or condition under study.
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