Monozygotic twins have double concordance rate for hysterectomy compared to dizygotic twins. The classic twin study compares monozygotic and dizygotic twins. The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. They used skin grafting to differentiate between monozygotic and dizygotic twins in cattle. Monozygotic (identical) but not dizygotic (fraternal) twins tend to have similar sleep habits. Monozygotic twins can have markedly different epigenetic profiles. This heritable variation is estimated from heritability studies based on monozygotic twins. The genetics are likely complex, however, given the inconsistent inheritance and discordance among monozygotic twins. When monozygotic twins are born with different sexes it is because of chromosomal defects. Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. Dizygotic (fraternal) and monozygotic (identical) twins show evidence of epigenetic influence in humans. Monozygotic twins always have the same genotype. Monozygotic twins may also be created artificially by embryo splitting. The predisposing factors of monozygotic twinning are unknown. Monoamniotic twins are "always" monozygotic (identical twins).