How does it work?
All humans share circa 99.9% of their DNA. The 0.1% that is not shared causes individual differences and are called segregating alleles *. Identical twin twins share 100% of their segregating alleles, because they develop from the same maternal egg, fertilised by a single sperm, which splits after fertilisation. These twins are therefore called monozygotic. Non-identical twins share on average 50% of their segregating alleles (just like any other couple of non-twin siblings), because two different eggs were fertilised by two different sperms (dizygotic twins).
Another special feature of twins is that both identical and non-identical twins grow up in the same home environment at the same age. Therefore, both types of twins share the home environment (making the twins more similar to each other) to the same extent. This is called the equal environments assumption and enables us to “control” for the environmental influences of the home environment (nurture) when estimating heritability (nature). In other words, identical and non-identical twins differ only in the genetic influences they share, but not in the environmental influences they share.
We estimate heritability by comparing the degree of similarity between identical and non-identical twins. If identical twins are more similar to each other than non-identical twins, we can conclude—due to the equal environments assumption—that this is due to genetic influence on the trait (i.e., the trait is heritable). The more similar identical twins are to each other and the less similar non-identical twins are, the higher the heritability. In the simplest twin study design, we estimate the proportions of heritability, shared environmental factors and non-shared environmental factors, based on the correlations between identical and non-identical twins.