With the classical twin design, trait variance can be partitioned into its genetic and environmental (shared within twin pairs and non-shared) components, by analysing the resemblance in MZ and DZ twin pairs. Additive genetic variance (A) results from the sum of allelic effects within and across multiple genes affecting a trait. Shared environmental variance (C) is due to environmental influences shared within twin pairs, such as the family environment, prenatal influences, parental style, and socio-economic status. Unshared environmental variance (E) results from environmental factors that are not shared within twin pairs (e.g., idiosyncratic events and experiences, unshared peers) and includes measurement error.
