The classical twin study design utilizes data from monozygotic (MZ) and dizygotic (DZ) twin pairs, reared together, to attempt to disentangle the role of genetic and environmental influences in population variation in a measured phenotype. Genetic variants are shared completely (100%) between members of MZ twin pairs while DZ twin pairs share, on average, 50% of their genetic variants. There are two sources of environmental variation—those latent environmental factors that members of a twin pair have in common (shared environment) and those environmental factors that are unique to individuals (non-shared environment). Shared environmental factors overlap 100% between members of MZ and DZ twin pairs, under the important assumption of equal environments. Non-shared environment is, by definition, uncorrelated between members of a twin pair. The caveats of a classical twin study include the equal environments assumption and the fact that epigenetic modifications (when not inherited) or sporadic structural change in DNA, which can occur in one member of an MZ pair but not the other, are not captured in a twin study. Reviews of the evidence from twin and adoption studies
