A further distinction exists in the study of GxE interaction: that of latent versus measured GxE interaction. Although they are both active fields of study it is actually quite striking how different and nonoverlapping the literatures are. Latent genetic influences are inferred by comparing similarity across individuals with different degrees of genetic sharing, for example, different types of family members. Twin studies are one of the most widely used methods for studying latent genetic effects. In the classic twin design, information about the relative importance of genetic and environmental influences is inferred by comparing the similarity of monozygotic (MZ) twins and dizygotic (DZ) twins. MZ twins arise from a single egg, fertilized by a single sperm, and therefore share all of their genetic variation identical by descent. DZ twins result from two eggs, fertilized by two sperm, and therefore share, on average, 50% of their segregating genetic variation, as do ordinary siblings. The basic genetically informative twin model partitions variance in a behavior into additive genetic influences (A), common environmental influences (C) [or dominant genetic influences (D); C and D
