In the classical twin design, monozygotic (identical) twins share their common environment and they are assumed to share 100% of their genes. Dizygotic (fraternal) twins also share their common environment and they are assumed to share on average 50% of their genes. By comparing the resemblance for aggressive behavior between monozygotic and dizygotic twins, the total phenotypic variance of aggression can be divided into additive genetic factors (or heritability, h2), shared environmental factors (c2), and nonshared environmental factors (e2). Shared environmental factors refer to nongenetic influences that contribute to similarity within pairs of twins. Nonshared environmental factors are those individual experiences that cause siblings to differ in their levels of aggressive behavior. Heritability is the proportion of total phenotypic variance due to genetic variation (Neale and Cardon, 1992). Genetic influences may also be divided into those that are additive (i.e., allelic effects add up across loci) and those that are nonadditive (i.e., due to dominance or epistasis). In twin studies, however, it is not possible to estimate both additive and nonadditive genetic effects (d2) simultaneously with shared twin environment effects.
