What are the advantages and disadvantages in using twins to analyze genetic heritability and Why might it be useful for future twin studies to separate out monozygotic (identical) from dizygotic (fraternal) twins?
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. Therefore, it seems likely that any differences between twins will have to be caused by environment rather than by genetics. It is also possible to make twin studies more robust by differentiating between identical and fraternal twins. For example, imagine that a study shows that a large percentage of identical twins share a common trait while a much smaller percentage of fraternal twins do. This would strongly imply that genes have a great deal to do with this particular trait. Thus, twin studies allow researchers to make comparisons between people whose genes are much more similar than usual.
There are, however, drawbacks to twin studies. The two main drawbacks are the low number of possible subjects for the studies and the fact that they may not be representative of the overall population. Twins are not terribly common and identical twins are even less so. Then you have to find twins where at least one twin has the condition you wish to study. This reduces the possible number of subjects greatly. When this happens, findings become less trustworthy. In addition, it is possible that there are differences between twins and the population as a whole. If this is the case, studies of twins may yield results that are more applicable to twins than to other people.
Thus, twin studies are very valuable tools, but they do have problems that can make them less reliable.