Do we really know that the values obtained from twin studies are correct? Are twins different from singletons? Do identical and non-identical twins differ in more ways than their genes?
Just like any other research design, twin studies may have some disadvantages. Twins are often born a few weeks earlier than singletons, and usually have a lower birth weight, so for some characteristics they may not be representative of the population as a whole. However, for other characteristics, twins seem to be very similar to singletons. Researchers have studied this by also including the non-twin siblings of twins in their study. If they find no differences between twins and their siblings, then we can be confident that the twins are representative of the population as a whole.
People often wonder if the environment that is shared between twins is really the same for identical and non-identical twins. Surely identical twins are treated more alike than non-identical twins? Many twin researchers have studied this issue, for instance by comparing non-identical twins who thought they were identical with twins who always knew they were non-identical (and the other way around). Most of these studies have not found evidence that identical and non-identical twins are treated differently.
What do you think?
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