How do twin studies work?
The logic of the twin study is quite simple. There are two types of twins, monozygotic or identical twins (who are from the same egg and therefore have 100% of their genes in common) and dizygotic or non-identical twins (who are from 2 separate eggs and therefore, like any sibling, share on average 50% of the same genes). Assuming that a trait is genetically influenced then it follows that identical twins will be more similar for that trait than non-identical twins. Conversely if a trait is heavily influenced by shared environmental factors it follows that identical and non-identical twins should be equally similar. Finally the difference between identical twins is a measure of non-shared environment, i.e. environmental influences that make the twins more different. Using this naturally occurring experimental design it is possible, with model fitting statistical techniques, to estimate the relative contribution of genes (heritability) and environment to variation for a given trait. Mor
