because MZ twins have identical genotypes, and therefore, differences between them are attributed to environmental influences. In a study by Fraga and colleagues (2005), MZ twins were found to be epigenetically indistinguishable during the early years of life, but older MZ twins exhibited remarkable differences in their epigenetic profiles. These findings suggest that epigenetic changes may be a mechanism by which environmental influences contribute to the differences in outcome observed for a variety of psychological traits of interest between genetically identical individuals.
